MONERA

I.Centrostigmatic Types.—The natural middle of the body is a mathematical point. Properly speaking, only one form is of this type, and that is the most regular of all, the sphere or ball. We may, however, distinguish two sub-classes, the smooth sphere and the flattened sphere. The smooth sphere (holospœra) is a mathematically pure sphere, in which all points at the surface are equally distant from the centre, and all axes drawn through the centre are of equal length. We find this realized in its purity in the ovum of many animals (for instance, that of man and the mammals) and the pollen cells of many plants; also cells that develop freelyfloating in a liquid, the simplest forms of the radiolaria (actissa), the spherical cœnobia of the volvocina and catallacta, and the corresponding pure embryonic form of theblastula. The smooth sphere is particularly important, because it is the only absolutely regular type, the sole form with a perfectly stable equilibrium, and at the same time the sole organic form which is susceptible of direct physical explanation. Inorganic fluids (drops of quicksilver, water, etc.) similarly assume the purely spherical form, as drops of oil do, for instance, when put in a watery fluid of the same specific weight (as a mixture of alcohol and water).The flattened sphere, or facetted sphere (platnosphæra), is known as an endospherical polyhedron; that is to say, a many-surfaced body, all the corners of which fall in the surface of a sphere. The axes or the diameters, which are drawn through the angles and the centre, are all unequal, and larger than all other axes (drawn through the facets). These facetted spheres are frequently found in the globular silicious skeletons of many of the radiolaria; the globular central capsule of many spheroidea is enclosed in a concentric gelatine envelope, on the round surface of which we find a net-work of fine silicious threads. The meshes of this net are sometimes regular (generally triangular or hexagonal), sometimes irregular; frequently starlike silicious needles rise from the knots of the net-work (A-f, 1, 51, 91). The pollen bodies in the flower-dust of many flowering plants also often assume the form of facetted spheres.II.Centraxonia Types.—The natural middle of the body is a straight line, the principal axis. This large group of fundamental forms consists of two classes, according as each axis is the sole fixed ideal axis of the body, or other fixed transverse axes may also be distinguished, cutting the first at right angles. We call the former uniaxial (monaxonia), and the latter transverse-axial (stauraxonia). The horizontal section (vertically to the chief axis) is round in the uniaxials and polygonal in the transverse-axial.In the monaxonia the form is determined by a single fixed axis, the principle axis; the two poles may be either equal (isopola) or unequal (allopola). To the isopola belong the familiar simple forms which are distinguished in geometry as spheroids, biconvex, ellipsoids, double cones, cylinders, etc. A horizontal section, passing through the middle of the vertical chief axis, divides the body into two corresponding halves. On the other hand, many of the parts are unequal in size and shape in theallopola. The upper pole or vertex differs fromthe basal pole or ground surface; as we find in the oval form, the planoconvex lens, the hemisphere, the cone, etc. Both sub-classes of the monaxonia, the allopola (conoidal) and the isopola (spheroidal), are found realized frequently in organic forms, both in the tissue-cells of the histona and the independently living protists (A-f, 4, 84).In the stauraxonia the vertical imaginary principal axis is cut by two or more horizontal cross-axes or radial-axes. This is the case in the forms which were formerly generally classed as regular or radial. Here also, as with the monaxonia, we may distinguish two sub-classes, isopola and allopola, according as the poles of the principal axis are equal or unequal.Of thestauraxonia isopolawe have, for instance, the double pyramids, one of the simplest forms of the octahedron. This form is exhibited very typically by most of the acantharia, the radiolaria in which twenty radial needles (consisting of silicated chalk) shoot out from the centre of the vertical chief axis. These twenty rays are (if we imagine the figure of the earth with its vertical axis) distributed in five horizontal zones, with four needles each, in this wise: two pairs cross at right angles in the equatorial zone, but on each side (in north and south hemispheres) the points of four needles fall in the tropical zone, and the points of four polar needles in the polar circles; twelve needles (the four equatorial and eight polar) lie in two meridian planes that are vertical to each other; and the eight tropical needles lie in two other meridian planes which cross the former at an angle of forty-five degrees. In most of the acantharia (the radial acanthometra and the mailed acanthophracta)—there are few exceptions—this remarkable structural law of twenty radial needles is faithfully maintained by heredity. Its origin is explained by adaptation to a regular attitude which the sea-dwelling unicellular body assumes in a certain stage of equilibrium (A-f, 21, 41). If the points of the real needles are connected by imaginary lines, we get a polyhedrical body, which may be reduced to the form of a regular double pyramid. This typical form of the equipolar stauraxonia is also found in other protists with a plastic skeleton, as in many diatomes and desmidiacea (A-f, 24). It is more rarely found embodied in the tissue-cells of the histona.Unequipolar stauraxonia are the pyramids, a fundamental form that plays an important part in the configuration of organic bodies. They were formerly described as regular or fundamental forms. Such are the regular blooms of flowering plants, the regular echinoderms, medusæ, corals, etc. We maydistinguish several groups of them according to the number of the horizontal transverse axes that cut the vertical main axis in the middle.Two totally different divisions of the pyramidal types are the regular and the amphithecta pyramids. In the regular pyramids the transverse axes are equal, and the ground-surface (or base) is a regular polygon, as in the three-rayed blooms of the iris and crocus, the four-rayed medusæ (A-f, 16, 28, 47, 48, etc.), the five-rayed "regular echinoderms," most of the star-fish, sea-urchins, etc. (A-f, 10, 40, 60), and the six-rayed "regular corals" (A-f, 9, 69).The amphithecta (or two-edged) pyramids, a special group of pyramidal types, are characterized by having as their basis a rhombus instead of a regular polygon. We may, therefore, draw two imaginary transverse axes, vertical to each other, through the ground-surface, both equipolar, but of unequal length. One of the two may be called the sagittal axis (with dorsal and ventral pole), and the other the transverse axis (with right and left pole); but the distinction is arbitrary, as the two are equipolar. In this lies the chief difference from the centroplane and dorsiventral forms, in which only the lateral axis is equipolar, the sagittal axis being unequipolar. We find the bisected pyramid in a very perfect form in the class of the ctenophora (or comb-medusæ, A-f, 27), where it is quite general. The striking typical form of these pelagic cnidaria is sometimes called biradial, sometimes four-rayed and bilateral, and sometimes eight-rayed-symmetrical. Closer study shows it to be a rhombus-pyramid. The originally four-rayed type, which it inherited from craspedote medusæ, has become bilateral by the development of different organs to the right and left from those before and behind.Similar rhombo-pyramidal forms to those of the ctenophora are also found in some of the medusæ and siphonophora, many of the corals and other cnidaria, and many flowers. The name "two-edged" which is given to this special type is taken from the ancient two-edged sword. Its chief axis is unequipolar, the handle being at the basic pole and the point at the verticle pole; but the two edges left and right are equal (poles of the lateral axis), and also the two broad surfaces (dorsal and ventral, joined by the sagittal axis).III.Centroplane Types.—The natural middle of the body is a plane, the median or chief plane (planum medianumorsagittale); it divides the bilateral body into two symmetrical halves, the right and the left. With this is associated thecharacteristic antithesis of back (dorsum) and belly (venter); hence, in botany this type (found, for instance, in most green leaves) is called the dorsiventral, and in zoology the bilateral in the narrower sense. One characteristic of this important and wide-spread type is the relation of three different axes, vertical to each other; of these three straight axes (enthyni) two are unequipolar and the third equipolar. Hence, the centroplanes may also be called tri-axial (triaxonia). In most of the higher animals (as in our own frame) the longest of the three axes is the principal one (axon principalis); its fore pole is the oral or mouth pole, and its hinder pole is the aboral or caudal (tail) pole. The shortest of the three enthyni is, in our body, the sagittal (arrow) or dorsiventral axis; its upper pole is at the back and its lower pole at the belly. The third axis—the transverse or lateral axis—is equipolar, one pole being called the right and the other the left. The various parts which make up the two halves of the body have relatively the same disposition in each half; but absolutely speaking (namely, in relation to the middle plane) they are oppositely arranged.Further, the centroplane or bilateral forms are also characterized by three vertical axes which may be drawn through each of the normal axes. The first of these normal planes is the median plane; it is defined by the chief axis and the sagittal axis, and divides the body into two symmetrical halves, the right and left. The second normal plane is the frontal plane; this passes through the chief axis and the transverse axis (which is parallel to the frontal surface in our body), and divides the dorsal half from the ventral half. The third normal plane is the cingular (waist) plane: this is defined by the sagittal and transverse axes. It divides the head half (or the vertical part) from the tail half (or the basal part).The name "bilateral symmetry," which is especially applied to the centroplane and dorsiventral types, is ambiguous, as I pointed out in 1866 in an exhaustive analysis and criticism of these fundamental forms in the fourth book of theGeneral Morphology. It is used in five different senses. For our present general purpose it suffices to distinguish two orders of centroplane types, the bilateral-radial and the bilateral-symmetrical; in the former the radial (pyramidal) form is combined with the bilateral, but not in the latter.The bilateral-radial type comprises those forms in which the radial structure is combined in a very characteristic fashion with the bilateral. We have striking examples in the three-rayed flowers of the orchids (A-f, 74), the five-rayed blooms of thelabiate and papilionaceous flowers, etc., in the plant world; and in the five-rayed "irregular" echinoderms, the bilateral sea-urchins (spatangida, clypeastrida, A-f, 30) in the animal world. In these cases the bilateral symmetry is recognizable at the first glance, as is also the radial structure, or the composition from three to five or more raylike parts (paramera), which are arranged bilaterally round a common central plane.The bilateral-symmetrical type is general among the higher animals which move about freely. The body consists of two antithetic parts (antimera), and has no trace of radial structure. In the free moving, creeping, or swimming animals (vertebrates, articulates, mollusks, annelids, etc.) the ventral side is underneath, against the ground, and the dorsal side upward. This form is clearly the most useful and practical of all conceivable types for the movement of the body in a definite direction and position. The burden is equally distributed between the two sides (right and left); the head (with the sense organs, the brain, and the mouth) faces frontward and the tail behind. For thousands of years all artificial vehicles (carts on land and ships in water) have been built on this type. Selection has recognized it to be the best and preserved it, while it has discarded the rest. There are, however, other causes that have produced the predominance of this type in green leaves—the relation to the supporting stalk, to the sunlight that falls from above, etc.Special notice must be taken of those bilateral forms which were originally symmetrical (by heredity), but have subsequently become asymmetrical (or of unequal halves), by adaptation to special conditions of life. The most familiar example among the vertebrates are the flat-fishes (pleuronectides), soles, flounders, turbots, etc. These high and narrow and flattened boney-fishes have a perfect bilateral symmetry when young, like ordinary fishes. Afterwards they form the habit of laying on one side (right or left) at the bottom of the sea; and in consequence the upper side, exposed to the light, is dark colored, and often marked with a design (sometimes very like the stony floor of the ocean—a protective coloring), while the side the flat-fish lies on remains without color. But, what is more curious, the eyefrom the under side travels to the upper side, and the two eyes lie together on one side (the right or left); while the bones of the skull and the softer parts of each side of the head grow quite crooked. Naturally, this ontogenetic process, in which a striking lack of symmetry succeeds to the early complete symmetry of each individual, can only be explained by our biogenetic law; it is a rapid and brief recapitulation (determined by heredity) of the long and slow phyletic process which the flat-fish has undergone for thousands of years in its ancestral history to bring about its gradual modification. At the same time, this interesting metamorphosis of thepleuronectidesgives us an excellent instance of the inheritance of acquired characteristics, as a consequence of constant œcological habit. It is quite impossible to explain it on Weismann's theory of the germ-plasm.We have another striking example among the invertebrates in the snails (gasteropoda). The great majority of these mollusks are characterized by the spiral shape of their shells. This variously shaped, and often prettily colored and marked, snail's house is in essence a spirally coiled tube, closed at the upper end and open at the lower (or mouth): the mollusk can at any moment withdraw into its tube. The comparative anatomy and ontogeny of the snails teach us that this spiral shell came originally from a simple discoid or cylindrical dorsal covering of the once bilateral-symmetrical mollusk, by the two sides of the body having an unequal growth. The cause of it was a purely mechanical factor—the sinking of the growing visceral sac, covered with the shell, to one side; one part of the viscera contained in it (the heart, kidneys, liver, etc.) grew more strongly on one side than the other in consequence of this; and this was accompanied by considerable displacement and modification of the neighboring parts, especially the gills. In most snails one of thegills and kidneys and the ventricle of the heart corresponding to these have disappeared altogether, only those of the opposite side remaining; and the latter have moved from the right side to the left, or vice versa. The conspicuous lack of symmetry between the two halves of the body which resulted from this finds expression in the spiral form of the snail's shell. This remarkable ontogenetic metamorphosis also can be fully explained by a corresponding phylogenetic process, and affords a very fine instance of the inheritance of acquired characters.There are also many examples of this asymmetry of bilateral forms in the plant world, such as the green foliage-leaves of the familiar begonia and the blooms ofcanna.IV.The Centraporia.—Few organic forms are completely irregular and without axes, as usually the attraction to the earth (geotaxis) or to the nearest object determines the special direction of growth, and so the formation of an axis in some direction or other. Nevertheless, we may instance as quite irregular the soft and ever-changing plasma-bodies of many rhizopods, the amœbinæ, mycetozoa, etc. Most of the sponges also—which we regard as stocks of gastræads—are completely irregular in structure; the most familiar example is the common bath-sponge.An impartial and thorough study of organic forms has convinced me that their actual, infinitely varied configurations may all be reduced to the few typical forms I have described. Comparative anatomy and ontogeny further teach us that the countless modifying processes which have led to the appearance of the various species have acted by adaptation to different environments, habits, and customs, and give us, in conjunction with heredity, a physiological explanation of this morphological transformation. But the question arises as tothe origin of these few geometrically definable types, and the cause of their divergence.In this important and difficult question we find a great variety of opinions and a strong leaning to dualistic and mystic theories. Educated laymen, who have only a partial and imperfect acquaintance with the biological facts, think that they are justified here in appealing to a supernatural creation of forms. They contend that only a wise creator, following a rational and conscious design, could produce such structures. Even distinguished and informed scientists lean in this matter towards mystic and transcendental ideas; they believe that the ordinary natural forces do not suffice to explain these phenomena, and that at least for the first construction of these fundamental types we must postulate a deliberate creative thought, a design, or some such teleological cause, and therefore consciously acting final causes. So say Nägeli and Alexander Braun.In direct opposition to this, I have ever maintained the view that the action of familiar physical forces—mechanical efficient causes—fully suffices to explain the origin and transformation of these fundamental types, as well as for all other biological and inorganic processes. In order to understand this monistic position thoroughly, and to meet the errors of dualism, we must bear in mind always the radical processes of growth which control all organic and inorganic configuration, and also the long chain of advancing stages of development, which lead us from the simplest protists, the monera, to the most advanced organisms.The unicellular organisms exhibit the greatest variety from the promorphological point of view. In the single class of the radiolaria we find all imaginable geometrical types represented. This is seen in a glance at the one hundred and forty plates on which I have depicted thousands of these graceful little protozoa in my monograph(Challenger Report, vol. xviii.). On the other hand, the monera, at the lowest stage of organic life, the structureless organisms without organs that live on the very frontier of the inorganic world, are very simple. Especially interesting in this connection are the chromacea, which have hitherto been so undeservedly and so incomprehensibly neglected. Among the well-known and widely distributed chroococcacea, the chroococcus, cœlosphærium, and aphanocapsa are quite the most primitive of all organisms known to us—and at the same time the organisms that enable us best to understand the origin of life by spontaneous generation (archigony). The whole organism is merely a tiny, bluish-green globule of plasm, without any structure, or only surrounded by a thin membrane; its fundamental form is the simplest of all, the centraxial smooth sphere. Next to these are the oscillaria and nostochina, social chromacea, which have the appearance of thin, bluish-green threads. They consist of simple primitive (unnucleated) cells joined to each other; they seem often to be flattened into a discoid shape as a result of close conjunction. Many protists are found in two conditions, one mobile with very varied and changeable forms, and one stationary with a globular shape. But when the separate living cell begins to form a firm skeleton or protective cover for itself, it may assume the most varied and often most complicated forms. In this respect the class of the radiolaria among the protozoa, and the class of the diatomes among the protophyta (both of which have flinty shells), surpass all the other groups of the diversified realm of the protists. In myArt-forms in NatureI have given a selection of their most beautiful forms (diatomes, A-f, 4, 84; radiolaria, A-f, 1, 11, 21, 22, 31, 41, 51, 61, 71, 95). The most remarkable and most important fact about them is that the artistic builders of these wonderful and often very ingenious andintricate flinty structures are merely the plastidules or micella, the molecular and microscopically invisible constituents of the soft viscous plasm (sarcode).The configuration of the histona differs essentially from that of the protists, since in the case of the latter the simple unicellular body produces for itself alone the whole form and vital action of the organism, while in the histona this is done by the cell state, or the social combination of a number of different cells, which make up the tissue body. Hence the ideal type which we can always define in the actual histonal form has quite a different significance from that in the unicellular protists. In the latter we find the utmost diversity in the configuration of the independent living cells and the protective cover it forms; among the histona the number of fundamental forms is limited. It is true that the cells themselves which make up the tissues may exhibit a great variety in form and structure; but the number of the different tissues which they make up is small, and so is the number of ideal types exhibited by the organism they combine to form—the sprout (culmus) in the plant kingdom and the person in the animal kingdom. The same may be said of the stock (cormus) in both kingdoms—that is to say, of the higher individual unity which is constituted by the union of several sprouts or persons.The two classes of fundamental forms which are especially found in the plant sprouts or the animal persons are the radial and bilateral. The one is determined by the stationary life, the other by free movement in a certain attitude and direction (swimming in water or creeping on the ground). Hence we find the radial form (as pyramidal) predominant in the blooms and fruits of the metaphyta, and the persons of the polyps, corals, and regular echinoderms. On the other hand, the bilateral or dorsiventral form preponderates in most free-movinganimals; though it is also found in many flowers (papilionaceous and labial flowers, orchids, and others that are fertilized by insects). Here we have to seek the cause of the bilateralism in different features, in the relations with the insects, in the mode of their fastening to and distribution on the stalk (for the green foliage leaves), and so on.The complex individuals of the first order, the stocks (cormi), are more dependent in their growth on the spatial conditions of their environment than the sprouts or persons; hence their typical form is generally more or less irregular, and rarely bilateral.The interest which we take in natural and artistic forms, and which has for thousands of years prompted men to reproduce the former in the latter, depends for the most part, if not altogether, on their beauty—that is to say, on the feeling of pleasure we experience in looking at them. The causes of this pleasure and joy in the beautiful and the naturalness of its development are explained in æsthetics. When we combine this science with the results of modern cerebral physiology, we may distinguish two classes of beauty—direct and indirect. In direct or sensible beauty the internal sense-organs, or the æsthetic neurona or sense-cells of the brain, are immediately affected with pleasure. But in indirect or associational beauty these impressions are combined with an excitement of the phronetic neurona—the rational brain-cells which effect presentation and thought.Direct or sensible beauty (the subject of sensual æsthetics) is the direct perception of agreeable stimuli by the sense-organs. We may distinguish the following stages of its perfection: 1. Simple beauty (the subject of primordial æsthetics); the pleasure is evoked by the direct sense-impression of a simple form or color. Thus, for instance, a wooden sphere makes an agreeable impressionas compared with a shapeless piece of wood, a crystal as compared with a stone, a sky-blue or golden-yellow spot as compared with a greenish-blue or dull-yellow one (in music a simple pure bell-tone as compared with a shrill whistle). 2. Rhythmic beauty (the subject of linear æsthetics); the æsthetic sensation is caused by the serial repetition of some simple form—for instance, a pearl necklace, a chainlike community of monera (nostoc) or of cells (diatomes, A-f, 84, figs. 7 and 9): in music a tasteful series of simple notes. 3. Actinal beauty (the subject of radial æsthetics); the pleasure is excited by the orderly arrangement of three or more homogeneous simple forms about a common centre, from which they radiate; for instance, a regular cross or a radiating star, the three counter-pieces in the iris-bloom, the four paramera in the body of the medusa, the five radial-pieces in the star-fish. The familiar experience of the kaleidoscope shows how amply the simple radial constellation of three or more simple figures may delight our æsthetic sense (in music we have the simple harmony of several simultaneous notes). 4. Symmetrical beauty (the subject of bilateral æsthetics); the pleasure is caused by the relation of a simple object to its like, the mutual completion of two similar halves (the right and left parts). When we fold a piece of paper over an ink-stain in such a way that it is equally impressed on both halves of the fold, we get a symmetrical figure which makes an agreeable impression on our natural sense of space or equilibrium.The æsthetic impressions in indirect associational beauty (the subject of associative or symbolical æsthetics) are not only much more varied and complex than those we have described, but they also play a much more important part in the life of man and the higher animals. The anatomic condition for this higher physiological function is the elaborate construction of the brain in thehigher animals and man, and particularly the development of the special association-centres (thought-centres, reason-sphere) and their differentiation from the internal sense-centres. In this millions of different neurona or psychic cells co-operate, the sensual æstheta acting in conjunction with the rational phroneta, and thus, by complex associations of ideas, much higher and more valuable functions arise. We may indicate four chief groups of this associational or indirect beauty. 5. Biological beauty (the subject of botanical and zoological æsthetics): the various forms of organisms and their organs (for instance, a flower, a butterfly) excite our æsthetic interest by association with their physiological significance, their movements, their bionomic relations, their practical use, and so on. 6. Anthropistic beauty (the subject of anthropomorphic æsthetics): man, as "the measure of all things," regards his own organism as the chief object of beauty, either morphologically considered (beauty of the whole body and its various organs—the eyes, mouth, hair, flesh-tint, etc.), or physiologically (beauty of movements or positions), or psychologically (the expression of the emotions in the physiognomy). As man transfers to the objective world this personal gratification he experiences from self-consideration, and anthropomorphically regards other beings in the light of them, this anthropistic æsthetic obtains a far-reaching significance. 7. Sexual beauty (the subject of erotic æsthetics): the pleasure is caused by the mutual attraction of the sexes. The supreme importance of love in the life of man and most other organisms, the powerful influence of the passions, the sexual selection that is associated with reproduction, have evoked an infinite number of æsthetic creations in every branch of art relating to the antithesis of man and woman. The special pleasure which is caused by the bodily and mental affinities of the sexes can be traced phylogenetically tothe cell-love of the two sexual cells, or the attraction of the sperm-cell to ovum. 8. Landscape beauty (the subject of regional æsthetics): the pleasure which is caused by the sight of a fine landscape, and that finds satisfaction in modern landscape-painting, is more comprehensive than that of any other æsthetic sensations. In point of space the object is larger and richer than any of the individual objects in nature which are beautiful and interesting in themselves. The varying forms of the clouds and the water, the outline of the blue mountains in the background, the woods and meadows in the middle-distance, and the living figures in the foreground, excite in the mind of the spectator a number of different impressions which are woven together into a harmonious whole by a most elaborate association of ideas. The physiological functions of the nerve-cells in the cortex which effect these æsthetic pleasures, and the interaction of the sensual æstheta with the rational phroneta, are among the most perfect achievements of organic life. This "regional æsthetics," which has to establish scientifically the laws of landscape beauty, is much younger than the other branches of the science of the beautiful. It is very remarkable that absolute irregularity, the absence of symmetry and mathematical forms, is the first condition for the beauty of a landscape (as contrasted with architecture, and the beauty of separate objects in nature). Symmetrical arrangement of things (such as a double row of poplars or houses) or radial figures (a flower-bed or artificial wood) do not please the finer taste for landscape; they seem tedious.A comparative survey of these eight kinds of beauty in natural forms discovers a connected development, rising from the simple to the complex, from the lower to the higher. This scale corresponds to the evolution of the sense of beauty in man, ontogenetically from the child to the adult, phylogenetically from the savage tothe civilized man and the art critic. The stem-history of man and his organs, which explains to us in anthropogeny the gradual rise from lower to higher forms by the interaction of heredity and adaptation, also finds an application in the history of æsthetics and ornamentation. It teaches us how feeling, taste, emotion, and art have been gradually evolved. On the other hand, we have corresponding to this evolutionary series the scale of the typical forms which lie at the root of the real forms of bodies both in nature and art.Seventh TableTHE MORPHOLOGICAL SYSTEM OF ORGANISMS (1869)Division of living things (plants and animals) into two kingdoms (protista and histona) on the ground of their cell-structure and body-structure.First organic kingdom:Unicellular, protista.Organisms which as a rule remain unicellular throughoutlife (monobia), less frequently they form loose cell communities (cœnobia) by repeated cleavage, but never real tissues.Sub-kingdom of the protista.A.Primitive Plants(protophyta).A. Character:Plasmodomous.Unicellulars with vegetal metabolism: Carbon-assimilation.Chief Groups:I. PhytomoneraProtophyta without nucleus(monera)ChromaceaII. Algariæ.Unicellular algæ with nucleus, without ciliary motion: Paulotomea diatomea.III. Algettæ.Unicellular algæ with nucleus, and with ciliary motion: Mastigota, melthallia, siphonea.B.Primitive Animals(protozoa).B. Character:Plasmophagous.Unicellulars with animal metabolism: Albumin-assimilation.Chief Groups:I. Zoomonera.Protozoa without nucleus(monera).Bacteria.II. Sporozoa.Nucleated protozoa without mobile processes: Gregarinæ, chytridinæ.III. Rhizopoda.Nucleated protozoa with pseudopodia: Labosa, radiolaria.IV. Infusoria.Nucleated protozoa with cilia or lashes: Flagellata, ciliata.Second organic kingdom:Multicellular, histona.Organisms which are only unicellular at the beginning of their existence, are later multicellular, and always form real tissueshistobia) by the firm conjunction of social cells.Sub-kingdom of the histona.C.Tissue Plants(metaphyta).C. Character:Plasmodomous.Multicellulars with vegetal metabolism: Carbon-assimilation.Chief Groups:I. Thallophyta.Thallus-plants. Metaphyta with thallus: Algæ, mycetæ (fungi).II. Mesophyta.Median plants, with prothallium: Mosses, ferns (muscinæ filicinæ).III. Anthophyta(phanerogams).Flowering plants, with blooms and seeds (spermophyta): Gymnosperms, angiosperms.D.Tissue Animals(metazoa).D. Character:Phasmophagous.Multicellulars with animal metabolism: Albumin-assimilation.Chief Groups:I. Cœlenteria(cœlenterata).Metazoa without body cavity and anus: Gastræada. Sponges, cnidaria, platodes.II. Cœlomaria(bilaterals).Metazoa with body cavity and anus (generally also blood-vessels). Vermalia, mollusca, echinoderma, articulata, tunicata, vertebrata.IXMONERAThe simplest forms of life—Cell theory and cell dogma—Precellular organisms: monera, cytodes, and cells—Actual monera—Chromacea (cyanophyceæ)—Chromatophora—Cœnobia of chromacea: vital phenomena—Bacteria—Relations of the bacteria to the chromacea, the fungi, and the protozoa—Rhizomonera (protamœba, protogenes, protomyxa, bathybius)—Problematic monera—Phytomonera (plasmodoma) and zoomonera (plasmophaga)—Transition between the two classes.In the study and explanation of all complex phenomena the first thing to do is to understand the simple parts, the manner of their combination, and the development of the compound from the simple. This principle applies generally to inorganic objects, such as minerals, artificially constructed machines, etc. It is also of general application in biological work. The efforts of comparative anatomy are directed to the comprehension of the intricate structure of the higher organisms from the rising scale of organization and life in the lower, and the origin of the former by historical development from the latter. The modern science of the cell (cytology), which has in a short time attained a considerable rank, pursues a method in opposition to this principle. The intricate composition of the unicellular organism, in many of the higher protists (such as the ciliata and infusoria) and many of the higher tissue-cells (such as the neurona) has led to the erroneous ascription of ahighly complex organization to the cell in general. One would be justified in saying that of late the cell-theory has established itself in the dangerous and misleading position of a cell-dogma.The modern treatment of the science, as we find it in numbers of recent works, even in some of the most distinguished manuals, and which we must resent on account of its dogmatism, culminates in something like the following theses:1. The nucleated cell is the general elementary organism; all living things are either unicellular, or made up of a number of cells and tissues.2. This elementary organism consists of at least two different organs (or, more correctly, organella), the internal nucleus and the outer cell-body (or cytoplasm).3. The matter in each of these cell-organs—the caryoplasm of the nucleus and the cytoplasm of the body—is never homogeneous (or consisting of a chemical substratum), but always "organized," or made up of several chemically and anatomically different elementary constituents.4. The plasm (or protoplasm) is, therefore, a morphological, not a chemical, unity.5. Every cell comes (and has come) only from a mother-cell, and every nucleus from a mother-nucleus (omnis cellula e cellula—omnis nucleus e nucleo).These five theses of the modern cell-dogma are by no means sound; they are incompatible with the theory of evolution. I have, therefore, consistently resisted them for thirty-eight years, and consider them to be so dangerous that I will briefly give my reasons. First, let us clearly understand the modern definition of the cell. It is now generally defined (in accordance with the second thesis) as being composed of two essentially different parts, the nucleus and the cell-body, and it is added that these organella differ constantly both inrespect of chemistry, morphology, and physiology. If that is really so, the cell cannot possibly be the primitive organism; if it were, we should have a miracle at the beginning of organic life on the earth. The theory of natural evolution clearly and distinctly demands that the cell (in this sense) is a secondary development from a simpler, primary, elementary organism, a homogeneous cytode. There are still living to-day very simple protists which do not tally with this definition, and which I designatedmonerain 1866. As they must necessarily have preceded the real cells, they may also be called "precellular organisms."The earliest organisms to live on the earth, with which the wonderful drama of life began, can, in the present condition of biological science, only be conceived as homogeneous particles of plasm—biogens or groups of biogens, in which there was not yet the division of nucleus and cell-body which characterizes the real cell. I gave the name "cytodes" to these unnucleated cells in 1866, and joined them with the real nucleated cells under the general head of "plastids." I also endeavored to prove that such cytodes still exist in the form of independent monera, and in 1870 I described in myMonograph on the Moneraa number of protists which do not tally with the above definition.Fifty years ago I made the first careful observations of living monera (protamœbaandprotogenes), and described them in myGeneral Morphology(vol. i., pp. 133-5; vol. ii., p. xxii.) as structureless organisms without organs and the real beginnings of organic life. Soon afterwards, during a stay in the Canary Islands, I succeeded in following the continuous life-history of a related organism of the rhizopod type, which behaved like a very simple mycetozoon, but differed in having no nucleus; I have reproduced the picture of it in the first plate of myHistory of Creation. The description of this orange-redglobule of plasm (protomyxa aurantiaca) appeared first in myMonograph on the Monera. Most of the organisms which I comprised under this name exhibited the same movements as true rhizopods (or sarcodina). It was afterwards proved of some of them that there was a nucleus hidden within the homogeneous particle of plasm, and that, therefore, they must be regarded as real cells. But this discovery was wrongly extended to the whole of the monera, and the existence of unnucleated organisms was denied altogether. Nevertheless, there are living to-day several kinds of these organisms without organs, some of them being very widely distributed. The chief examples are the chromacea and the bacteria, the former with vegetal and the latter with animal metabolism (or the former plasmodomous = plasma-forming, and the latter plasmophagous = plasma-feeding). On the ground of this important chemical difference, I distinguished two principal groups of the monera in mySystematic Phylogenytwenty years ago—the phytomonera and the zoomonera, the former being unnucleated protophyta and the latter unnucleated protozoa.Among living organisms the chromacea are certainly the most primitive and the nearest to the oldest inhabitants of the earth. Their simplest forms, the chroococcacea, are nothing but small structureless particles of plasm, growing by plasmodomism (formation of plasm) and multiplying by simple cleavage as soon as their growth passes a certain limit of individual size. Many of them are surrounded by a thin membrane or somewhat thicker gelatinous covering, and this circumstance had prevented me for some time from counting the chromacea as monera. However, I became convinced afterwards that the formation of a protective cover of this kind around the homogeneous particle of plasm may indeed be regarded from the physiological stand-point as a "purposive" structure, but at the same time may belooked upon, from the purely physical stand-point, as a result of superficial strain. On the other hand, the physiological character of these plasmodomous monera is especially important, as it gives us the simple key to the solution of the great question of spontaneous generation (or archigony,cf.chapter xv.).The chromacea are to-day found in every part of the earth, living sometimes in fresh water and sometimes in the sea. Many species form blue-green, violet, or reddish deposits on rocks, stones, wood, and other objects. In these thin gelatinous plates millions of small homogeneous cytodes are packed close together. Their tint is due to a peculiar coloring matter (phycocyan), which is chemically connected with the substance of the plasma-particle. The shade of this color differs a good deal in the various species of chromacea (of which more than eight hundred have been distinguished); in the native species it is generally blue-green or sage-green, sometimes blue, cyanine blue, or violet. Hence the common name cyanophyceæ (i.e., blue algæ). It is incorrect, for two reasons; firstly, because only a part of these protophyta are blue, and, secondly, because they (as simple, primitive plants without tissue) must be distinguished from the real algæ (phyceæ), which are multicellular, tissue-forming plants. Other chromacea are red, orange, or yellow in color, as the interestingtrichodesmium erythræum, for instance, the flaky masses of which, gathering in enormous quantities, cause at certain times the yellow or red coloring of the sea-water in the tropics; it is these that are responsible for the name "Red Sea" on the Arabian and "Yellow Sea" on the Chinese coast. When I passed the equator in the Sunda Straits on March 10, 1901, the boat sailed through colossal accumulations, several miles in width, of this trichodesmium. The yellow or reddish surface of the water looked as if it were strewn with sawdust.In the same way, the surface of the Arctic Ocean is often colored brown or reddish-brown by masses of the brownprocytella primordialis(formerly described asprotococcus marinus).It is clearly quite illogical to regard the chromacea as a class or family of the algæ, as is still done in most manuals of botany. The real algæ—excluding the unicellular diatomes and paulotomes, which belong to the protophyta—are multicellular plants that form athallusor bed of a certain form and characteristic tissue. The chromacea, which have not advanced as far as the real nucleated cell, are unnucleated cytodes of a lower and earlier stage of plant-life. If one would compare the chromacea with algæ or other plants at all, the comparison cannot be with their constituent cells, but merely with the chromatophora or chromatella, which are found in all green plant-cells, and formpartof their contents. To be more precise, these green granules of chlorophyll must be regarded as organella of the plant-cell, or separated plasma-formations which arise beside the nucleus in the cytoplasm. In the embryonic cells of the germs of plants and in their vegetation points the chromatophora are as yet colorless, and are developed, as solid, very refractive, globular, or roundish granules, from the firm layer of plasm which immediately surrounds the nucleus. Afterwards they are converted, by a chemical process, into the green chlorophyll granules or chloroplasts, which have the most important function in the plasmodomism or carbon-assimilation of the plant.The fact that the green chlorophyll granules grow independently within the living plant-cell and multiply by segmentation is very important and interesting. The globular chloroplasts are constricted in the middle, and split into two equal daughter-globules. These daughter-plastids grow, and multiply in turn in the same way.Hence they behave within the plant-cell just like the free-living chromacea in the water. On the strength of this significant comparison, one of our ablest and most open-minded scientists, Fritz Müller-Desterro, of Brazil, pointed out in 1893 that we may see in every green vegetal cell a symbiosis between plasmodomous green and plasmophagous not-green companions (cf.myAnthropogeny, figs. 277 and 278, and in the text).Many species of the simplest chromacea live as monobia (individually). When the tiny plasma globules have split into two equal halves by simple segmentation, they separate, and live their lives apart. This is the case with the common, ubiquitous chroococcus. However, most species live in common, the plasma granules forming more or less thick cœnobia, or communities or colonies of cells. In the simplest case (aphanocapsa) the social cytodes secrete a structureless gelatinous mass, in which numbers of blue-green plasma globules are irregularly distributed. In theglœocapsa, which forms a thin blue-green gelatinous deposit on damp walls and rocks, the constituent cytodes cover themselves immediately after cleavage with a fresh gelatinous envelope, and these run together into large masses. But the majority of the chromacea form firm, threadlike cell communities or chains of plastids (catenal cœnobia.) As the transverse cleavage of the rapidly multiplying cytodes always follows the same direction, and the new daughter-cytodes remain joined at the cleavage surfaces, and are flattened into discoid shape, we get stringlike formations or articulated threads of considerable length, as in the oscillaria and nostochina. When a number of these threads are joined together in gelatinous masses, we often get large, irregular, jelly-like bodies, as in the common "shooting-star jellies" (nostoc communis). They attain the size of a plum.In view of the extreme importance which I attach tothe chromacea as the earliest and simplest of all organisms, it is necessary to put clearly the following facts with regard to their anatomic structure and physiological activity:1. The organism of the simplest chromacea isnotcomposed of different organella or organs; and it shows no trace of purposive construction or definite architecture.2. The homogeneous tinted plasma granule which makes up the entire organism in the simplest case (chroococcus) exhibits no plasma structure (honeycomb, threads, etc.) whatever.3. The original globular form of the plasma particle is the simplest of all fundamental types, and is also that assumed by the inorganic body (such as a drop of rain) in a condition of stable equilibrium.4. The formation of a thin membrane at the surface of the structureless plasma granule may be explained as a purely physical process—that of surface strain.5. The gelatinous envelope which is secreted by many of the chromacea is also formed by a simple physical (or chemical) process.6. The sole essential vital function that is common to all the chromacea is self-maintenance, and growth by means of their vegetal metabolism, or plasmodomism (=carbon assimilation); this purely chemical process is on a level with the catalysis of inorganic compounds (chapter x.).7. The growth of the cytodes, in virtue of their continuous plasmodomism, is on a level with the physical process of crystal growth.8. The reproduction of the chromacea by simple cleavage is merely the continuation of this simple growth process, when it passes the limit of individual size.9. All the other vital phenomena which are to be seen in some of the chromacea can also be explained byphysical or chemical causes on mechanical principles. Not a single fact compels us to assume a "vital force."Especially noteworthy in regard to the physiological character of these lowest organisms are their bionomic peculiarities, especially the indifference to external influences, higher and lower temperatures, etc. Many of the chromacea live in hot springs, with a temperature of fifty to eighty degrees centigrade, in which no other organism is found. Other species may remain for a long time frozen in ice, and resume their vital activity as soon as it thaws. Many chromacea may be completely dried up, and then resume their life if put in water after several years.Next in order to the chromacea we have the bacteria, the remarkable little organisms which have been well known in the last few decades as the causes of fatal diseases, and the agents of fermentation, putrefaction, etc. The special science which is concerned with them—modern bacteriology—has attained so important a position in a short period—especially as regards practical and theoretical medicine—that it is now represented by separate chairs at most of the universities. We may admire the penetration and the perseverance with which scientists have succeeded, with the aid of the best modern microscopes and methods of preparation and coloring, in making so close a study of the organism of the bacteria, determining their physiological properties, and explaining their great importance for organic life by careful experiments and methods of culture. The bionomic or economic position of the bacteria in nature's household has thus secured for these tiny organisms the greatest scientific and practical interest.However, we find that certain general views have been maintained by specialists in bacteriology up to our own time which are in curious contrast with these brilliant results. The biologist who studies the systematicrelations of the bacteria from the modern point of view of the theory of descent is bewildered at the extraordinary views as to the place of the bacteria in the plant-world (as segmentation-fungi), their relations to other classes of plants, and the formation of their species. When we carefully consider the morphological properties that are common to all true bacteria and compare them with other organisms, we are forced to the conclusion that I urged years ago in various writings: the bacteria are not real (nucleated) cells, but unnucleated cytodes of the rank of the monera; they are not real (tissue-forming) fungi, but simple protists; their nearest relatives are the chromacea.The individual organisms of the simplest kind, which bacteriologists call "bacteria-cells," are not real nucleated cells. That is the clear negative result of a number of most careful investigations which have been made up to date with the object of finding a nucleus in the plasma-body of the bacteria. Among recent exact investigations we must especially note those of the botanist Reinke, of Kiel, who sought in vain to detect a nucleus in one of the largest and most easily studied genera of the bacteria, thebeggiatoa, using every modern technical aid. His conviction that this important cell-structure is really lacking is the more valuable, as it is very prejudicial to his own theory of "dominants." Other scientists (especially Schaudinn) have recently claimed, as equivalent to a nucleus in some of the larger bacteria, a number of very small granules, which are irregularly distributed in the plasm, and are strongly tinted under certain coloring processes. But even if the chemical identity of these substances which take the same color were proved—which is certainly not the case—and even if the appearance of scattered nuclein-granules in the plasm could be regarded as a preliminary to, or a beginning of, the differentiation of an individual,morphologically distinct nucleus, we should not yet have shown its independence as an organellum of the cell.Nor is this any more proved from the circumstance that in some bacteria (not all) we find a severance of the plasm into an inner and outer layer, or a frothy structure with vacuole-formation, or a special sharply outlined membrane on the plastid. Many bacteria (but not all) have such a membrane, like the nearly related chromacea, and also the secretion of a gelatine envelope. Both classes have also in common an exclusively monogenetic reproduction. The bacteria multiply, like the chromacea, by simple segmentation; as soon as the structureless plasma-granule has reached a certain size by simple growth, it is constricted and splits into two halves. In the long-bodied bacteria (the rod-shaped bacilli) the constriction always goes through the middle of the long axis, and is, therefore, simple transverse cleavage. Many bacteria have also been said to multiply by the formation of spores. But these so-called "spores" are really permanent quiescent forms (without any multiplication of individuals); the central part of the plastid (endoplasm) condenses, separates from the peripheral part (exoplasm), and undergoes a chemical change which makes it very indifferent to external influences (such as a high temperature).The great majority of the bacteria differ so little morphologically from the chromacea that we can only distinguish these two classes of monera by the difference in their metabolism. The chromacea, as protophyta, are plasmodomous. They form new plasm by synthesis and reduction from simple inorganic compounds—water, carbonic acid, ammonia, nitric acid, etc. But the bacteria, as protozoa, are plasmophagous. They cannot, as a rule, form new plasm, but have to take it from other organisms (as parasites, saprophytes, etc.); they decompose it by analysis and oxydation. Hence thecolorless bacteria are without the important green, blue, or red coloring matter (phycocyan) which tints the plastids of the chromacea, and is the real instrument of the carbon-assimilation. However, there are exceptions in this respect:bacillus virensis tinted green with chlorophyll,micrococcus prodigiosusis blood-red, other bacteria purple, and so on. Certain earth-dwelling bacteria (nitro-bacteria) have the vegetal property of plasmodomism; they convert ammonia by oxydation into nitrous acid, and this into nitric acid, using as their source of carbon the carbonic acid gas in the atmosphere. They are thus quite independent of organic substances, and feed, like the chromacea, on simple inorganic compounds.Hence the affinity between the plasmodomous chromacea and plasmophagous bacteria is so close that it is impossible to give a single safe criterion that will effectually separate the two classes. Many botanists accordingly combine both groups in a single class with the name ofschizophyta, and within this distinguish as "orders" the blue-green chromacea asschizophycæ(cleavage-algæ) and the colorless bacteria asschizomycetes(cleavage-fungi). However, we must not take this division too rigidly; and the absolute lack of a nucleus and tissue-formation separates the chromacea just as widely from the multicellular tissue-forming algæ as the bacteria from the fungi. The simple multiplication by the halving of the cell, which is expressed in the name "cleavage-plants" (schizophyta), is also found in many other protists.The number of forms that can be distinguished as species in the technical sense is very great in the case of the bacteria, in spite of the extreme simplicity of their outward appearance; many biologists speak of several hundred, and even of more than a thousand, species. But when we look solely to the outer form of the livingplasma-granule, we can only distinguish three fundamental types: (1) Micrococci, or spherobacteria (briefly, cocci), globular or ellipsoid; (2) bacilli, or rhabdo-bacteria (also called eubacteria, or bacteria in the narrower sense), rod-shaped, cylindrical, and often twisted like worms (comma-bacilli); (3) spirilla, or spirobacteria, screw-shaped rods (vibriones when the screw is slight, and spirochæta when it has many coils). Besides this threefold difference in the forms of the cytodes, we have a ground of distinction in many bacilli and spirilla in the possession of one or more very thin lashes (flagella), which proceed from one of both poles of the lengthened plastid. The construction and vibration of these serves for locomotion in the swimming bacteria; but they are only found for a time in many species, and in many others are altogether wanting.Since, then, neither the simple outer form of the bacterium-cytodes nor their homogeneous internal structure provides a satisfactory ground for the systematic distinction of the numerous species, their physiological properties are generally used for the purpose, especially their different behavior towards organic foods (albumin, gelatine, etc.), their chemical actions, and the various effects of poisoning and decomposition which they produce in the living organism. No bacteriologist now doubts that all the vital activities of the bacteria are of a chemical nature, and precisely on this account these microbes are of extreme importance. When we bear in mind how complicated are the relations of the various species of bacteria to the tissues of the human body, in which they cause the diseases of typhus, hypochondriasis, cholera, and tuberculosis, we are bound to admit that the real cause of these maladies must be sought in the peculiar molecular structure of the bacterium-plasm, or the particular arrangement of its molecules and the innumerable atoms (more than a thousand)which are, in a very loose way, made up into special groups of molecules. The chemical products of their mutual action are what we call ptomaines, which are partly very virulent poisons (toxins). We have succeeded in producing several of these poisonous matters in large quantities by artificial culture, and isolating them and experimentally ascertaining their nature; as, for instance, tetanin, which causes tetanus, typhotoxin, the poison of typhus, etc.In thus declaring the action of bacteria to be purely chemical and analogous to that of well-known inorganic poisons, I would particularly point out that this very justifiable statement is a pure hypothesis; it is an excellent illustration of the fact that we cannot get on in the explanation of the most important natural phenomena without hypotheses. We can see nothing whatever of the chemical molecular structure of the plasm, even under the highest power of the microscope; it lies far below the limit of microscopic perception. Nevertheless, no expert scientist has the slightest doubt of its existence, or that the complicated movements of the sensitive atoms and the molecules and groups of molecules they make up are the causes of the vast changes which these tiny organisms effect in the tissues of the human and the higher animal body.Moreover, the distinction of the many species of bacteria is of interest in connection with the general question of the nature and constancy of a species. Whereas formerly in biological classification only definite morphological characters, or definable differences in outer form or inner structure, were regarded as of any moment in the distinction of species, here, in view of the vagueness or total lack of these characters, we have to look mainly to the physiological properties, and these are based on the chemical differences in their hypothetical molecular structure. But even these are not absolutelyconstant; on the contrary, many bacteria lose their specific qualities by progressive culture under changed food-conditions. By a change in the temperature and the nutritive field in which a number of poisonous bacteria have been reared, or by the action of certain chemicals, not only the growth and multiplication are altered, but also the injurious effect they have on other organisms by the generation of poisons. This poisonous effect is weakened, and—what is most important—the weakening is transmitted by heredity to the following generations. On this is based the familiar process of inoculation, an admirable example of the inheritance of acquired characteristics.As the bacteria are still often described as "cleavage-fungi" and classified along with the real fungi, we must particularly point out the wide gulf that separates the two groups. The real fungi (ormycetes) are metaphyta, their multicellular body (thallus) forming a very characteristic sort of tissue, the mycelium; this is composed of a number of interlaced and interwoven threads (or hyphens). Each fungus-thread consists of a row of lengthened cells, which have a thin membrane and enclose a number of small nuclei in the colorless plasm. Moreover, the two sub-classes of the real fungi, the ascomycetes and basimycetes, form peculiar fruit-bodies which generate spores (ascodia and basidia). There is no trace whatever of these real characteristics of the true fungus in the bacteria. Nor is it less incorrect to class them with the fungilli, the so-called unicellular fungi or phycomycetes (ovomycetes and zygomycetes); these form a special class of protists which has the closest affinity to the gregarinæ.Like the closely related chromacea, many of the bacteria show a marked tendency to form communities or cell-colonies. These cell-communities arise, as elsewhere, from the fact that the individuals, which multiplyrapidly by continuous cleavage, remain joined together. This may happen in two ways. When the social bacteria secrete large quantities of gelatine, and remain distributed in this, we have thezooglœa(as in the case of theaphanocapsaandglœocapsaamong the chromacea). If, on the other hand, the long-bodied bacilli remain fastened together in rows, we get the knotted threads ofleptothrixandbeggiatoa(which may be compared with the oscillaria). And, if these threads go into branches, we havecladothrix. Other cœnobia of bacteria have the appearance of disks, the cytodes dividing in a plane, usually in groups of four (as inmerismopedia), or of cube-shaped packets when they are in all three directions of space (sarcina).The two classes of bacteria and chromacea seem, in the present condition of our knowledge, on account of their simple organization, to be the simplest of all living things, real monera, or organisms without organs. Hence we have to put them at the lowest stage of the protist kingdom, and must regard the difference between them and the most highly differentiated unicellular beings (such as the radiolaria, ciliated infusoria, diatomes, or siphonea) as no smaller than the difference (in the realm of the histona) between a lower polyp (hydra) and a vertebrate, or between a simple alga (ulva) and a palm. But if the kingdom of the protists is badly divided, on the older rule, into a plant kingdom and an animal kingdom, the only discriminating mark we have left is the difference in metabolism; in that case we have to include the plasmophagous bacteria in the animal kingdom (as Ehrenberg did in 1838) and the plasmodomous chromacea in the plant kingdom. The remarkable class of the flagellata, which includes ciliated unicellulars of both groups, contains several forms which are only distinguished from the typical bacterium by the possession of a nucleus. If it is true that in some ofthe protists which were counted as bacteria a real nucleus has been detected, these must be separated from the others (unnucleated) and included in the nucleated flagellata.The monera which I described in 1866, and on which I based the theory of the monera in my monograph, belong to a different division of the protists from the classes of bacteria and chromacea. These are the forms which I described asprotamœba,protogenes,protomyxa, etc. Their naked mobile plasma-bodies thrust out pseudopodia, or variable "false feet," from their surface, like the (nucleated) real rhizopods (=sarcodinæ); but they differ essentially from the latter in the absence of a nucleus. Afterwards (in mySystematic Phytogeny) I proposed to separate these unnucleated rhizopods from the others, giving the name oflobomonera(protamœba) to the amœba-like monera with flap-shaped feet, and the name ofrhizomonera(protomyxa,pontomyxa,biomyxa,arachnula, etc.) to the gromia-like, root-feet forming monera. However, of late years, real nuclei have been detected in each of these large monera, and so they have been proved to be true cells. This discovery was made possible by the improved modern methods of coloring the nucleus which I had not the use of thirty years ago in my first observations. On the strength of these recent discoveries many scientists claim that all the monera I described are true cells, and must have nuclei. This baseless assertion is much employed by the opponents of the theory of evolution in order to deny the existence of the monera altogether.Of the genus of monera which we call protamœba I have given an illustration in myHistory of Creation(tenth edition), which has been frequently reproduced. Several species (at least two or three) of this genus still exist, and are distinguished by the shape of their flap-formation and their method of motion. They resembleordinary simple amœbæ, and only differ from these to any extent in the absence of a nucleus. Theprotamœba primitivaseems to be pretty widely distributed; it has been found repeatedly by observers (Gruber, Cienkowski, Leidy, etc.) in inland waters. In the zoological demonstrations which I have given at the University of Jena for forty years, and in the course of which the lowly inhabitants of our fresh water are regularly examined with the microscope, theprotamœba primitivahas been found four or five times. It always had the same form, as I described it, moved about by the slow formation of flaps at its surface, multiplied by simple cleavage, and showed no trace of a nucleus in its homogeneous plasma-body even with the most careful application of the modern methods of tinting the nucleus. A larger number of very fine granules (microsoma) that were irregularly distributed in the plasm, and were more or less colored by nucleus-reagents, cannot be reckoned as clear equivalents of the nucleus in this or in similar cases; they are probably products of metabolism. The same may be said of the larger marine form of rhizomoneron, which A. Gruber has recently calledpelomyxa pallida.The large marine form of rhizomoneron to which Huxley gave the name ofbathybius Haeckeliiin 1868, and as to the real nature of which many opinions have been expressed, seems, according to the latest investigation, not to have the significance ascribed to it. However, the much-discussed question of the bathybius is superfluous as far as our monera theory and the associated hypothesis of archigony (chapter xv.) are concerned, since we have now a better knowledge of the much more important monera-forms of the chromacea and bacteria.In the case of some of the protists I described in myMonograph on the Monera, it is at present doubtful whether their plasma-body contains a nucleus or not,and, therefore, whether they are to be classed as true cells or cytodes. This applies especially to the forms which only happened to come under observation once, such asprotomyxaandmyxastrum. In these obscure cases we must wait for fresh investigations and the application of the modern methods of tinting the nucleus. I may, however, point out, in passing, that these famous methods of nucleus-coloring give by no means the absolute certainty which is ascribed to them; there are other substances which take color in the same way as chromatin. As far as my monera theory is concerned, or the great general importance which I attach to these unnucleated living granules of plasm, it does not matter whether a nucleus is detected in these problematic monera or not. The chromacea alone—the most important of all monera—completely suffice to provide a base for the far-reaching theoretical conclusions which I draw from it.At the close of these observations on the monera I will briefly recapitulate the weighty inferences which we can deduce from their simple organization. They serve as a solid foundation for the chief theses of our monistic biology; and they are inconsistent with the dualistic views of modern vitalists. In the first place, I emphasize the fact that the structureless plasm-body of the simple monera has no sort of organization and no composition from dissimilar parts co-operating for definite vital aims. Reinke's conscious "dominant"—as well as Weismann's mechanical "determinants"—have nothing to do here. The whole vital activity of the simplest monera, especially of the chromacea, is confined to their metabolism, and is therefore a purely chemical process, that may be compared to the catalysis of inorganic compounds. The simple formation of individuals in this primitive living matter is merely a question of the cleavage of plasma globules of a certain size (chroococcus);and their primitive multiplication (by simple self-division) is only a continued growth (analogous to that of the crystal). When this simple growth passes a certain limit, that is fixed by the chemical constitution, it leads to the independent existence of the redundant growth-products.X

I.Centrostigmatic Types.—The natural middle of the body is a mathematical point. Properly speaking, only one form is of this type, and that is the most regular of all, the sphere or ball. We may, however, distinguish two sub-classes, the smooth sphere and the flattened sphere. The smooth sphere (holospœra) is a mathematically pure sphere, in which all points at the surface are equally distant from the centre, and all axes drawn through the centre are of equal length. We find this realized in its purity in the ovum of many animals (for instance, that of man and the mammals) and the pollen cells of many plants; also cells that develop freelyfloating in a liquid, the simplest forms of the radiolaria (actissa), the spherical cœnobia of the volvocina and catallacta, and the corresponding pure embryonic form of theblastula. The smooth sphere is particularly important, because it is the only absolutely regular type, the sole form with a perfectly stable equilibrium, and at the same time the sole organic form which is susceptible of direct physical explanation. Inorganic fluids (drops of quicksilver, water, etc.) similarly assume the purely spherical form, as drops of oil do, for instance, when put in a watery fluid of the same specific weight (as a mixture of alcohol and water).The flattened sphere, or facetted sphere (platnosphæra), is known as an endospherical polyhedron; that is to say, a many-surfaced body, all the corners of which fall in the surface of a sphere. The axes or the diameters, which are drawn through the angles and the centre, are all unequal, and larger than all other axes (drawn through the facets). These facetted spheres are frequently found in the globular silicious skeletons of many of the radiolaria; the globular central capsule of many spheroidea is enclosed in a concentric gelatine envelope, on the round surface of which we find a net-work of fine silicious threads. The meshes of this net are sometimes regular (generally triangular or hexagonal), sometimes irregular; frequently starlike silicious needles rise from the knots of the net-work (A-f, 1, 51, 91). The pollen bodies in the flower-dust of many flowering plants also often assume the form of facetted spheres.II.Centraxonia Types.—The natural middle of the body is a straight line, the principal axis. This large group of fundamental forms consists of two classes, according as each axis is the sole fixed ideal axis of the body, or other fixed transverse axes may also be distinguished, cutting the first at right angles. We call the former uniaxial (monaxonia), and the latter transverse-axial (stauraxonia). The horizontal section (vertically to the chief axis) is round in the uniaxials and polygonal in the transverse-axial.In the monaxonia the form is determined by a single fixed axis, the principle axis; the two poles may be either equal (isopola) or unequal (allopola). To the isopola belong the familiar simple forms which are distinguished in geometry as spheroids, biconvex, ellipsoids, double cones, cylinders, etc. A horizontal section, passing through the middle of the vertical chief axis, divides the body into two corresponding halves. On the other hand, many of the parts are unequal in size and shape in theallopola. The upper pole or vertex differs fromthe basal pole or ground surface; as we find in the oval form, the planoconvex lens, the hemisphere, the cone, etc. Both sub-classes of the monaxonia, the allopola (conoidal) and the isopola (spheroidal), are found realized frequently in organic forms, both in the tissue-cells of the histona and the independently living protists (A-f, 4, 84).In the stauraxonia the vertical imaginary principal axis is cut by two or more horizontal cross-axes or radial-axes. This is the case in the forms which were formerly generally classed as regular or radial. Here also, as with the monaxonia, we may distinguish two sub-classes, isopola and allopola, according as the poles of the principal axis are equal or unequal.Of thestauraxonia isopolawe have, for instance, the double pyramids, one of the simplest forms of the octahedron. This form is exhibited very typically by most of the acantharia, the radiolaria in which twenty radial needles (consisting of silicated chalk) shoot out from the centre of the vertical chief axis. These twenty rays are (if we imagine the figure of the earth with its vertical axis) distributed in five horizontal zones, with four needles each, in this wise: two pairs cross at right angles in the equatorial zone, but on each side (in north and south hemispheres) the points of four needles fall in the tropical zone, and the points of four polar needles in the polar circles; twelve needles (the four equatorial and eight polar) lie in two meridian planes that are vertical to each other; and the eight tropical needles lie in two other meridian planes which cross the former at an angle of forty-five degrees. In most of the acantharia (the radial acanthometra and the mailed acanthophracta)—there are few exceptions—this remarkable structural law of twenty radial needles is faithfully maintained by heredity. Its origin is explained by adaptation to a regular attitude which the sea-dwelling unicellular body assumes in a certain stage of equilibrium (A-f, 21, 41). If the points of the real needles are connected by imaginary lines, we get a polyhedrical body, which may be reduced to the form of a regular double pyramid. This typical form of the equipolar stauraxonia is also found in other protists with a plastic skeleton, as in many diatomes and desmidiacea (A-f, 24). It is more rarely found embodied in the tissue-cells of the histona.Unequipolar stauraxonia are the pyramids, a fundamental form that plays an important part in the configuration of organic bodies. They were formerly described as regular or fundamental forms. Such are the regular blooms of flowering plants, the regular echinoderms, medusæ, corals, etc. We maydistinguish several groups of them according to the number of the horizontal transverse axes that cut the vertical main axis in the middle.Two totally different divisions of the pyramidal types are the regular and the amphithecta pyramids. In the regular pyramids the transverse axes are equal, and the ground-surface (or base) is a regular polygon, as in the three-rayed blooms of the iris and crocus, the four-rayed medusæ (A-f, 16, 28, 47, 48, etc.), the five-rayed "regular echinoderms," most of the star-fish, sea-urchins, etc. (A-f, 10, 40, 60), and the six-rayed "regular corals" (A-f, 9, 69).The amphithecta (or two-edged) pyramids, a special group of pyramidal types, are characterized by having as their basis a rhombus instead of a regular polygon. We may, therefore, draw two imaginary transverse axes, vertical to each other, through the ground-surface, both equipolar, but of unequal length. One of the two may be called the sagittal axis (with dorsal and ventral pole), and the other the transverse axis (with right and left pole); but the distinction is arbitrary, as the two are equipolar. In this lies the chief difference from the centroplane and dorsiventral forms, in which only the lateral axis is equipolar, the sagittal axis being unequipolar. We find the bisected pyramid in a very perfect form in the class of the ctenophora (or comb-medusæ, A-f, 27), where it is quite general. The striking typical form of these pelagic cnidaria is sometimes called biradial, sometimes four-rayed and bilateral, and sometimes eight-rayed-symmetrical. Closer study shows it to be a rhombus-pyramid. The originally four-rayed type, which it inherited from craspedote medusæ, has become bilateral by the development of different organs to the right and left from those before and behind.Similar rhombo-pyramidal forms to those of the ctenophora are also found in some of the medusæ and siphonophora, many of the corals and other cnidaria, and many flowers. The name "two-edged" which is given to this special type is taken from the ancient two-edged sword. Its chief axis is unequipolar, the handle being at the basic pole and the point at the verticle pole; but the two edges left and right are equal (poles of the lateral axis), and also the two broad surfaces (dorsal and ventral, joined by the sagittal axis).III.Centroplane Types.—The natural middle of the body is a plane, the median or chief plane (planum medianumorsagittale); it divides the bilateral body into two symmetrical halves, the right and the left. With this is associated thecharacteristic antithesis of back (dorsum) and belly (venter); hence, in botany this type (found, for instance, in most green leaves) is called the dorsiventral, and in zoology the bilateral in the narrower sense. One characteristic of this important and wide-spread type is the relation of three different axes, vertical to each other; of these three straight axes (enthyni) two are unequipolar and the third equipolar. Hence, the centroplanes may also be called tri-axial (triaxonia). In most of the higher animals (as in our own frame) the longest of the three axes is the principal one (axon principalis); its fore pole is the oral or mouth pole, and its hinder pole is the aboral or caudal (tail) pole. The shortest of the three enthyni is, in our body, the sagittal (arrow) or dorsiventral axis; its upper pole is at the back and its lower pole at the belly. The third axis—the transverse or lateral axis—is equipolar, one pole being called the right and the other the left. The various parts which make up the two halves of the body have relatively the same disposition in each half; but absolutely speaking (namely, in relation to the middle plane) they are oppositely arranged.Further, the centroplane or bilateral forms are also characterized by three vertical axes which may be drawn through each of the normal axes. The first of these normal planes is the median plane; it is defined by the chief axis and the sagittal axis, and divides the body into two symmetrical halves, the right and left. The second normal plane is the frontal plane; this passes through the chief axis and the transverse axis (which is parallel to the frontal surface in our body), and divides the dorsal half from the ventral half. The third normal plane is the cingular (waist) plane: this is defined by the sagittal and transverse axes. It divides the head half (or the vertical part) from the tail half (or the basal part).The name "bilateral symmetry," which is especially applied to the centroplane and dorsiventral types, is ambiguous, as I pointed out in 1866 in an exhaustive analysis and criticism of these fundamental forms in the fourth book of theGeneral Morphology. It is used in five different senses. For our present general purpose it suffices to distinguish two orders of centroplane types, the bilateral-radial and the bilateral-symmetrical; in the former the radial (pyramidal) form is combined with the bilateral, but not in the latter.The bilateral-radial type comprises those forms in which the radial structure is combined in a very characteristic fashion with the bilateral. We have striking examples in the three-rayed flowers of the orchids (A-f, 74), the five-rayed blooms of thelabiate and papilionaceous flowers, etc., in the plant world; and in the five-rayed "irregular" echinoderms, the bilateral sea-urchins (spatangida, clypeastrida, A-f, 30) in the animal world. In these cases the bilateral symmetry is recognizable at the first glance, as is also the radial structure, or the composition from three to five or more raylike parts (paramera), which are arranged bilaterally round a common central plane.The bilateral-symmetrical type is general among the higher animals which move about freely. The body consists of two antithetic parts (antimera), and has no trace of radial structure. In the free moving, creeping, or swimming animals (vertebrates, articulates, mollusks, annelids, etc.) the ventral side is underneath, against the ground, and the dorsal side upward. This form is clearly the most useful and practical of all conceivable types for the movement of the body in a definite direction and position. The burden is equally distributed between the two sides (right and left); the head (with the sense organs, the brain, and the mouth) faces frontward and the tail behind. For thousands of years all artificial vehicles (carts on land and ships in water) have been built on this type. Selection has recognized it to be the best and preserved it, while it has discarded the rest. There are, however, other causes that have produced the predominance of this type in green leaves—the relation to the supporting stalk, to the sunlight that falls from above, etc.

I.Centrostigmatic Types.—The natural middle of the body is a mathematical point. Properly speaking, only one form is of this type, and that is the most regular of all, the sphere or ball. We may, however, distinguish two sub-classes, the smooth sphere and the flattened sphere. The smooth sphere (holospœra) is a mathematically pure sphere, in which all points at the surface are equally distant from the centre, and all axes drawn through the centre are of equal length. We find this realized in its purity in the ovum of many animals (for instance, that of man and the mammals) and the pollen cells of many plants; also cells that develop freelyfloating in a liquid, the simplest forms of the radiolaria (actissa), the spherical cœnobia of the volvocina and catallacta, and the corresponding pure embryonic form of theblastula. The smooth sphere is particularly important, because it is the only absolutely regular type, the sole form with a perfectly stable equilibrium, and at the same time the sole organic form which is susceptible of direct physical explanation. Inorganic fluids (drops of quicksilver, water, etc.) similarly assume the purely spherical form, as drops of oil do, for instance, when put in a watery fluid of the same specific weight (as a mixture of alcohol and water).

The flattened sphere, or facetted sphere (platnosphæra), is known as an endospherical polyhedron; that is to say, a many-surfaced body, all the corners of which fall in the surface of a sphere. The axes or the diameters, which are drawn through the angles and the centre, are all unequal, and larger than all other axes (drawn through the facets). These facetted spheres are frequently found in the globular silicious skeletons of many of the radiolaria; the globular central capsule of many spheroidea is enclosed in a concentric gelatine envelope, on the round surface of which we find a net-work of fine silicious threads. The meshes of this net are sometimes regular (generally triangular or hexagonal), sometimes irregular; frequently starlike silicious needles rise from the knots of the net-work (A-f, 1, 51, 91). The pollen bodies in the flower-dust of many flowering plants also often assume the form of facetted spheres.

II.Centraxonia Types.—The natural middle of the body is a straight line, the principal axis. This large group of fundamental forms consists of two classes, according as each axis is the sole fixed ideal axis of the body, or other fixed transverse axes may also be distinguished, cutting the first at right angles. We call the former uniaxial (monaxonia), and the latter transverse-axial (stauraxonia). The horizontal section (vertically to the chief axis) is round in the uniaxials and polygonal in the transverse-axial.

In the monaxonia the form is determined by a single fixed axis, the principle axis; the two poles may be either equal (isopola) or unequal (allopola). To the isopola belong the familiar simple forms which are distinguished in geometry as spheroids, biconvex, ellipsoids, double cones, cylinders, etc. A horizontal section, passing through the middle of the vertical chief axis, divides the body into two corresponding halves. On the other hand, many of the parts are unequal in size and shape in theallopola. The upper pole or vertex differs fromthe basal pole or ground surface; as we find in the oval form, the planoconvex lens, the hemisphere, the cone, etc. Both sub-classes of the monaxonia, the allopola (conoidal) and the isopola (spheroidal), are found realized frequently in organic forms, both in the tissue-cells of the histona and the independently living protists (A-f, 4, 84).

In the stauraxonia the vertical imaginary principal axis is cut by two or more horizontal cross-axes or radial-axes. This is the case in the forms which were formerly generally classed as regular or radial. Here also, as with the monaxonia, we may distinguish two sub-classes, isopola and allopola, according as the poles of the principal axis are equal or unequal.

Of thestauraxonia isopolawe have, for instance, the double pyramids, one of the simplest forms of the octahedron. This form is exhibited very typically by most of the acantharia, the radiolaria in which twenty radial needles (consisting of silicated chalk) shoot out from the centre of the vertical chief axis. These twenty rays are (if we imagine the figure of the earth with its vertical axis) distributed in five horizontal zones, with four needles each, in this wise: two pairs cross at right angles in the equatorial zone, but on each side (in north and south hemispheres) the points of four needles fall in the tropical zone, and the points of four polar needles in the polar circles; twelve needles (the four equatorial and eight polar) lie in two meridian planes that are vertical to each other; and the eight tropical needles lie in two other meridian planes which cross the former at an angle of forty-five degrees. In most of the acantharia (the radial acanthometra and the mailed acanthophracta)—there are few exceptions—this remarkable structural law of twenty radial needles is faithfully maintained by heredity. Its origin is explained by adaptation to a regular attitude which the sea-dwelling unicellular body assumes in a certain stage of equilibrium (A-f, 21, 41). If the points of the real needles are connected by imaginary lines, we get a polyhedrical body, which may be reduced to the form of a regular double pyramid. This typical form of the equipolar stauraxonia is also found in other protists with a plastic skeleton, as in many diatomes and desmidiacea (A-f, 24). It is more rarely found embodied in the tissue-cells of the histona.

Unequipolar stauraxonia are the pyramids, a fundamental form that plays an important part in the configuration of organic bodies. They were formerly described as regular or fundamental forms. Such are the regular blooms of flowering plants, the regular echinoderms, medusæ, corals, etc. We maydistinguish several groups of them according to the number of the horizontal transverse axes that cut the vertical main axis in the middle.

Two totally different divisions of the pyramidal types are the regular and the amphithecta pyramids. In the regular pyramids the transverse axes are equal, and the ground-surface (or base) is a regular polygon, as in the three-rayed blooms of the iris and crocus, the four-rayed medusæ (A-f, 16, 28, 47, 48, etc.), the five-rayed "regular echinoderms," most of the star-fish, sea-urchins, etc. (A-f, 10, 40, 60), and the six-rayed "regular corals" (A-f, 9, 69).

The amphithecta (or two-edged) pyramids, a special group of pyramidal types, are characterized by having as their basis a rhombus instead of a regular polygon. We may, therefore, draw two imaginary transverse axes, vertical to each other, through the ground-surface, both equipolar, but of unequal length. One of the two may be called the sagittal axis (with dorsal and ventral pole), and the other the transverse axis (with right and left pole); but the distinction is arbitrary, as the two are equipolar. In this lies the chief difference from the centroplane and dorsiventral forms, in which only the lateral axis is equipolar, the sagittal axis being unequipolar. We find the bisected pyramid in a very perfect form in the class of the ctenophora (or comb-medusæ, A-f, 27), where it is quite general. The striking typical form of these pelagic cnidaria is sometimes called biradial, sometimes four-rayed and bilateral, and sometimes eight-rayed-symmetrical. Closer study shows it to be a rhombus-pyramid. The originally four-rayed type, which it inherited from craspedote medusæ, has become bilateral by the development of different organs to the right and left from those before and behind.

Similar rhombo-pyramidal forms to those of the ctenophora are also found in some of the medusæ and siphonophora, many of the corals and other cnidaria, and many flowers. The name "two-edged" which is given to this special type is taken from the ancient two-edged sword. Its chief axis is unequipolar, the handle being at the basic pole and the point at the verticle pole; but the two edges left and right are equal (poles of the lateral axis), and also the two broad surfaces (dorsal and ventral, joined by the sagittal axis).

III.Centroplane Types.—The natural middle of the body is a plane, the median or chief plane (planum medianumorsagittale); it divides the bilateral body into two symmetrical halves, the right and the left. With this is associated thecharacteristic antithesis of back (dorsum) and belly (venter); hence, in botany this type (found, for instance, in most green leaves) is called the dorsiventral, and in zoology the bilateral in the narrower sense. One characteristic of this important and wide-spread type is the relation of three different axes, vertical to each other; of these three straight axes (enthyni) two are unequipolar and the third equipolar. Hence, the centroplanes may also be called tri-axial (triaxonia). In most of the higher animals (as in our own frame) the longest of the three axes is the principal one (axon principalis); its fore pole is the oral or mouth pole, and its hinder pole is the aboral or caudal (tail) pole. The shortest of the three enthyni is, in our body, the sagittal (arrow) or dorsiventral axis; its upper pole is at the back and its lower pole at the belly. The third axis—the transverse or lateral axis—is equipolar, one pole being called the right and the other the left. The various parts which make up the two halves of the body have relatively the same disposition in each half; but absolutely speaking (namely, in relation to the middle plane) they are oppositely arranged.

Further, the centroplane or bilateral forms are also characterized by three vertical axes which may be drawn through each of the normal axes. The first of these normal planes is the median plane; it is defined by the chief axis and the sagittal axis, and divides the body into two symmetrical halves, the right and left. The second normal plane is the frontal plane; this passes through the chief axis and the transverse axis (which is parallel to the frontal surface in our body), and divides the dorsal half from the ventral half. The third normal plane is the cingular (waist) plane: this is defined by the sagittal and transverse axes. It divides the head half (or the vertical part) from the tail half (or the basal part).

The name "bilateral symmetry," which is especially applied to the centroplane and dorsiventral types, is ambiguous, as I pointed out in 1866 in an exhaustive analysis and criticism of these fundamental forms in the fourth book of theGeneral Morphology. It is used in five different senses. For our present general purpose it suffices to distinguish two orders of centroplane types, the bilateral-radial and the bilateral-symmetrical; in the former the radial (pyramidal) form is combined with the bilateral, but not in the latter.

The bilateral-radial type comprises those forms in which the radial structure is combined in a very characteristic fashion with the bilateral. We have striking examples in the three-rayed flowers of the orchids (A-f, 74), the five-rayed blooms of thelabiate and papilionaceous flowers, etc., in the plant world; and in the five-rayed "irregular" echinoderms, the bilateral sea-urchins (spatangida, clypeastrida, A-f, 30) in the animal world. In these cases the bilateral symmetry is recognizable at the first glance, as is also the radial structure, or the composition from three to five or more raylike parts (paramera), which are arranged bilaterally round a common central plane.

The bilateral-symmetrical type is general among the higher animals which move about freely. The body consists of two antithetic parts (antimera), and has no trace of radial structure. In the free moving, creeping, or swimming animals (vertebrates, articulates, mollusks, annelids, etc.) the ventral side is underneath, against the ground, and the dorsal side upward. This form is clearly the most useful and practical of all conceivable types for the movement of the body in a definite direction and position. The burden is equally distributed between the two sides (right and left); the head (with the sense organs, the brain, and the mouth) faces frontward and the tail behind. For thousands of years all artificial vehicles (carts on land and ships in water) have been built on this type. Selection has recognized it to be the best and preserved it, while it has discarded the rest. There are, however, other causes that have produced the predominance of this type in green leaves—the relation to the supporting stalk, to the sunlight that falls from above, etc.

Special notice must be taken of those bilateral forms which were originally symmetrical (by heredity), but have subsequently become asymmetrical (or of unequal halves), by adaptation to special conditions of life. The most familiar example among the vertebrates are the flat-fishes (pleuronectides), soles, flounders, turbots, etc. These high and narrow and flattened boney-fishes have a perfect bilateral symmetry when young, like ordinary fishes. Afterwards they form the habit of laying on one side (right or left) at the bottom of the sea; and in consequence the upper side, exposed to the light, is dark colored, and often marked with a design (sometimes very like the stony floor of the ocean—a protective coloring), while the side the flat-fish lies on remains without color. But, what is more curious, the eyefrom the under side travels to the upper side, and the two eyes lie together on one side (the right or left); while the bones of the skull and the softer parts of each side of the head grow quite crooked. Naturally, this ontogenetic process, in which a striking lack of symmetry succeeds to the early complete symmetry of each individual, can only be explained by our biogenetic law; it is a rapid and brief recapitulation (determined by heredity) of the long and slow phyletic process which the flat-fish has undergone for thousands of years in its ancestral history to bring about its gradual modification. At the same time, this interesting metamorphosis of thepleuronectidesgives us an excellent instance of the inheritance of acquired characteristics, as a consequence of constant œcological habit. It is quite impossible to explain it on Weismann's theory of the germ-plasm.

We have another striking example among the invertebrates in the snails (gasteropoda). The great majority of these mollusks are characterized by the spiral shape of their shells. This variously shaped, and often prettily colored and marked, snail's house is in essence a spirally coiled tube, closed at the upper end and open at the lower (or mouth): the mollusk can at any moment withdraw into its tube. The comparative anatomy and ontogeny of the snails teach us that this spiral shell came originally from a simple discoid or cylindrical dorsal covering of the once bilateral-symmetrical mollusk, by the two sides of the body having an unequal growth. The cause of it was a purely mechanical factor—the sinking of the growing visceral sac, covered with the shell, to one side; one part of the viscera contained in it (the heart, kidneys, liver, etc.) grew more strongly on one side than the other in consequence of this; and this was accompanied by considerable displacement and modification of the neighboring parts, especially the gills. In most snails one of thegills and kidneys and the ventricle of the heart corresponding to these have disappeared altogether, only those of the opposite side remaining; and the latter have moved from the right side to the left, or vice versa. The conspicuous lack of symmetry between the two halves of the body which resulted from this finds expression in the spiral form of the snail's shell. This remarkable ontogenetic metamorphosis also can be fully explained by a corresponding phylogenetic process, and affords a very fine instance of the inheritance of acquired characters.

There are also many examples of this asymmetry of bilateral forms in the plant world, such as the green foliage-leaves of the familiar begonia and the blooms ofcanna.

IV.The Centraporia.—Few organic forms are completely irregular and without axes, as usually the attraction to the earth (geotaxis) or to the nearest object determines the special direction of growth, and so the formation of an axis in some direction or other. Nevertheless, we may instance as quite irregular the soft and ever-changing plasma-bodies of many rhizopods, the amœbinæ, mycetozoa, etc. Most of the sponges also—which we regard as stocks of gastræads—are completely irregular in structure; the most familiar example is the common bath-sponge.

An impartial and thorough study of organic forms has convinced me that their actual, infinitely varied configurations may all be reduced to the few typical forms I have described. Comparative anatomy and ontogeny further teach us that the countless modifying processes which have led to the appearance of the various species have acted by adaptation to different environments, habits, and customs, and give us, in conjunction with heredity, a physiological explanation of this morphological transformation. But the question arises as tothe origin of these few geometrically definable types, and the cause of their divergence.

In this important and difficult question we find a great variety of opinions and a strong leaning to dualistic and mystic theories. Educated laymen, who have only a partial and imperfect acquaintance with the biological facts, think that they are justified here in appealing to a supernatural creation of forms. They contend that only a wise creator, following a rational and conscious design, could produce such structures. Even distinguished and informed scientists lean in this matter towards mystic and transcendental ideas; they believe that the ordinary natural forces do not suffice to explain these phenomena, and that at least for the first construction of these fundamental types we must postulate a deliberate creative thought, a design, or some such teleological cause, and therefore consciously acting final causes. So say Nägeli and Alexander Braun.

In direct opposition to this, I have ever maintained the view that the action of familiar physical forces—mechanical efficient causes—fully suffices to explain the origin and transformation of these fundamental types, as well as for all other biological and inorganic processes. In order to understand this monistic position thoroughly, and to meet the errors of dualism, we must bear in mind always the radical processes of growth which control all organic and inorganic configuration, and also the long chain of advancing stages of development, which lead us from the simplest protists, the monera, to the most advanced organisms.

The unicellular organisms exhibit the greatest variety from the promorphological point of view. In the single class of the radiolaria we find all imaginable geometrical types represented. This is seen in a glance at the one hundred and forty plates on which I have depicted thousands of these graceful little protozoa in my monograph(Challenger Report, vol. xviii.). On the other hand, the monera, at the lowest stage of organic life, the structureless organisms without organs that live on the very frontier of the inorganic world, are very simple. Especially interesting in this connection are the chromacea, which have hitherto been so undeservedly and so incomprehensibly neglected. Among the well-known and widely distributed chroococcacea, the chroococcus, cœlosphærium, and aphanocapsa are quite the most primitive of all organisms known to us—and at the same time the organisms that enable us best to understand the origin of life by spontaneous generation (archigony). The whole organism is merely a tiny, bluish-green globule of plasm, without any structure, or only surrounded by a thin membrane; its fundamental form is the simplest of all, the centraxial smooth sphere. Next to these are the oscillaria and nostochina, social chromacea, which have the appearance of thin, bluish-green threads. They consist of simple primitive (unnucleated) cells joined to each other; they seem often to be flattened into a discoid shape as a result of close conjunction. Many protists are found in two conditions, one mobile with very varied and changeable forms, and one stationary with a globular shape. But when the separate living cell begins to form a firm skeleton or protective cover for itself, it may assume the most varied and often most complicated forms. In this respect the class of the radiolaria among the protozoa, and the class of the diatomes among the protophyta (both of which have flinty shells), surpass all the other groups of the diversified realm of the protists. In myArt-forms in NatureI have given a selection of their most beautiful forms (diatomes, A-f, 4, 84; radiolaria, A-f, 1, 11, 21, 22, 31, 41, 51, 61, 71, 95). The most remarkable and most important fact about them is that the artistic builders of these wonderful and often very ingenious andintricate flinty structures are merely the plastidules or micella, the molecular and microscopically invisible constituents of the soft viscous plasm (sarcode).

The configuration of the histona differs essentially from that of the protists, since in the case of the latter the simple unicellular body produces for itself alone the whole form and vital action of the organism, while in the histona this is done by the cell state, or the social combination of a number of different cells, which make up the tissue body. Hence the ideal type which we can always define in the actual histonal form has quite a different significance from that in the unicellular protists. In the latter we find the utmost diversity in the configuration of the independent living cells and the protective cover it forms; among the histona the number of fundamental forms is limited. It is true that the cells themselves which make up the tissues may exhibit a great variety in form and structure; but the number of the different tissues which they make up is small, and so is the number of ideal types exhibited by the organism they combine to form—the sprout (culmus) in the plant kingdom and the person in the animal kingdom. The same may be said of the stock (cormus) in both kingdoms—that is to say, of the higher individual unity which is constituted by the union of several sprouts or persons.

The two classes of fundamental forms which are especially found in the plant sprouts or the animal persons are the radial and bilateral. The one is determined by the stationary life, the other by free movement in a certain attitude and direction (swimming in water or creeping on the ground). Hence we find the radial form (as pyramidal) predominant in the blooms and fruits of the metaphyta, and the persons of the polyps, corals, and regular echinoderms. On the other hand, the bilateral or dorsiventral form preponderates in most free-movinganimals; though it is also found in many flowers (papilionaceous and labial flowers, orchids, and others that are fertilized by insects). Here we have to seek the cause of the bilateralism in different features, in the relations with the insects, in the mode of their fastening to and distribution on the stalk (for the green foliage leaves), and so on.

The complex individuals of the first order, the stocks (cormi), are more dependent in their growth on the spatial conditions of their environment than the sprouts or persons; hence their typical form is generally more or less irregular, and rarely bilateral.

The interest which we take in natural and artistic forms, and which has for thousands of years prompted men to reproduce the former in the latter, depends for the most part, if not altogether, on their beauty—that is to say, on the feeling of pleasure we experience in looking at them. The causes of this pleasure and joy in the beautiful and the naturalness of its development are explained in æsthetics. When we combine this science with the results of modern cerebral physiology, we may distinguish two classes of beauty—direct and indirect. In direct or sensible beauty the internal sense-organs, or the æsthetic neurona or sense-cells of the brain, are immediately affected with pleasure. But in indirect or associational beauty these impressions are combined with an excitement of the phronetic neurona—the rational brain-cells which effect presentation and thought.

Direct or sensible beauty (the subject of sensual æsthetics) is the direct perception of agreeable stimuli by the sense-organs. We may distinguish the following stages of its perfection: 1. Simple beauty (the subject of primordial æsthetics); the pleasure is evoked by the direct sense-impression of a simple form or color. Thus, for instance, a wooden sphere makes an agreeable impressionas compared with a shapeless piece of wood, a crystal as compared with a stone, a sky-blue or golden-yellow spot as compared with a greenish-blue or dull-yellow one (in music a simple pure bell-tone as compared with a shrill whistle). 2. Rhythmic beauty (the subject of linear æsthetics); the æsthetic sensation is caused by the serial repetition of some simple form—for instance, a pearl necklace, a chainlike community of monera (nostoc) or of cells (diatomes, A-f, 84, figs. 7 and 9): in music a tasteful series of simple notes. 3. Actinal beauty (the subject of radial æsthetics); the pleasure is excited by the orderly arrangement of three or more homogeneous simple forms about a common centre, from which they radiate; for instance, a regular cross or a radiating star, the three counter-pieces in the iris-bloom, the four paramera in the body of the medusa, the five radial-pieces in the star-fish. The familiar experience of the kaleidoscope shows how amply the simple radial constellation of three or more simple figures may delight our æsthetic sense (in music we have the simple harmony of several simultaneous notes). 4. Symmetrical beauty (the subject of bilateral æsthetics); the pleasure is caused by the relation of a simple object to its like, the mutual completion of two similar halves (the right and left parts). When we fold a piece of paper over an ink-stain in such a way that it is equally impressed on both halves of the fold, we get a symmetrical figure which makes an agreeable impression on our natural sense of space or equilibrium.

The æsthetic impressions in indirect associational beauty (the subject of associative or symbolical æsthetics) are not only much more varied and complex than those we have described, but they also play a much more important part in the life of man and the higher animals. The anatomic condition for this higher physiological function is the elaborate construction of the brain in thehigher animals and man, and particularly the development of the special association-centres (thought-centres, reason-sphere) and their differentiation from the internal sense-centres. In this millions of different neurona or psychic cells co-operate, the sensual æstheta acting in conjunction with the rational phroneta, and thus, by complex associations of ideas, much higher and more valuable functions arise. We may indicate four chief groups of this associational or indirect beauty. 5. Biological beauty (the subject of botanical and zoological æsthetics): the various forms of organisms and their organs (for instance, a flower, a butterfly) excite our æsthetic interest by association with their physiological significance, their movements, their bionomic relations, their practical use, and so on. 6. Anthropistic beauty (the subject of anthropomorphic æsthetics): man, as "the measure of all things," regards his own organism as the chief object of beauty, either morphologically considered (beauty of the whole body and its various organs—the eyes, mouth, hair, flesh-tint, etc.), or physiologically (beauty of movements or positions), or psychologically (the expression of the emotions in the physiognomy). As man transfers to the objective world this personal gratification he experiences from self-consideration, and anthropomorphically regards other beings in the light of them, this anthropistic æsthetic obtains a far-reaching significance. 7. Sexual beauty (the subject of erotic æsthetics): the pleasure is caused by the mutual attraction of the sexes. The supreme importance of love in the life of man and most other organisms, the powerful influence of the passions, the sexual selection that is associated with reproduction, have evoked an infinite number of æsthetic creations in every branch of art relating to the antithesis of man and woman. The special pleasure which is caused by the bodily and mental affinities of the sexes can be traced phylogenetically tothe cell-love of the two sexual cells, or the attraction of the sperm-cell to ovum. 8. Landscape beauty (the subject of regional æsthetics): the pleasure which is caused by the sight of a fine landscape, and that finds satisfaction in modern landscape-painting, is more comprehensive than that of any other æsthetic sensations. In point of space the object is larger and richer than any of the individual objects in nature which are beautiful and interesting in themselves. The varying forms of the clouds and the water, the outline of the blue mountains in the background, the woods and meadows in the middle-distance, and the living figures in the foreground, excite in the mind of the spectator a number of different impressions which are woven together into a harmonious whole by a most elaborate association of ideas. The physiological functions of the nerve-cells in the cortex which effect these æsthetic pleasures, and the interaction of the sensual æstheta with the rational phroneta, are among the most perfect achievements of organic life. This "regional æsthetics," which has to establish scientifically the laws of landscape beauty, is much younger than the other branches of the science of the beautiful. It is very remarkable that absolute irregularity, the absence of symmetry and mathematical forms, is the first condition for the beauty of a landscape (as contrasted with architecture, and the beauty of separate objects in nature). Symmetrical arrangement of things (such as a double row of poplars or houses) or radial figures (a flower-bed or artificial wood) do not please the finer taste for landscape; they seem tedious.

A comparative survey of these eight kinds of beauty in natural forms discovers a connected development, rising from the simple to the complex, from the lower to the higher. This scale corresponds to the evolution of the sense of beauty in man, ontogenetically from the child to the adult, phylogenetically from the savage tothe civilized man and the art critic. The stem-history of man and his organs, which explains to us in anthropogeny the gradual rise from lower to higher forms by the interaction of heredity and adaptation, also finds an application in the history of æsthetics and ornamentation. It teaches us how feeling, taste, emotion, and art have been gradually evolved. On the other hand, we have corresponding to this evolutionary series the scale of the typical forms which lie at the root of the real forms of bodies both in nature and art.

Seventh Table

THE MORPHOLOGICAL SYSTEM OF ORGANISMS (1869)

Division of living things (plants and animals) into two kingdoms (protista and histona) on the ground of their cell-structure and body-structure.

Organisms which as a rule remain unicellular throughoutlife (monobia), less frequently they form loose cell communities (cœnobia) by repeated cleavage, but never real tissues.

Unicellulars with vegetal metabolism: Carbon-assimilation.

Unicellular algæ with nucleus, without ciliary motion: Paulotomea diatomea.

Unicellular algæ with nucleus, and with ciliary motion: Mastigota, melthallia, siphonea.

Unicellulars with animal metabolism: Albumin-assimilation.

Nucleated protozoa without mobile processes: Gregarinæ, chytridinæ.

Nucleated protozoa with pseudopodia: Labosa, radiolaria.

Nucleated protozoa with cilia or lashes: Flagellata, ciliata.

Multicellulars with vegetal metabolism: Carbon-assimilation.

Thallus-plants. Metaphyta with thallus: Algæ, mycetæ (fungi).

Median plants, with prothallium: Mosses, ferns (muscinæ filicinæ).

Flowering plants, with blooms and seeds (spermophyta): Gymnosperms, angiosperms.

Multicellulars with animal metabolism: Albumin-assimilation.

Metazoa without body cavity and anus: Gastræada. Sponges, cnidaria, platodes.

Metazoa with body cavity and anus (generally also blood-vessels). Vermalia, mollusca, echinoderma, articulata, tunicata, vertebrata.

IX

The simplest forms of life—Cell theory and cell dogma—Precellular organisms: monera, cytodes, and cells—Actual monera—Chromacea (cyanophyceæ)—Chromatophora—Cœnobia of chromacea: vital phenomena—Bacteria—Relations of the bacteria to the chromacea, the fungi, and the protozoa—Rhizomonera (protamœba, protogenes, protomyxa, bathybius)—Problematic monera—Phytomonera (plasmodoma) and zoomonera (plasmophaga)—Transition between the two classes.

In the study and explanation of all complex phenomena the first thing to do is to understand the simple parts, the manner of their combination, and the development of the compound from the simple. This principle applies generally to inorganic objects, such as minerals, artificially constructed machines, etc. It is also of general application in biological work. The efforts of comparative anatomy are directed to the comprehension of the intricate structure of the higher organisms from the rising scale of organization and life in the lower, and the origin of the former by historical development from the latter. The modern science of the cell (cytology), which has in a short time attained a considerable rank, pursues a method in opposition to this principle. The intricate composition of the unicellular organism, in many of the higher protists (such as the ciliata and infusoria) and many of the higher tissue-cells (such as the neurona) has led to the erroneous ascription of ahighly complex organization to the cell in general. One would be justified in saying that of late the cell-theory has established itself in the dangerous and misleading position of a cell-dogma.

The modern treatment of the science, as we find it in numbers of recent works, even in some of the most distinguished manuals, and which we must resent on account of its dogmatism, culminates in something like the following theses:

1. The nucleated cell is the general elementary organism; all living things are either unicellular, or made up of a number of cells and tissues.

2. This elementary organism consists of at least two different organs (or, more correctly, organella), the internal nucleus and the outer cell-body (or cytoplasm).

3. The matter in each of these cell-organs—the caryoplasm of the nucleus and the cytoplasm of the body—is never homogeneous (or consisting of a chemical substratum), but always "organized," or made up of several chemically and anatomically different elementary constituents.

4. The plasm (or protoplasm) is, therefore, a morphological, not a chemical, unity.

5. Every cell comes (and has come) only from a mother-cell, and every nucleus from a mother-nucleus (omnis cellula e cellula—omnis nucleus e nucleo).

These five theses of the modern cell-dogma are by no means sound; they are incompatible with the theory of evolution. I have, therefore, consistently resisted them for thirty-eight years, and consider them to be so dangerous that I will briefly give my reasons. First, let us clearly understand the modern definition of the cell. It is now generally defined (in accordance with the second thesis) as being composed of two essentially different parts, the nucleus and the cell-body, and it is added that these organella differ constantly both inrespect of chemistry, morphology, and physiology. If that is really so, the cell cannot possibly be the primitive organism; if it were, we should have a miracle at the beginning of organic life on the earth. The theory of natural evolution clearly and distinctly demands that the cell (in this sense) is a secondary development from a simpler, primary, elementary organism, a homogeneous cytode. There are still living to-day very simple protists which do not tally with this definition, and which I designatedmonerain 1866. As they must necessarily have preceded the real cells, they may also be called "precellular organisms."

The earliest organisms to live on the earth, with which the wonderful drama of life began, can, in the present condition of biological science, only be conceived as homogeneous particles of plasm—biogens or groups of biogens, in which there was not yet the division of nucleus and cell-body which characterizes the real cell. I gave the name "cytodes" to these unnucleated cells in 1866, and joined them with the real nucleated cells under the general head of "plastids." I also endeavored to prove that such cytodes still exist in the form of independent monera, and in 1870 I described in myMonograph on the Moneraa number of protists which do not tally with the above definition.

Fifty years ago I made the first careful observations of living monera (protamœbaandprotogenes), and described them in myGeneral Morphology(vol. i., pp. 133-5; vol. ii., p. xxii.) as structureless organisms without organs and the real beginnings of organic life. Soon afterwards, during a stay in the Canary Islands, I succeeded in following the continuous life-history of a related organism of the rhizopod type, which behaved like a very simple mycetozoon, but differed in having no nucleus; I have reproduced the picture of it in the first plate of myHistory of Creation. The description of this orange-redglobule of plasm (protomyxa aurantiaca) appeared first in myMonograph on the Monera. Most of the organisms which I comprised under this name exhibited the same movements as true rhizopods (or sarcodina). It was afterwards proved of some of them that there was a nucleus hidden within the homogeneous particle of plasm, and that, therefore, they must be regarded as real cells. But this discovery was wrongly extended to the whole of the monera, and the existence of unnucleated organisms was denied altogether. Nevertheless, there are living to-day several kinds of these organisms without organs, some of them being very widely distributed. The chief examples are the chromacea and the bacteria, the former with vegetal and the latter with animal metabolism (or the former plasmodomous = plasma-forming, and the latter plasmophagous = plasma-feeding). On the ground of this important chemical difference, I distinguished two principal groups of the monera in mySystematic Phylogenytwenty years ago—the phytomonera and the zoomonera, the former being unnucleated protophyta and the latter unnucleated protozoa.

Among living organisms the chromacea are certainly the most primitive and the nearest to the oldest inhabitants of the earth. Their simplest forms, the chroococcacea, are nothing but small structureless particles of plasm, growing by plasmodomism (formation of plasm) and multiplying by simple cleavage as soon as their growth passes a certain limit of individual size. Many of them are surrounded by a thin membrane or somewhat thicker gelatinous covering, and this circumstance had prevented me for some time from counting the chromacea as monera. However, I became convinced afterwards that the formation of a protective cover of this kind around the homogeneous particle of plasm may indeed be regarded from the physiological stand-point as a "purposive" structure, but at the same time may belooked upon, from the purely physical stand-point, as a result of superficial strain. On the other hand, the physiological character of these plasmodomous monera is especially important, as it gives us the simple key to the solution of the great question of spontaneous generation (or archigony,cf.chapter xv.).

The chromacea are to-day found in every part of the earth, living sometimes in fresh water and sometimes in the sea. Many species form blue-green, violet, or reddish deposits on rocks, stones, wood, and other objects. In these thin gelatinous plates millions of small homogeneous cytodes are packed close together. Their tint is due to a peculiar coloring matter (phycocyan), which is chemically connected with the substance of the plasma-particle. The shade of this color differs a good deal in the various species of chromacea (of which more than eight hundred have been distinguished); in the native species it is generally blue-green or sage-green, sometimes blue, cyanine blue, or violet. Hence the common name cyanophyceæ (i.e., blue algæ). It is incorrect, for two reasons; firstly, because only a part of these protophyta are blue, and, secondly, because they (as simple, primitive plants without tissue) must be distinguished from the real algæ (phyceæ), which are multicellular, tissue-forming plants. Other chromacea are red, orange, or yellow in color, as the interestingtrichodesmium erythræum, for instance, the flaky masses of which, gathering in enormous quantities, cause at certain times the yellow or red coloring of the sea-water in the tropics; it is these that are responsible for the name "Red Sea" on the Arabian and "Yellow Sea" on the Chinese coast. When I passed the equator in the Sunda Straits on March 10, 1901, the boat sailed through colossal accumulations, several miles in width, of this trichodesmium. The yellow or reddish surface of the water looked as if it were strewn with sawdust.In the same way, the surface of the Arctic Ocean is often colored brown or reddish-brown by masses of the brownprocytella primordialis(formerly described asprotococcus marinus).

It is clearly quite illogical to regard the chromacea as a class or family of the algæ, as is still done in most manuals of botany. The real algæ—excluding the unicellular diatomes and paulotomes, which belong to the protophyta—are multicellular plants that form athallusor bed of a certain form and characteristic tissue. The chromacea, which have not advanced as far as the real nucleated cell, are unnucleated cytodes of a lower and earlier stage of plant-life. If one would compare the chromacea with algæ or other plants at all, the comparison cannot be with their constituent cells, but merely with the chromatophora or chromatella, which are found in all green plant-cells, and formpartof their contents. To be more precise, these green granules of chlorophyll must be regarded as organella of the plant-cell, or separated plasma-formations which arise beside the nucleus in the cytoplasm. In the embryonic cells of the germs of plants and in their vegetation points the chromatophora are as yet colorless, and are developed, as solid, very refractive, globular, or roundish granules, from the firm layer of plasm which immediately surrounds the nucleus. Afterwards they are converted, by a chemical process, into the green chlorophyll granules or chloroplasts, which have the most important function in the plasmodomism or carbon-assimilation of the plant.

The fact that the green chlorophyll granules grow independently within the living plant-cell and multiply by segmentation is very important and interesting. The globular chloroplasts are constricted in the middle, and split into two equal daughter-globules. These daughter-plastids grow, and multiply in turn in the same way.Hence they behave within the plant-cell just like the free-living chromacea in the water. On the strength of this significant comparison, one of our ablest and most open-minded scientists, Fritz Müller-Desterro, of Brazil, pointed out in 1893 that we may see in every green vegetal cell a symbiosis between plasmodomous green and plasmophagous not-green companions (cf.myAnthropogeny, figs. 277 and 278, and in the text).

Many species of the simplest chromacea live as monobia (individually). When the tiny plasma globules have split into two equal halves by simple segmentation, they separate, and live their lives apart. This is the case with the common, ubiquitous chroococcus. However, most species live in common, the plasma granules forming more or less thick cœnobia, or communities or colonies of cells. In the simplest case (aphanocapsa) the social cytodes secrete a structureless gelatinous mass, in which numbers of blue-green plasma globules are irregularly distributed. In theglœocapsa, which forms a thin blue-green gelatinous deposit on damp walls and rocks, the constituent cytodes cover themselves immediately after cleavage with a fresh gelatinous envelope, and these run together into large masses. But the majority of the chromacea form firm, threadlike cell communities or chains of plastids (catenal cœnobia.) As the transverse cleavage of the rapidly multiplying cytodes always follows the same direction, and the new daughter-cytodes remain joined at the cleavage surfaces, and are flattened into discoid shape, we get stringlike formations or articulated threads of considerable length, as in the oscillaria and nostochina. When a number of these threads are joined together in gelatinous masses, we often get large, irregular, jelly-like bodies, as in the common "shooting-star jellies" (nostoc communis). They attain the size of a plum.

In view of the extreme importance which I attach tothe chromacea as the earliest and simplest of all organisms, it is necessary to put clearly the following facts with regard to their anatomic structure and physiological activity:

1. The organism of the simplest chromacea isnotcomposed of different organella or organs; and it shows no trace of purposive construction or definite architecture.

2. The homogeneous tinted plasma granule which makes up the entire organism in the simplest case (chroococcus) exhibits no plasma structure (honeycomb, threads, etc.) whatever.

3. The original globular form of the plasma particle is the simplest of all fundamental types, and is also that assumed by the inorganic body (such as a drop of rain) in a condition of stable equilibrium.

4. The formation of a thin membrane at the surface of the structureless plasma granule may be explained as a purely physical process—that of surface strain.

5. The gelatinous envelope which is secreted by many of the chromacea is also formed by a simple physical (or chemical) process.

6. The sole essential vital function that is common to all the chromacea is self-maintenance, and growth by means of their vegetal metabolism, or plasmodomism (=carbon assimilation); this purely chemical process is on a level with the catalysis of inorganic compounds (chapter x.).

7. The growth of the cytodes, in virtue of their continuous plasmodomism, is on a level with the physical process of crystal growth.

8. The reproduction of the chromacea by simple cleavage is merely the continuation of this simple growth process, when it passes the limit of individual size.

9. All the other vital phenomena which are to be seen in some of the chromacea can also be explained byphysical or chemical causes on mechanical principles. Not a single fact compels us to assume a "vital force."

Especially noteworthy in regard to the physiological character of these lowest organisms are their bionomic peculiarities, especially the indifference to external influences, higher and lower temperatures, etc. Many of the chromacea live in hot springs, with a temperature of fifty to eighty degrees centigrade, in which no other organism is found. Other species may remain for a long time frozen in ice, and resume their vital activity as soon as it thaws. Many chromacea may be completely dried up, and then resume their life if put in water after several years.

Next in order to the chromacea we have the bacteria, the remarkable little organisms which have been well known in the last few decades as the causes of fatal diseases, and the agents of fermentation, putrefaction, etc. The special science which is concerned with them—modern bacteriology—has attained so important a position in a short period—especially as regards practical and theoretical medicine—that it is now represented by separate chairs at most of the universities. We may admire the penetration and the perseverance with which scientists have succeeded, with the aid of the best modern microscopes and methods of preparation and coloring, in making so close a study of the organism of the bacteria, determining their physiological properties, and explaining their great importance for organic life by careful experiments and methods of culture. The bionomic or economic position of the bacteria in nature's household has thus secured for these tiny organisms the greatest scientific and practical interest.

However, we find that certain general views have been maintained by specialists in bacteriology up to our own time which are in curious contrast with these brilliant results. The biologist who studies the systematicrelations of the bacteria from the modern point of view of the theory of descent is bewildered at the extraordinary views as to the place of the bacteria in the plant-world (as segmentation-fungi), their relations to other classes of plants, and the formation of their species. When we carefully consider the morphological properties that are common to all true bacteria and compare them with other organisms, we are forced to the conclusion that I urged years ago in various writings: the bacteria are not real (nucleated) cells, but unnucleated cytodes of the rank of the monera; they are not real (tissue-forming) fungi, but simple protists; their nearest relatives are the chromacea.

The individual organisms of the simplest kind, which bacteriologists call "bacteria-cells," are not real nucleated cells. That is the clear negative result of a number of most careful investigations which have been made up to date with the object of finding a nucleus in the plasma-body of the bacteria. Among recent exact investigations we must especially note those of the botanist Reinke, of Kiel, who sought in vain to detect a nucleus in one of the largest and most easily studied genera of the bacteria, thebeggiatoa, using every modern technical aid. His conviction that this important cell-structure is really lacking is the more valuable, as it is very prejudicial to his own theory of "dominants." Other scientists (especially Schaudinn) have recently claimed, as equivalent to a nucleus in some of the larger bacteria, a number of very small granules, which are irregularly distributed in the plasm, and are strongly tinted under certain coloring processes. But even if the chemical identity of these substances which take the same color were proved—which is certainly not the case—and even if the appearance of scattered nuclein-granules in the plasm could be regarded as a preliminary to, or a beginning of, the differentiation of an individual,morphologically distinct nucleus, we should not yet have shown its independence as an organellum of the cell.

Nor is this any more proved from the circumstance that in some bacteria (not all) we find a severance of the plasm into an inner and outer layer, or a frothy structure with vacuole-formation, or a special sharply outlined membrane on the plastid. Many bacteria (but not all) have such a membrane, like the nearly related chromacea, and also the secretion of a gelatine envelope. Both classes have also in common an exclusively monogenetic reproduction. The bacteria multiply, like the chromacea, by simple segmentation; as soon as the structureless plasma-granule has reached a certain size by simple growth, it is constricted and splits into two halves. In the long-bodied bacteria (the rod-shaped bacilli) the constriction always goes through the middle of the long axis, and is, therefore, simple transverse cleavage. Many bacteria have also been said to multiply by the formation of spores. But these so-called "spores" are really permanent quiescent forms (without any multiplication of individuals); the central part of the plastid (endoplasm) condenses, separates from the peripheral part (exoplasm), and undergoes a chemical change which makes it very indifferent to external influences (such as a high temperature).

The great majority of the bacteria differ so little morphologically from the chromacea that we can only distinguish these two classes of monera by the difference in their metabolism. The chromacea, as protophyta, are plasmodomous. They form new plasm by synthesis and reduction from simple inorganic compounds—water, carbonic acid, ammonia, nitric acid, etc. But the bacteria, as protozoa, are plasmophagous. They cannot, as a rule, form new plasm, but have to take it from other organisms (as parasites, saprophytes, etc.); they decompose it by analysis and oxydation. Hence thecolorless bacteria are without the important green, blue, or red coloring matter (phycocyan) which tints the plastids of the chromacea, and is the real instrument of the carbon-assimilation. However, there are exceptions in this respect:bacillus virensis tinted green with chlorophyll,micrococcus prodigiosusis blood-red, other bacteria purple, and so on. Certain earth-dwelling bacteria (nitro-bacteria) have the vegetal property of plasmodomism; they convert ammonia by oxydation into nitrous acid, and this into nitric acid, using as their source of carbon the carbonic acid gas in the atmosphere. They are thus quite independent of organic substances, and feed, like the chromacea, on simple inorganic compounds.

Hence the affinity between the plasmodomous chromacea and plasmophagous bacteria is so close that it is impossible to give a single safe criterion that will effectually separate the two classes. Many botanists accordingly combine both groups in a single class with the name ofschizophyta, and within this distinguish as "orders" the blue-green chromacea asschizophycæ(cleavage-algæ) and the colorless bacteria asschizomycetes(cleavage-fungi). However, we must not take this division too rigidly; and the absolute lack of a nucleus and tissue-formation separates the chromacea just as widely from the multicellular tissue-forming algæ as the bacteria from the fungi. The simple multiplication by the halving of the cell, which is expressed in the name "cleavage-plants" (schizophyta), is also found in many other protists.

The number of forms that can be distinguished as species in the technical sense is very great in the case of the bacteria, in spite of the extreme simplicity of their outward appearance; many biologists speak of several hundred, and even of more than a thousand, species. But when we look solely to the outer form of the livingplasma-granule, we can only distinguish three fundamental types: (1) Micrococci, or spherobacteria (briefly, cocci), globular or ellipsoid; (2) bacilli, or rhabdo-bacteria (also called eubacteria, or bacteria in the narrower sense), rod-shaped, cylindrical, and often twisted like worms (comma-bacilli); (3) spirilla, or spirobacteria, screw-shaped rods (vibriones when the screw is slight, and spirochæta when it has many coils). Besides this threefold difference in the forms of the cytodes, we have a ground of distinction in many bacilli and spirilla in the possession of one or more very thin lashes (flagella), which proceed from one of both poles of the lengthened plastid. The construction and vibration of these serves for locomotion in the swimming bacteria; but they are only found for a time in many species, and in many others are altogether wanting.

Since, then, neither the simple outer form of the bacterium-cytodes nor their homogeneous internal structure provides a satisfactory ground for the systematic distinction of the numerous species, their physiological properties are generally used for the purpose, especially their different behavior towards organic foods (albumin, gelatine, etc.), their chemical actions, and the various effects of poisoning and decomposition which they produce in the living organism. No bacteriologist now doubts that all the vital activities of the bacteria are of a chemical nature, and precisely on this account these microbes are of extreme importance. When we bear in mind how complicated are the relations of the various species of bacteria to the tissues of the human body, in which they cause the diseases of typhus, hypochondriasis, cholera, and tuberculosis, we are bound to admit that the real cause of these maladies must be sought in the peculiar molecular structure of the bacterium-plasm, or the particular arrangement of its molecules and the innumerable atoms (more than a thousand)which are, in a very loose way, made up into special groups of molecules. The chemical products of their mutual action are what we call ptomaines, which are partly very virulent poisons (toxins). We have succeeded in producing several of these poisonous matters in large quantities by artificial culture, and isolating them and experimentally ascertaining their nature; as, for instance, tetanin, which causes tetanus, typhotoxin, the poison of typhus, etc.

In thus declaring the action of bacteria to be purely chemical and analogous to that of well-known inorganic poisons, I would particularly point out that this very justifiable statement is a pure hypothesis; it is an excellent illustration of the fact that we cannot get on in the explanation of the most important natural phenomena without hypotheses. We can see nothing whatever of the chemical molecular structure of the plasm, even under the highest power of the microscope; it lies far below the limit of microscopic perception. Nevertheless, no expert scientist has the slightest doubt of its existence, or that the complicated movements of the sensitive atoms and the molecules and groups of molecules they make up are the causes of the vast changes which these tiny organisms effect in the tissues of the human and the higher animal body.

Moreover, the distinction of the many species of bacteria is of interest in connection with the general question of the nature and constancy of a species. Whereas formerly in biological classification only definite morphological characters, or definable differences in outer form or inner structure, were regarded as of any moment in the distinction of species, here, in view of the vagueness or total lack of these characters, we have to look mainly to the physiological properties, and these are based on the chemical differences in their hypothetical molecular structure. But even these are not absolutelyconstant; on the contrary, many bacteria lose their specific qualities by progressive culture under changed food-conditions. By a change in the temperature and the nutritive field in which a number of poisonous bacteria have been reared, or by the action of certain chemicals, not only the growth and multiplication are altered, but also the injurious effect they have on other organisms by the generation of poisons. This poisonous effect is weakened, and—what is most important—the weakening is transmitted by heredity to the following generations. On this is based the familiar process of inoculation, an admirable example of the inheritance of acquired characteristics.

As the bacteria are still often described as "cleavage-fungi" and classified along with the real fungi, we must particularly point out the wide gulf that separates the two groups. The real fungi (ormycetes) are metaphyta, their multicellular body (thallus) forming a very characteristic sort of tissue, the mycelium; this is composed of a number of interlaced and interwoven threads (or hyphens). Each fungus-thread consists of a row of lengthened cells, which have a thin membrane and enclose a number of small nuclei in the colorless plasm. Moreover, the two sub-classes of the real fungi, the ascomycetes and basimycetes, form peculiar fruit-bodies which generate spores (ascodia and basidia). There is no trace whatever of these real characteristics of the true fungus in the bacteria. Nor is it less incorrect to class them with the fungilli, the so-called unicellular fungi or phycomycetes (ovomycetes and zygomycetes); these form a special class of protists which has the closest affinity to the gregarinæ.

Like the closely related chromacea, many of the bacteria show a marked tendency to form communities or cell-colonies. These cell-communities arise, as elsewhere, from the fact that the individuals, which multiplyrapidly by continuous cleavage, remain joined together. This may happen in two ways. When the social bacteria secrete large quantities of gelatine, and remain distributed in this, we have thezooglœa(as in the case of theaphanocapsaandglœocapsaamong the chromacea). If, on the other hand, the long-bodied bacilli remain fastened together in rows, we get the knotted threads ofleptothrixandbeggiatoa(which may be compared with the oscillaria). And, if these threads go into branches, we havecladothrix. Other cœnobia of bacteria have the appearance of disks, the cytodes dividing in a plane, usually in groups of four (as inmerismopedia), or of cube-shaped packets when they are in all three directions of space (sarcina).

The two classes of bacteria and chromacea seem, in the present condition of our knowledge, on account of their simple organization, to be the simplest of all living things, real monera, or organisms without organs. Hence we have to put them at the lowest stage of the protist kingdom, and must regard the difference between them and the most highly differentiated unicellular beings (such as the radiolaria, ciliated infusoria, diatomes, or siphonea) as no smaller than the difference (in the realm of the histona) between a lower polyp (hydra) and a vertebrate, or between a simple alga (ulva) and a palm. But if the kingdom of the protists is badly divided, on the older rule, into a plant kingdom and an animal kingdom, the only discriminating mark we have left is the difference in metabolism; in that case we have to include the plasmophagous bacteria in the animal kingdom (as Ehrenberg did in 1838) and the plasmodomous chromacea in the plant kingdom. The remarkable class of the flagellata, which includes ciliated unicellulars of both groups, contains several forms which are only distinguished from the typical bacterium by the possession of a nucleus. If it is true that in some ofthe protists which were counted as bacteria a real nucleus has been detected, these must be separated from the others (unnucleated) and included in the nucleated flagellata.

The monera which I described in 1866, and on which I based the theory of the monera in my monograph, belong to a different division of the protists from the classes of bacteria and chromacea. These are the forms which I described asprotamœba,protogenes,protomyxa, etc. Their naked mobile plasma-bodies thrust out pseudopodia, or variable "false feet," from their surface, like the (nucleated) real rhizopods (=sarcodinæ); but they differ essentially from the latter in the absence of a nucleus. Afterwards (in mySystematic Phytogeny) I proposed to separate these unnucleated rhizopods from the others, giving the name oflobomonera(protamœba) to the amœba-like monera with flap-shaped feet, and the name ofrhizomonera(protomyxa,pontomyxa,biomyxa,arachnula, etc.) to the gromia-like, root-feet forming monera. However, of late years, real nuclei have been detected in each of these large monera, and so they have been proved to be true cells. This discovery was made possible by the improved modern methods of coloring the nucleus which I had not the use of thirty years ago in my first observations. On the strength of these recent discoveries many scientists claim that all the monera I described are true cells, and must have nuclei. This baseless assertion is much employed by the opponents of the theory of evolution in order to deny the existence of the monera altogether.

Of the genus of monera which we call protamœba I have given an illustration in myHistory of Creation(tenth edition), which has been frequently reproduced. Several species (at least two or three) of this genus still exist, and are distinguished by the shape of their flap-formation and their method of motion. They resembleordinary simple amœbæ, and only differ from these to any extent in the absence of a nucleus. Theprotamœba primitivaseems to be pretty widely distributed; it has been found repeatedly by observers (Gruber, Cienkowski, Leidy, etc.) in inland waters. In the zoological demonstrations which I have given at the University of Jena for forty years, and in the course of which the lowly inhabitants of our fresh water are regularly examined with the microscope, theprotamœba primitivahas been found four or five times. It always had the same form, as I described it, moved about by the slow formation of flaps at its surface, multiplied by simple cleavage, and showed no trace of a nucleus in its homogeneous plasma-body even with the most careful application of the modern methods of tinting the nucleus. A larger number of very fine granules (microsoma) that were irregularly distributed in the plasm, and were more or less colored by nucleus-reagents, cannot be reckoned as clear equivalents of the nucleus in this or in similar cases; they are probably products of metabolism. The same may be said of the larger marine form of rhizomoneron, which A. Gruber has recently calledpelomyxa pallida.

The large marine form of rhizomoneron to which Huxley gave the name ofbathybius Haeckeliiin 1868, and as to the real nature of which many opinions have been expressed, seems, according to the latest investigation, not to have the significance ascribed to it. However, the much-discussed question of the bathybius is superfluous as far as our monera theory and the associated hypothesis of archigony (chapter xv.) are concerned, since we have now a better knowledge of the much more important monera-forms of the chromacea and bacteria.

In the case of some of the protists I described in myMonograph on the Monera, it is at present doubtful whether their plasma-body contains a nucleus or not,and, therefore, whether they are to be classed as true cells or cytodes. This applies especially to the forms which only happened to come under observation once, such asprotomyxaandmyxastrum. In these obscure cases we must wait for fresh investigations and the application of the modern methods of tinting the nucleus. I may, however, point out, in passing, that these famous methods of nucleus-coloring give by no means the absolute certainty which is ascribed to them; there are other substances which take color in the same way as chromatin. As far as my monera theory is concerned, or the great general importance which I attach to these unnucleated living granules of plasm, it does not matter whether a nucleus is detected in these problematic monera or not. The chromacea alone—the most important of all monera—completely suffice to provide a base for the far-reaching theoretical conclusions which I draw from it.

At the close of these observations on the monera I will briefly recapitulate the weighty inferences which we can deduce from their simple organization. They serve as a solid foundation for the chief theses of our monistic biology; and they are inconsistent with the dualistic views of modern vitalists. In the first place, I emphasize the fact that the structureless plasm-body of the simple monera has no sort of organization and no composition from dissimilar parts co-operating for definite vital aims. Reinke's conscious "dominant"—as well as Weismann's mechanical "determinants"—have nothing to do here. The whole vital activity of the simplest monera, especially of the chromacea, is confined to their metabolism, and is therefore a purely chemical process, that may be compared to the catalysis of inorganic compounds. The simple formation of individuals in this primitive living matter is merely a question of the cleavage of plasma globules of a certain size (chroococcus);and their primitive multiplication (by simple self-division) is only a continued growth (analogous to that of the crystal). When this simple growth passes a certain limit, that is fixed by the chemical constitution, it leads to the independent existence of the redundant growth-products.

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