When the socially joined epidermic cells at the surface of the tissue-body thrust forth in common a protective covering, we get the cuticles, which are often thick and solid armor-plates. In many of the metaphyta wax and flinty matter are deposited in the cellulose cuticles. The strongest formation is found in the invertebrate animals, where the cuticle often determines the whole shape and articulation, as in the calcareous shells of mollusks (mussel-shells, snail-shells, cockle-shells, etc.); and especially the coats of the articulata (the crab's coat of mail, and the skins of spiders and insects).VIIUNITIES OF LIFEUnits of life—Simple and complex organisms—Morphological and physiological individuals—Morphonta and bionta—Stages of individuality: cell, person, stem—Actual and virtual bionta—Partial and genealogical bionta—Metaphysical individuals—Cells (elementary organisms)—Cell membranes—Unnucleated cells—Plastids (cytodes and cells)—Primitive cells and nucleated cells—Organella (cell organs)—Cell communities (cœnobia)—Tissues of histona—Systems of organs—Organic apparatus—Histonal individuals (sprouts and persons)—Articulation of the histona (metamerism)—Stems of the histona—Animal states.The dissection of the body of the higher animal and plant into its various organs soon prompted comparative anatomists to draw a distinction between simple and complex organisms. Then, when the cell-theory developed in the course of the last half-century, the common anatomic groundwork of all living forms was recognized in the cell; and the conception of the cell as the elementary organism led to the further belief that our own frame, like that of all the higher animals and plants, is a cell-state, composed of millions of microscopic citizens, the individual cells, which work more or less independently therein, and co-operate for the common purposes of the entire community. This fundamental principle of the modern cell-theory was applied with great success by Rudolph Virchow to the diseased organism, and led to most important reforms in medicine. The cells are, in his view, independent "life-unitiesor individual life-centres," and the unified life of the whole man is the combined result of the work of his component cells. In this way the cells are the real life-unities of the organism. Their individual independence is at once seen in the permanently unicellular protists, of which several thousand species are already known to us.On the other hand, we find among the lower animals and the higher plants a composition of homogeneous parts, which represents a higher stage of life-unity. The tree is an individual, but it is made up of a number of branches or individual sprouts, each of which consists in like manner of an axial stem with leaves attached. If we detach such a branch and plant it in the ground, it takes root and grows into an independent plant. So the coral-stem is made up of a number of individual animals or persons, each of which has its own stomach and mouth with a crown of tentacles. Each several coral-individual is equivalent to a single living polyp (actinia). Thus the stem (cormus) is a higher unity, both in the animal and the plant world. Even the herds of gregarious animals, the swarms of bees and ants, and the communities of human beings, are similar unities; with the difference that the individual persons or citizens are not physically connected, but held together by common interests. We can, therefore, distinguish three stages of organic individuality, one building upon the other—the cell, the person (or sprout), and the stem or state (cormus). Each higher unity represents an intimate union of lower individuals. Morphologically, in relation to their anatomic structure, the latter are independent; but physiologically, in respect of the life-unity of the whole, they are subordinated to the former.This relation is quite clear in the familiar examples I have quoted. But there are other organisms in which this is not so, and where the question of the real individualityis very difficult to answer. Thus, fifty years ago, we came to recognize floating animal-stems in the remarkable siphonophora, or social medusæ, which had hitherto been regarded as individual animals, or medusæ with a multiplicity of organs; further study proved that each of these apparent organs is really a modified medusa, and the whole united structure a stem. This example throws a good deal of light on the important question of association and division of labor. The whole floating siphonophoron is, physiologically considered (in respect of its vital activity), a harmoniously organized animal with a number of different organs; but from the morphological point of view (in respect of form and structure) each dependent organ is really an independent medusa.It is clear, from these few illustrations, that the question of organic individuality is by no means so simple as it seems at first sight, and that it receives different answers according as we look at the form and structure (morphologically) or the vital and psychic activity (physiologically). We must, therefore, distinguish at once between morphological (morphonta) and physiological (bionta) individuals. The tree and the siphonophoron are bionta, or individuals of the highest order, made up of a number of similar branches or persons, the social morphonta. But, when we further dissect the latter anatomically into their various organs, and these again into their microscopic elements, the cells, each branch or person seems to be a bion, and their cells to be morphonta. Each multicellular organism is, however, developed in the beginning from a single cell, the stem-cell (cytula) or fertilized ovum; this is at once a morphon and a bion, a simple individual both morphologically and physiologically. The whole process of its development into a multicellular organism consists in a repeated cleavage of the stem-cell, the resultantcells being joined in a higher unity, and assuming different forms in consequence of the division of work.The complicated modern state, with its remarkable achievements, may be regarded as the highest stage of individual perfection which is known to us in organic nature. But we can only understand the structure of this extremely complex "organism of the highest order," and its social forms and functions, when we have a sociological knowledge of the various classes that compose it, and the laws of their association and division of labor; and when we have made an anthropological study of the nature of the persons who have united, under the same laws, for the formation of a community and are distributed in its various classes. The familiar arrangement of these classes, and the settling of the rank in the mass and the governing body, show us how this complex social organism is built up step by step.But we have to look in the same way on the cell-state, which is made up from the separate individualities in human society or in the kingdom of the tissue-animals, or the branches in the kingdom of the tissue-plants. Their complex organism, composed of various organs and tissues, can only be understood when we are acquainted with their constituent elements, the cells, and the laws according to which these elementary organisms unite to form cell-communities and tissues, and are in turn modified in the divers organs in the division of labor. We must, therefore, first establish the scale of the morphonta, and the laws of their association and ergonomy, according to which the several stages or conditions of morphological individuality build on each other. Three such stages may be at once distinguished: (1) the cell (or, more correctly, the plastid), (2) the person (animal) or branch (vegetal), and (3) the stem or cormus. But we shall find that there are further subordinate stages under each of these three. It is only in the caseof the protists that the morphological unity is bound up with the physiological. In the case of the histona, the multicellular, tissue-forming organisms, this is only so at the beginning of individual existence (at the stage of the stem-cell). As soon as the multicellular body arises from this cytula by repeated segmentation, it is raised to the stage of a higher individuality, the cell-state.Our own human frame is, in its mature condition, like that of all the higher animals, a very complete cell-state, but a single cell at the beginning of its existence. We speak of the life-unity of the former as an actual bion, and that of the latter as a virtual bion; in other words, the physiological individual or the life-unity has in the first case reached the highest stage of individual development that pertains to its species, while in the second case it remains at the lowest stage of virtual formation, and has only the capacity of rising to the higher stage. In the higher plants and animals only one cell of the organism, or the two combined sexual cells (ovum and spermium), are the potential bion which may develop into an actual one. There are, however, exceptions. In the fresh-water polyp (hydra) and cognate cnidaria each piece of the body-wall, in the bath-sponge (euspongia) and similar sponges each piece of tissue, and in many plants (for instance, marchantia among the crytogams and bryophyllum among the phanerogams) each portion of a branch or leaf, has the power to develop into a mature organism, and is, therefore, a virtual bion.From these virtual bionta (parts of the body that may grow into whole organisms) we must distinguish the partial bionta which have not this property. These are separated parts of the body that live for a time after being cut off from the whole organism, but then die off. Thus, for instance, the heart of a tortoise beats for a long time after being cut out. A flower that has beenplucked may, if put in water, keep fresh and alive for many days. In some highly organized cephalopods one of the eight arms of the male develops into an independent body, swims about, and accomplishes the fertilization of the female (hectocotylusamong theargonauta,philonexis, etc.). It was at first thought to be an independent animal parasite. The same thing happens with the remarkable foldlike dorsal appendages of a large naked snail (thetys), which get detached and creep about. The body of many of the lower animals may be cut in pieces and yet may live for weeks. The life-properties of these partial bionta are important in view of the general question of the nature of life and its apparent unity in most of the higher organisms. As a fact, even here the cells and organs lead their separate individual life, though they are subordinate to and dependent on the whole.It has been attempted to answer this question of organic individuality in the sense of counting all organisms individuals which develop from a single fertilized ovum. Thus, the Italian botanist Gallesio, in 1816, regarded all plants that arise by asexual generation (budding or segmentation)—sprouts, branches, slips, bulbs, etc.—as merely portions of a single individual that came from an egg (the seed). So also Huxley, in 1855, considered the sum of all the animals that have been produced by asexual propagation, but from a single sexually generated animal, to be parts of one individual. In practice, however, this principle is useless. We should have to say that the millions of plant-lice which arise parthenogenetically from unfertilized germ-cells, but are originally descended from one impregnated ovum, are one single individual; so also all the weeping-willows in Europe, because they all came from shoots of one single sexually-produced tree.Many attempts have been made in the course of thenineteenth century to give a generally satisfactory answer to this difficult question of the content and connotation of the idea of the organic individual. None of these has found general favor. I have compared and criticised them in the third book of myGeneral Morphology. I there paid special attention to the views of Goethe, Alexander Braun, and Nägeli among the botanists, and Johannes Müller, Leuckart, and Victor Carus among the zoologists. When we consider the striking divergence of the views of such distinguished scientists and thinkers on so important a biological question, we can understand that opinions are still very divided to-day. Hence we must not be too hard on the metaphysical philosophers when—in complete ignorance of the real facts—they rear the most extraordinary theories in their airy speculations on "the principle of individuation". Compare, for instance, the opinions of the school-men and those of recent thinkers such as Arthur Schopenhauer and Edward Hartmann. As a rule, the psychological side of the problem—the question of the individual soul—is very prominent, without much attention being paid to its material substratum—the anatomic basis of the organism. Many metaphysicians, who, in their one-sided anthropism, make man here also the measure of all things, would assign personal consciousness as the basis of the idea of individuality. It is obvious that this is not a practicable test even for the higher animals, to say nothing of the lower animals and plants. In these we have a far greater variety of individuality on the one hand, and a far greater simplicity of construction on the other. I have tried to show, in my essay on "The Individuality of the Animal Body" (1878), the easiest way to answer these complicated tectological questions, and to support it by the science of structure. It suffices to distinguish the three chief stages I have mentioned, and to explainclearly their physiological significance on the one hand and morphological on the other. We will therefore consider the cell first, then the person (or sprout), and, finally, the stock (or cormus).Ever since the middle of the nineteenth century the cell theory has been generally and rightly considered one of the most important theories in biology. Every anatomical, histological, physiological, and ontogenetic work must build on the idea of the cell as the elementary organism. Nevertheless, we are still very far from having a general and clear agreement as to this universal and fundamental idea. On the contrary, the ablest biologists still differ considerably as to the nature of the cell or the elementary individual, its relation to the whole of the multicellular organism, and so on. This divergence of views is partly due to the intricacy of the phenomena we find in the life of the cell, and partly to the many and extensive changes that have been made in the meaning of the term in the course of its employment. Let us first cast a glance at the various stages of its history.When in the last third of the seventeenth century a number of scientists, especially Malpighi in Italy and Crew in England, used the microscope for the first time in the anatomic study of plant structure, they noticed a certain build of the tissue that closely resembled the honeycomb. The closely packed wax cells, filled with honey, of the hive, which show a hexagonal appearance in section, are like the wood cells that contain the sap in the plant. It was the great merit of Schleiden, the real founder of the cell theory, to prove thatallthe different tissues of plants are originally composed of such cells (1838). Theodor Schwann soon afterwards proved the same for the animal tissues; in 1839 he extended the theory to the whole organic world. Both these scientists regarded the cell as essentially a vesicle, the firm membraneof which enclosed a fluid content, and a solid smaller body inside this, which R. Brown had recognized as the nucleus in 1833. They compared the cell, as a microscopic individual, to an organic crystal, and thought it arose by a sort of crystallization in an organic medium (cytoblastema); in this the central nucleus would serve as starting-point like the nucleus of the crystal.In the first twenty years (1839-59) of the cell theory it was a fixed principle that there were three essential parts of the cell. Firstly, there was the strong outer membrane, which was not only regarded as a protective covering, but also credited with a great deal of importance as an element in the building of the organism. In the second place, there was the fluid or semi-fluid content (the sap); and, thirdly, the firm nucleus enclosed in the sap. In order to give a clearer idea of the relative thickness and disposition of these parts, the cell was compared to a cherry or a plum. The soft flesh of this fruit (corresponding to the cell sap) can, with difficulty, be separated from the external firm skin or from the hard stone within. A great step in advance was made in 1860, when Max Schultze showed that the external membrane was an unessential and secondarily formed part of the cell. It is, as a fact, altogether wanting in many, especially young, cells of the animal body. They are naked cells without any membrane. The distinguished anatomist also proved that the so-called "cell sap"—the real body of the cell—is not a simple fluid, but a viscous, albuminous substance, the independent movements of which had long been known in the rhizopods, and which the first to study it carefully, Felix Dujardin, had described assarcodein 1835. Max Schultze further showed that this "sarcode" was identical with the "cell mucus" of the plant cells which Hugo Mohl had designated "protoplasm" in 1846, andthat this living matter must be regarded as the real vehicle of the phenomena of life. As the membrane was now recognized to be non-essential, of secondary growth, and completely wanting in some cases, there remained only two essential parts of the cell—the outer soft cell body, consisting of protoplasm, and the inner firm nucleus, consisting of a similar substance called nuclein. The original naked cell was now like a cherry or plum without the skin. This new idea of the cell, formulated forty years ago, which I endeavored to confirm in my monograph on the radiolaria (1862), is now generally accepted, and the cell is defined as a granule or particle of protoplasm (= cytoplasm) enclosing a firm and definite nucleus (or caryon, consisting of caryoplasm).This would be a good occasion to glance at the errors to which microscopic investigation and the conclusions based on it are liable. Although Kölliker in 1845, and Remak in 1851, had drawn attention to the existence of naked cells, and had compared their movements (for instance, in lymph-cells) to those of the protoplasm in plant-cells, the majority of the leading microscopists clung for twenty years to the dogma that every cell must have a membrane; the definite outline which even a naked cell must show in a different refracting medium was taken to be the sign of a special and anatomically separable membrane. It would be just as correct to talk of a protective membrane on a homogeneous glass ball; its outline is sharply defined. In the long controversy that "exact" observers sustained as to the presence or absence of a membrane, this optical error—the false interpretation of a sharp contour—counted for a good deal. It is much the same with other conflicts of "exact" observers who give their "certain observations" as facts, whereas they are really inferences from imperfect observations on which different interpretations may be put.Forty years ago (1864) I tried in vain to detect a nucleus in the naked, living, mobile protoplasm of a few small rhizopod-like protists (protamœba and protogenes). Other observers, who afterwards studied similar unnucleated cells (Gruber, Cienkowski, and others), were no more successful. On the ground of these observations, which were often repeated afterwards, I formed the class of themonera—the simplest unnucleated organisms—in myGeneral Morphologyin 1866, and pointed out their great importance in solving some of the chief problems of biology. This importance has been much enhanced of late, since the chromacea and bacteria have also been recognized as unnucleated cells. Bütschli has, it is true, raised the objection that their homogeneous plasma-body behaves, not as cytoplasm, but as caryoplasm (or nuclein), and so that these simplest plastids correspond, not to the cell-body, but to the nucleus of other cells. On this view the bacteria and chromacea are not cells without nuclei, but nuclei without cell-bodies. This idea agrees with my own in conceiving the plasma-body of the monera (apart from its molecular structure) as homogeneous and not yet advanced as far as the characteristic differentiation of inner nucleus and outer cell-body. Bearing in mind that these essential parts of the cell (in the view of most cytologists) are chemically related yet different from each other, we have three possible cases of the original formation of the nucleated cell from the unnucleated cytode: (i) The nucleus and cell-body have arisen by differentiation of a homogeneous plasm (monera); (2) the cell-body is a secondary growth from the primary nucleus; (3) the nucleus is a secondary development from the cell-body.On the first view, which I hold, the plasm, or living matter, of the earliest organisms on the earth (which can only be conceived as archigonous monera) was ahomogeneousplassonor archiplasm—that is to say, a plasma-compound that was not yet differentiated into outer cytoplasm and inner caryoplasm. The rise of this chemical distinction—and the accompanying morphological division of cell-body and nucleus—was due to a phyletic differentiation; it was the outcome of a very early and most important division of labor. The hereditary matter gathered in the nucleus, the outer cell-matter controlling the intercourse with the external world. Thus, by this first ergonomy, the nucleus became the vehicle of heredity and the cell-body the organ of adaptation. Opposed to this view is the second, the hypothesis which the founder of the cell-theory, Schleiden, had put forward—that the nucleus is the original base of the cell, and the cell-body a secondary development from it. This opinion (which, in the main, corresponds to that of Bütschli) raises a number of difficulties; as does also the third hypothesis, that the unnucleated "protoplasm-body" (the outer cytoplasm-body) is the original formation, and that the nucleus arose secondarily by condensation and chemical modification of it. At the bottom, however, the difference between the three hypotheses on the primary cytogenesis is not as great as it seems at first sight. However, I am more inclined to adhere to the first; it supposes that the physiological and chemical differences between nucleus and cell-body, which afterwards became so important, were not originally present. The phenomena of caryolysis in indirect cell-division show us still how close are the relations of the two substances.If the organic population of our planet has arisen naturally, and not by a miracle, as Reinke and other vitalists suppose, the earliest elementary organisms, produced by the chemical process of archigony (spontaneous generation), could not be real nucleated cells, but unnucleated cytodes of the type of the chromacea(cf.chapter ii.). The nucleated real cell, as Oscar Hertwig and others define it to-day, can only have arisen by phylogenetic differentiation of nucleus and cell-body from the simple cytode of the monera. In that case it is a matter of simple logic to distinguish the older cytode from the later cell. The two may then best be comprised (as I proposed in vain in 1866) under the name of "plastids" (formative principles)—that is, the elementary organism in the broader sense. But if it is preferred to call the lattercells(in the broader sense), the wrong modern idea of the cell must be altered, and the nucleus-feature omitted from it. The cell is then simply the living particle of plasm, and its two stages of development must be described by other names. The unnucleated plastid might be calledprimitive cell(protocytos), and the ordinary nucleated one the nuclear cell (caryocytos).A long gradation of cellular organization leads from the simplest primitive cells (monera) to the highest developed protists. While no morphological organization whatever is discoverable in the homogeneous plasma-body of the chromacea and bacteria, we find a composition from different parts in the highly differentiated body of the advanced protophyta (diatomes, siphonea) and protozoa (radiolaria, infusoria). The manifold parts of the unicellular organism, developed by division of work in the plasm, discharge various functions, and behave physiologically like the organs of the multicellular histona. But as the idea of "organ" in the latter is morphologically fixed as a multicellular part of the body, made up of numerous tissues, we cannot call these similarly functioning parts "organs of the cell," and had better describe them as organella (or organoids).The great majority of the protists are, in the developed condition, as actual individuals, equivalent morphologicallyto real nucleated cells. By means of adaptation to the most varied conditions and the inheritance of the properties thus acquired such a variety of unicellular forms has been evolved in the course of millions of years that we can distinguish thousands of living species, both of plasmodomous protophyta and plasmophagous protozoa. The number of known and named species is already as high as this in several distinct classes, as, for instance, in the diatomes of the primitive plants and the radiolaria of the primitive animals. These solitary living unicellulars, or "hermit-cells," may be calledmonobia.Many other protists have abandoned this original solitary life; they follow their social instincts and form communities or colonies of cells (cœnobia). These are usually formed by the daughter-cells which arise from the cleavage of a mother-cell remaining united after the division, and so on with the succeeding generations which come from their repeated segmentation. The following are the chief forms of these cœnobia:1.Gelatinous Cœnobia.—The social cells secrete a structureless mass of jelly, and remain associated in the common gelatinous mass, without actual contact. Sometimes they are regularly, at other times irregularly, distributed in it. We find cœnobia of this kind even among the monera, such as thezooglœaof many bacteria and chromacea. They are common among the protophyta and protozoa.2.Spherical Cœnobia.—The cell-community forms a sort of ball, the cells lying close together at its surface, touching each other or even forming a continuous layer; such areholosphæraandvolvoxamong the protophyta,magosphæraandsynuraamong the protozoa. The latter are particularly interesting because they resemble theblastula, an important embryological stage of the metazoa, of which the simple, epithelial cell-layer atthe surface of the hollow sphere is called theblastoderm(or germinal membrane).3.Arboreal Cœnobia.—The cell-community takes the form of a small tree or shrub, the fixed cells secreting jelly-like stalks at their base and these forming branches. At the top of each stalk or branch is an independent cell; so in the case of thegomphonemaand many other diatomes, thecodonocladiumamong the flagellata, and thecarchesiumamong the ciliata.4.Catenal Cœnobia.—The cell-community forms a chain, the links of which (the individual cells) are joined in a row. We find chainlike cell-communities of this sort, or "articulated threads," even among the monera (oscillariaandnosticamong the chromacea,leptothrixamong the bacteria). Among the diatomes we have thebacillaria, among the thalamophoranodosaria, as examples. Many of the lower protophyta (algaria and algetta) form the direct transition to the true algæ among the metaphyta, as the threadlike layer of the latter (for instance,cladophora) is only a higher development of the catenal cœnobium, with polymorphism of the co-ordinated cells. We may also regard these articulated multicellular threads as the first sketch for the formation of tissues in the metaphyta.The stable communities of cells which make up the body of the histona, or multicellular plants and animals, are called tissues (telaorhista). They differ from the cœnobia of the protists in that the social cells give up their independence, assume different forms in the division of labor, and subordinate themselves to the higher unity of the organ. However, it would be just as difficult to lay down a sharp limit between the cœnobia and the tissues as between the protists and the histona which possess them; the latter have been developed phylogenetically from the former. The original physiological independence of the cells which have combined to formtissues is more completely lost in proportion to the closeness of their combination, the complexity of their division of labor, and the differentiation and centralization of the tissue-organism. Hence the various kinds of tissue in the body of the histona behave like the various classes and professions in a state. The higher the civilization and the more varied the classes of workers, the more they are dependent on each other, and the state is centralized.In the lower tissue-forming plants, the algæ and fungi, the plant-body has the appearance of a layer of cells, the tissues of which show little or no division of labor. In thesethallophytathere are none of the conducting or vascular fibres, the formation of which is of great importance in the higher plants in connection with their physiological function of circulation of the sap. These more advanced vascular plants comprehend the two great groups of ferns (pteridophyta) and flowering plants (anthophyta, or phanerogams). Their body is always composed of two chief organs, the axial stem and the lateral leaves. This is also the case with the mosses (bryophyta), which have no vascular fibres; they lie between the two chief groups of the non-vascular thallophyta and the vascular cormophyta. However, this histological and organological division of the two great groups of tissue-plants must not be pressed; there are many exceptions and intermediate forms. In general their manifold tissue-forms may be brought under two chief groups, which we may call primary and secondary. The primary tissues are the phylogenetically older and histologically simple "cell-tissues," such as we have in the thallophyta (algæ, fungi, and mosses); in these there are no conducting fibres, or, at least, only rudimentary ones. The secondary tissues are a later development from these; they form conducting and vascular fibres and other highly differentiated forms of tissue (cambium,wood, etc.). They make up the bodies of the more complex vascular plants, the ferns and flowering plants.In the bodies of the tissue-animals we may similarly distinguish two chief groups of tissues, the primary and secondary. The former are phylogenetically and ontogenetically older than the latter. The primary tissues of the metazoa are theepitelia, simple layers of cells or forms of tissue directly derived from such (glands, etc.). Secondary tissues, evolved from the former by physiological change of work and morphological differentiation, are theapotelia; of these "derivative tissues" we may distinguish the three leading groups of connective tissue, muscular tissue, and nerve tissue. These three great groups of tissue in the animal world may be subdivided, like the plant groups, into lower and higher sub-sections. The cœlenteria (gastræads, sponges, cnidaria) are predominantly built up of epitelia, as are also the phyletically older group of the cœlomaria; in the vast majority of the latter, however, the great mass of the body is formed of apotelia, and they are subject to the most extensive differentiation. The embryo of all the metazoa consists solely of epitelia (the germ-layers) at first; apotelia are developed from these afterwards by differentiation of the tissues.Comparative anatomy distinguishes in the multicellular body of the tissue-forming organisms a great number of different parts, which are regularly adapted to discharge definite vital functions, and have been most intricately developed in virtue of the division of labor. They are called "organs" in the stricter sense in opposition to the organella (or organoids) of the protists; the latter have, it is true, a similar physiological purport, but are not (being parts of a cell) equal to the former morphologically. The remarkable efficiency that we find in the structure of the various organs inview of the functions they have to discharge, and the regularity of their construction in the unity of the histon—in other words, their adaptive organization—is explained mechanically by the theory of selection, while the teleological hypotheses of dualistic biology (for instance, the "intelligent dominants" of Reinke) completely fail to account for their origin. The gradual advance of the organs and their physiological division of labor have many analogies in the two kingdoms of the histona. While at the lowest stages the simple organ represents only a separate individual piece of primitive tissue, we find special systems of organs and organic apparatus in the higher stages.The idea of a particular system of organs is determined by the unity of one tissue which forms the characteristic element in the totality of the organs that belong to it. Of such systems in the kingdom of the metaphyta we have: the skin-system (with the tissue of the epidermis), the vascular system (with its conducting and vascular fibres), and the complementary tissue system (with the basic tissue). In the kingdom of the metazoa we may similarly distinguish: the skin-system (integument of the epidermis), the vascular system (with the mesenchyma-tissue of the blood and blood-vessels), the muscular system (with the muscle-tissue), and the nervous system (with the neurona of the nerve-tissue).In contrast with the histological idea of a system of organs, we have the physiological conception of an apparatus of organs. This is not determined by the unity of the constituent tissue, but by the unity of the lifework that is accomplished by the particular group of organs in the histona. Such an apparatus of organs is, for instance, the flowers and the fruit developing therefrom in the phanerogams, or the eye or the gut of an animal. In these apparatus the most diverse organsand systems of organs may be associated for the fulfilment of a definite physiological task.In the higher animals and plants we usually regard as the "real individual" (in the wider sense of the word) the tissue-forming organism made up of various organs; and we may here briefly and instructively call this the histonal individual (or, more briefly, the "histonal"). Botanists call this individual phenomenon among the metaphyta a sprout (blastus). Zoologists give the title of "person" (prosopon) to the corresponding unity among the animals. The two forms agree very much in their general features, and may be called "individuals of the second order," if we take the cells to be the first and the stock the third stage in the hierarchy of organic individuality. In comprising them here under the general head of histonals, or histonal individuals, I mean by this to designate the definite physiological unity of the multicellular and tissue-forming organism, as contrasted with the unicellular protist on the one hand, and the higher stem, made up of several histonals, on the other.The plant-histonal, which Alexander Braun especially clearly marked out and described as the sprout, is found in two principal forms in the kingdom of the metaphyta—the lower form of the layer-sprout (thallus) and the higher form of the stalk-sprout (culmus). The thallus predominates in the lower and older sub-kingdom of the layer-plants (thallophyta), in the classes of the algæ and fungi; the culmus in the higher and younger sub-kingdom of the stalk-plants (cormophyta), in the classes of the mosses, ferns, and flowering plants. The culmus presents in general the characteristic form of an axial central organ, the stalk, with lateral organs, the leaves, attached to this at the sides, the former having an unlimited vertical growth and the latter an unlimited basal growth. The thallus does not yet show this importantmorphological division. There are, however, exceptions in both groups of the metaphyta. The large and highly developedfucoideaamong the algæ exhibit similar differentiations of organs to those we distinguish as stalk and leaves in the higher cormophyta. On the other hand, they are wanting in the lower liverworts, which form a thallus like many of the algæ; thus, for instance, the liverwortriccia fluitansis just like the brown algadictyota dichotoma. Other primitive liverworts (such as theanthoceros) have also a very simple thallus; but most of them have a separation of the thallus into an axial organ (stalk) and several lateral organs (leaves). In the distribution of labor among the leaves there then emerge the differences between the lower leaves, foliage leaves, higher leaves, and flower leaves. A simple poppy-plant (papaver) or a single-floweredgentiana ciliata, which has only one bloom at the top of its branchless stalk, is a good example of a highly developed culmus.To the plant-sprout corresponds in the animal world theperson. All the tissue-animals pass in the course of their embryonic development through the important stage of thegastrula, or "cup-shaped embryo." The whole body of the tissue-animal at this stage forms at first a simple gut-sac or gastric sac (the primitive gut), the cavity of which opens outward by a primitive mouth. The thin wall of the sac is formed by two superimposed layers of cells, the two primary germinal layers. This gastrula is the simplest form of the "person," and the two germinal layers are its sole organs.The diverse animal forms which develop along different lines from this common embryonic form of the gastrula may be grouped into two sub-kingdoms, the lower (cœlenteria) and the upper (cœlomaria) animals. The former correspond in the simplicity of their structure in many respects to the thallophyta, and the latter to thecormophyta. Of the four stems of the cœlenteria (which have only a ventral opening and no gut-cavity) the gastræads remain at the gastrula stage, and the sponges are formed by multiplication of the same stems of gastræads. On the other hand, the cnidaria develop into higher radial (star-shaped) persons, and the platodes into lower bilateral persons. From the latter are derived the worms (vermalia), the common stem-groups of the five higher animal stems, the unarticulated mollusks, echinoderms, and tunicates, and the limb-forming articulates and vertebrates.A large part of the physiological advantages and morphological perfection which the higher histona have, as contrasted with the lower, may be traced to the circumstance that the tissue-forming organism articulates—that is to say, divides on its long axis into several sections. With this multiplication of groups of organs there goes, as a rule, a more or less extensive division of work among them, a leading factor of higher development. In this point also we see the biogenetic parallelism between the two great groups of the tissue-plants and tissue-animals.In the kingdom of the tissue-plants the articulated cormophyta rise high above the unarticulated thallophyta. While the articulation of the stem of the former proceeds and leaves are developed at the knots (nodi) between each two sections of the stalk, far greater play is offered to polymorphic differentiation than in the thallophyta, which are generally without this metamerism. The formation of the bloom in the flowering plants or phanerogams consists in a sexual division of labor among the thickly gathered leaves in a short section of a stem.To the two groups of unarticulated and articulated sprouts in the kingdom of the tissue-plants correspond, in many respects, the two sections of the tissue-animals, the unarticulated and the articulated. The two stems ofthe articulates and vertebrates rise above all the other metazoa by the perfection of their organism and the variety of their functions. In the articulates the metamerism is chiefly external—an articulation of the body wall. In the vertebrates it mainly affects the internal organs, the skeleton, and the muscular system. The vertebration (articulation) of the vertebrates is not outwardly visible like that of the articulates. In both stems the articulation is similar in the lower and upper forms, as we find in the annelids and myriapods, the acrania and cyclostoma. On the other hand, the higher the organization the greater is the unlikeness of the members or articulated parts, as in the arachnida and insects, the amphibia and amniotes. The same antithesis is found in the lower and higher crustacea. This metamerism of the higher metazoa is of a motor character, having been acquired through the manner of movement of the lengthened body; but we find in some groups of the lower, and usually unarticulated, metazoa a propagative metamerism, determined by budding at the end; such is the strobilation of the chain-worms and the scyphostoma polyps. The individual metamera (parts) that are released from the end of the chain in these cases immediately show their individuality. This is also the case with many of the annelids, in which every member that is separated has the power to reproduce the whole chain of metamera.The third and highest stage of individuality to which the multicellular organism attains is the stock or colony (cormus). It is usually formed by a permanent association of histonals that are produced by cleavage (imperfect segmentation or budding) from one histonal individual. The great majority of the metaphyta form complex plants in this sense. But among the metazoa we find this form of individuality only in the lower (and generally stationary) stages of development. Here alsothere is a striking parallelism of development between the two chief groups of the histona. At the lower stages of stock-formation there is equality of the social histonals. But in the higher grades they become unequally developed in the division of labor; and the greater the differences between them become, the greater is the centralization of the whole stock (as in the case of the siphonophora). We may therefore distinguish two principal forms of stocks—the homonomous and heteronomous, the one without, and the other with, division of labor among the histonals.The history of civilization teaches us that its gradual evolution is bound up with three different processes: (1) Association of individuals in a community; (2) division of labor (ergonomy) among the social elements, and a consequent differentiation of structure (polymorphism); (3) centralization or integration of the unified whole, or rigid organization of the community. The same fundamental laws of sociology hold good for association throughout the entire organic world; and also for the gradual evolution of the several organs out of the tissues and cell-communities. The formation of human societies is directly connected with the gregariousness of the nearest related mammals. The herds of apes and ungulates, the packs of wolves, the flocks of birds, often controlled by a single leader, exhibit various stages of social formation; as also the swarms of the higher articulates (insects, crustacea), especially communities of ants and termites, swarms of bees, etc. These organized communities of free individuals are distinguished from the stationary colonies of the lower animals chiefly by the circumstance that the social elements are not bodily connected, but held together by the ideal link of common interest.VIIIFORMS OF LIFEMorphology—Laws of symmetry—Fundamental forms of animals and plants—Fundamental forms of protists and histona—Four chief classes of fundamental forms: (1) Centrostigma: vesicles (smooth vesicle and tabular vesicle); (2) Centraxonia: typical forms with central axis—Uniaxial (monaxonia, equipolar and unequipolar)—Transverse-axial (stauraxonia, double-pyramidal and pyramidal); (3) Centroplana: fundamental forms with central plane—Bilateral symmetry—Bilateral-radial and bilateral-symmetrical fundamental forms—Asymmetrical fundamental forms; (4) Anaxonia: irregular fundamental forms—- Causes of form-construction—Fundamental forms of monera, protists, and histona—Fundamental form and mode of life—Beauty of natural forms—Æsthetics of organic forms—Art forms in nature.The infinite variety of forms which we observe in the realm of organic life not only delight our senses with their beauty and diversity, but also excite our curiosity, in suggesting the problem of their origin and connection. While the æsthetic study of the forms of life provides inexhaustible material for the plastic arts, the scientific study of their relations, their structures, their origin and evolution, forms a special branch of biology, the science of forms or morphology. I expounded the principles of this science in myGeneral Morphologythirty-eight years ago. They are so remote from the ordinary curriculum of education, and are so difficult to explain without the aid of numerous illustrations, that I cannot think ofgoing fully into them here. In the present chapter I will only briefly describe those features of living things which relate to the difficult question of their ideal fundamental forms, the laws of their symmetry, and their relation to crystal-formation. I have treated these intricate questions somewhat fully in the last (eleventh) part ofArt-forms in Nature. The hundred plates contained in this work may serve as illustrations of morphological relations. In the following pages the respective plates are indicated by the letters A-f, with the number of each.The unity of the organic structure, which expresses itself everywhere in the fundamental features of living things and in the chemical composition and constructive power of their plasm, is also seen in the laws of symmetry in their typical forms. The infinite variety of the species may, both in the animal and plant worlds, be reduced to a few principal groups or classes of fundamental forms, and these show no difference in the two kingdoms (cf.plate 6). The lily has the same regular typical form as the hexaradial coral or anemone (A-f, 9, 49), and the bilateral-radial form is the same in the violet and the sea-urchin (clypeaster, A-f, 30). The dorsiventral or bilateral-symmetrical form of most green leaves is repeated in the frame of most of the higher animals (the cœlomaria); the distinction of right and left determines in each the characteristic antithesis of back and belly.The distinction between protists and histons is much more important than the familiar division of organisms into plants and animals, in respect of their fundamental forms and their configuration. For the protists, the unicellular organisms (without tissue) exhibit a much greater freedom and variety in the development of their fundamental forms than the histons, the multicellular tissue-forming organisms. In the protists (both protophytaand protozoa) the constructive force of the elementary organism, the individual cell, determines the symmetry of the typical form and the special form of its supplementation; but in the histons (both metaphyta and metazoa) it is the plasticity of the tissue, made up of a number of socially combined cells, that determines this. On the ground of this tectological distinction we may divide the whole organic world into four kingdoms (or sub-kingdoms), as the morphological system in the seventh table shows.In respect of the general science of fundamental forms (promorphology), the most interesting and varied group of living things is the class of the radiolaria. All the various fundamental forms that can be distinguished and defined mathematically are found realized in the graceful flinty skeletons of these unicellular sea-dwelling protozoa. I have distinguished more than four thousand forms of them, and illustrated by one hundred and forty plates, in my monograph on theChallengerradiolaria [translated].Only a very few organic forms seem to be quite irregular, without any trace of symmetry, or constantly changing their formless shape, as we find, for instance, in the amœbæ and the similar amœboid cells of the plasmodia. The great majority of organic bodies show a certain regularity both in their outer configuration and the construction of their various parts, which we may call "symmetry" in the wider sense of the word. The regularity of this symmetrical construction often expresses itself at first sight in the arrangement side by side of similar parts in a certain number and of a certain size, and in the possibility of distinguishing certain ideal axes and planes cutting each other at measurable angles. In this respect many organic forms are like inorganic crystals. The important branch of mineralogy that describes these crystalline forms, and gives themmathematical formulæ, is called crystallography. There is a parallel branch of the science of biological forms, promorphology, which has been greatly neglected. These two branches of investigation have the common aim of detecting an ideal law of symmetry in the bodies they deal with and expressing this in a definite mathematical formula.The number of ideal fundamental forms, to which we may reduce the symmetries of the innumerable living organisms, is comparatively small. Formerly it was thought sufficient to distinguish two or three chief groups: (1) radial (or actinomorphic) types, (2) bilateral (or zygomorphic) types, and (3) irregular (or amorphic) types. But when we study the distinctive marks and differences of these types more closely, and take due account of the relations of the ideal axes and their poles, we are led to distinguish the nine groups or types which are found in the sixth table. In this promorphological system the determining factor is the disposition of the parts to the natural middle of the body. On this basis we make a first distinction into four classes or types: (1) the centrostigma have apointas the natural middle of the body; (2) the centraxonia a straight line (axis); (3) the centroplana a plane (median plane); and (4) the centraporia (acentra or anaxonia), the wholly irregular forms, have no distinguishable middle or symmetry.
When the socially joined epidermic cells at the surface of the tissue-body thrust forth in common a protective covering, we get the cuticles, which are often thick and solid armor-plates. In many of the metaphyta wax and flinty matter are deposited in the cellulose cuticles. The strongest formation is found in the invertebrate animals, where the cuticle often determines the whole shape and articulation, as in the calcareous shells of mollusks (mussel-shells, snail-shells, cockle-shells, etc.); and especially the coats of the articulata (the crab's coat of mail, and the skins of spiders and insects).
VII
Units of life—Simple and complex organisms—Morphological and physiological individuals—Morphonta and bionta—Stages of individuality: cell, person, stem—Actual and virtual bionta—Partial and genealogical bionta—Metaphysical individuals—Cells (elementary organisms)—Cell membranes—Unnucleated cells—Plastids (cytodes and cells)—Primitive cells and nucleated cells—Organella (cell organs)—Cell communities (cœnobia)—Tissues of histona—Systems of organs—Organic apparatus—Histonal individuals (sprouts and persons)—Articulation of the histona (metamerism)—Stems of the histona—Animal states.
The dissection of the body of the higher animal and plant into its various organs soon prompted comparative anatomists to draw a distinction between simple and complex organisms. Then, when the cell-theory developed in the course of the last half-century, the common anatomic groundwork of all living forms was recognized in the cell; and the conception of the cell as the elementary organism led to the further belief that our own frame, like that of all the higher animals and plants, is a cell-state, composed of millions of microscopic citizens, the individual cells, which work more or less independently therein, and co-operate for the common purposes of the entire community. This fundamental principle of the modern cell-theory was applied with great success by Rudolph Virchow to the diseased organism, and led to most important reforms in medicine. The cells are, in his view, independent "life-unitiesor individual life-centres," and the unified life of the whole man is the combined result of the work of his component cells. In this way the cells are the real life-unities of the organism. Their individual independence is at once seen in the permanently unicellular protists, of which several thousand species are already known to us.
On the other hand, we find among the lower animals and the higher plants a composition of homogeneous parts, which represents a higher stage of life-unity. The tree is an individual, but it is made up of a number of branches or individual sprouts, each of which consists in like manner of an axial stem with leaves attached. If we detach such a branch and plant it in the ground, it takes root and grows into an independent plant. So the coral-stem is made up of a number of individual animals or persons, each of which has its own stomach and mouth with a crown of tentacles. Each several coral-individual is equivalent to a single living polyp (actinia). Thus the stem (cormus) is a higher unity, both in the animal and the plant world. Even the herds of gregarious animals, the swarms of bees and ants, and the communities of human beings, are similar unities; with the difference that the individual persons or citizens are not physically connected, but held together by common interests. We can, therefore, distinguish three stages of organic individuality, one building upon the other—the cell, the person (or sprout), and the stem or state (cormus). Each higher unity represents an intimate union of lower individuals. Morphologically, in relation to their anatomic structure, the latter are independent; but physiologically, in respect of the life-unity of the whole, they are subordinated to the former.
This relation is quite clear in the familiar examples I have quoted. But there are other organisms in which this is not so, and where the question of the real individualityis very difficult to answer. Thus, fifty years ago, we came to recognize floating animal-stems in the remarkable siphonophora, or social medusæ, which had hitherto been regarded as individual animals, or medusæ with a multiplicity of organs; further study proved that each of these apparent organs is really a modified medusa, and the whole united structure a stem. This example throws a good deal of light on the important question of association and division of labor. The whole floating siphonophoron is, physiologically considered (in respect of its vital activity), a harmoniously organized animal with a number of different organs; but from the morphological point of view (in respect of form and structure) each dependent organ is really an independent medusa.
It is clear, from these few illustrations, that the question of organic individuality is by no means so simple as it seems at first sight, and that it receives different answers according as we look at the form and structure (morphologically) or the vital and psychic activity (physiologically). We must, therefore, distinguish at once between morphological (morphonta) and physiological (bionta) individuals. The tree and the siphonophoron are bionta, or individuals of the highest order, made up of a number of similar branches or persons, the social morphonta. But, when we further dissect the latter anatomically into their various organs, and these again into their microscopic elements, the cells, each branch or person seems to be a bion, and their cells to be morphonta. Each multicellular organism is, however, developed in the beginning from a single cell, the stem-cell (cytula) or fertilized ovum; this is at once a morphon and a bion, a simple individual both morphologically and physiologically. The whole process of its development into a multicellular organism consists in a repeated cleavage of the stem-cell, the resultantcells being joined in a higher unity, and assuming different forms in consequence of the division of work.
The complicated modern state, with its remarkable achievements, may be regarded as the highest stage of individual perfection which is known to us in organic nature. But we can only understand the structure of this extremely complex "organism of the highest order," and its social forms and functions, when we have a sociological knowledge of the various classes that compose it, and the laws of their association and division of labor; and when we have made an anthropological study of the nature of the persons who have united, under the same laws, for the formation of a community and are distributed in its various classes. The familiar arrangement of these classes, and the settling of the rank in the mass and the governing body, show us how this complex social organism is built up step by step.
But we have to look in the same way on the cell-state, which is made up from the separate individualities in human society or in the kingdom of the tissue-animals, or the branches in the kingdom of the tissue-plants. Their complex organism, composed of various organs and tissues, can only be understood when we are acquainted with their constituent elements, the cells, and the laws according to which these elementary organisms unite to form cell-communities and tissues, and are in turn modified in the divers organs in the division of labor. We must, therefore, first establish the scale of the morphonta, and the laws of their association and ergonomy, according to which the several stages or conditions of morphological individuality build on each other. Three such stages may be at once distinguished: (1) the cell (or, more correctly, the plastid), (2) the person (animal) or branch (vegetal), and (3) the stem or cormus. But we shall find that there are further subordinate stages under each of these three. It is only in the caseof the protists that the morphological unity is bound up with the physiological. In the case of the histona, the multicellular, tissue-forming organisms, this is only so at the beginning of individual existence (at the stage of the stem-cell). As soon as the multicellular body arises from this cytula by repeated segmentation, it is raised to the stage of a higher individuality, the cell-state.
Our own human frame is, in its mature condition, like that of all the higher animals, a very complete cell-state, but a single cell at the beginning of its existence. We speak of the life-unity of the former as an actual bion, and that of the latter as a virtual bion; in other words, the physiological individual or the life-unity has in the first case reached the highest stage of individual development that pertains to its species, while in the second case it remains at the lowest stage of virtual formation, and has only the capacity of rising to the higher stage. In the higher plants and animals only one cell of the organism, or the two combined sexual cells (ovum and spermium), are the potential bion which may develop into an actual one. There are, however, exceptions. In the fresh-water polyp (hydra) and cognate cnidaria each piece of the body-wall, in the bath-sponge (euspongia) and similar sponges each piece of tissue, and in many plants (for instance, marchantia among the crytogams and bryophyllum among the phanerogams) each portion of a branch or leaf, has the power to develop into a mature organism, and is, therefore, a virtual bion.
From these virtual bionta (parts of the body that may grow into whole organisms) we must distinguish the partial bionta which have not this property. These are separated parts of the body that live for a time after being cut off from the whole organism, but then die off. Thus, for instance, the heart of a tortoise beats for a long time after being cut out. A flower that has beenplucked may, if put in water, keep fresh and alive for many days. In some highly organized cephalopods one of the eight arms of the male develops into an independent body, swims about, and accomplishes the fertilization of the female (hectocotylusamong theargonauta,philonexis, etc.). It was at first thought to be an independent animal parasite. The same thing happens with the remarkable foldlike dorsal appendages of a large naked snail (thetys), which get detached and creep about. The body of many of the lower animals may be cut in pieces and yet may live for weeks. The life-properties of these partial bionta are important in view of the general question of the nature of life and its apparent unity in most of the higher organisms. As a fact, even here the cells and organs lead their separate individual life, though they are subordinate to and dependent on the whole.
It has been attempted to answer this question of organic individuality in the sense of counting all organisms individuals which develop from a single fertilized ovum. Thus, the Italian botanist Gallesio, in 1816, regarded all plants that arise by asexual generation (budding or segmentation)—sprouts, branches, slips, bulbs, etc.—as merely portions of a single individual that came from an egg (the seed). So also Huxley, in 1855, considered the sum of all the animals that have been produced by asexual propagation, but from a single sexually generated animal, to be parts of one individual. In practice, however, this principle is useless. We should have to say that the millions of plant-lice which arise parthenogenetically from unfertilized germ-cells, but are originally descended from one impregnated ovum, are one single individual; so also all the weeping-willows in Europe, because they all came from shoots of one single sexually-produced tree.
Many attempts have been made in the course of thenineteenth century to give a generally satisfactory answer to this difficult question of the content and connotation of the idea of the organic individual. None of these has found general favor. I have compared and criticised them in the third book of myGeneral Morphology. I there paid special attention to the views of Goethe, Alexander Braun, and Nägeli among the botanists, and Johannes Müller, Leuckart, and Victor Carus among the zoologists. When we consider the striking divergence of the views of such distinguished scientists and thinkers on so important a biological question, we can understand that opinions are still very divided to-day. Hence we must not be too hard on the metaphysical philosophers when—in complete ignorance of the real facts—they rear the most extraordinary theories in their airy speculations on "the principle of individuation". Compare, for instance, the opinions of the school-men and those of recent thinkers such as Arthur Schopenhauer and Edward Hartmann. As a rule, the psychological side of the problem—the question of the individual soul—is very prominent, without much attention being paid to its material substratum—the anatomic basis of the organism. Many metaphysicians, who, in their one-sided anthropism, make man here also the measure of all things, would assign personal consciousness as the basis of the idea of individuality. It is obvious that this is not a practicable test even for the higher animals, to say nothing of the lower animals and plants. In these we have a far greater variety of individuality on the one hand, and a far greater simplicity of construction on the other. I have tried to show, in my essay on "The Individuality of the Animal Body" (1878), the easiest way to answer these complicated tectological questions, and to support it by the science of structure. It suffices to distinguish the three chief stages I have mentioned, and to explainclearly their physiological significance on the one hand and morphological on the other. We will therefore consider the cell first, then the person (or sprout), and, finally, the stock (or cormus).
Ever since the middle of the nineteenth century the cell theory has been generally and rightly considered one of the most important theories in biology. Every anatomical, histological, physiological, and ontogenetic work must build on the idea of the cell as the elementary organism. Nevertheless, we are still very far from having a general and clear agreement as to this universal and fundamental idea. On the contrary, the ablest biologists still differ considerably as to the nature of the cell or the elementary individual, its relation to the whole of the multicellular organism, and so on. This divergence of views is partly due to the intricacy of the phenomena we find in the life of the cell, and partly to the many and extensive changes that have been made in the meaning of the term in the course of its employment. Let us first cast a glance at the various stages of its history.
When in the last third of the seventeenth century a number of scientists, especially Malpighi in Italy and Crew in England, used the microscope for the first time in the anatomic study of plant structure, they noticed a certain build of the tissue that closely resembled the honeycomb. The closely packed wax cells, filled with honey, of the hive, which show a hexagonal appearance in section, are like the wood cells that contain the sap in the plant. It was the great merit of Schleiden, the real founder of the cell theory, to prove thatallthe different tissues of plants are originally composed of such cells (1838). Theodor Schwann soon afterwards proved the same for the animal tissues; in 1839 he extended the theory to the whole organic world. Both these scientists regarded the cell as essentially a vesicle, the firm membraneof which enclosed a fluid content, and a solid smaller body inside this, which R. Brown had recognized as the nucleus in 1833. They compared the cell, as a microscopic individual, to an organic crystal, and thought it arose by a sort of crystallization in an organic medium (cytoblastema); in this the central nucleus would serve as starting-point like the nucleus of the crystal.
In the first twenty years (1839-59) of the cell theory it was a fixed principle that there were three essential parts of the cell. Firstly, there was the strong outer membrane, which was not only regarded as a protective covering, but also credited with a great deal of importance as an element in the building of the organism. In the second place, there was the fluid or semi-fluid content (the sap); and, thirdly, the firm nucleus enclosed in the sap. In order to give a clearer idea of the relative thickness and disposition of these parts, the cell was compared to a cherry or a plum. The soft flesh of this fruit (corresponding to the cell sap) can, with difficulty, be separated from the external firm skin or from the hard stone within. A great step in advance was made in 1860, when Max Schultze showed that the external membrane was an unessential and secondarily formed part of the cell. It is, as a fact, altogether wanting in many, especially young, cells of the animal body. They are naked cells without any membrane. The distinguished anatomist also proved that the so-called "cell sap"—the real body of the cell—is not a simple fluid, but a viscous, albuminous substance, the independent movements of which had long been known in the rhizopods, and which the first to study it carefully, Felix Dujardin, had described assarcodein 1835. Max Schultze further showed that this "sarcode" was identical with the "cell mucus" of the plant cells which Hugo Mohl had designated "protoplasm" in 1846, andthat this living matter must be regarded as the real vehicle of the phenomena of life. As the membrane was now recognized to be non-essential, of secondary growth, and completely wanting in some cases, there remained only two essential parts of the cell—the outer soft cell body, consisting of protoplasm, and the inner firm nucleus, consisting of a similar substance called nuclein. The original naked cell was now like a cherry or plum without the skin. This new idea of the cell, formulated forty years ago, which I endeavored to confirm in my monograph on the radiolaria (1862), is now generally accepted, and the cell is defined as a granule or particle of protoplasm (= cytoplasm) enclosing a firm and definite nucleus (or caryon, consisting of caryoplasm).
This would be a good occasion to glance at the errors to which microscopic investigation and the conclusions based on it are liable. Although Kölliker in 1845, and Remak in 1851, had drawn attention to the existence of naked cells, and had compared their movements (for instance, in lymph-cells) to those of the protoplasm in plant-cells, the majority of the leading microscopists clung for twenty years to the dogma that every cell must have a membrane; the definite outline which even a naked cell must show in a different refracting medium was taken to be the sign of a special and anatomically separable membrane. It would be just as correct to talk of a protective membrane on a homogeneous glass ball; its outline is sharply defined. In the long controversy that "exact" observers sustained as to the presence or absence of a membrane, this optical error—the false interpretation of a sharp contour—counted for a good deal. It is much the same with other conflicts of "exact" observers who give their "certain observations" as facts, whereas they are really inferences from imperfect observations on which different interpretations may be put.
Forty years ago (1864) I tried in vain to detect a nucleus in the naked, living, mobile protoplasm of a few small rhizopod-like protists (protamœba and protogenes). Other observers, who afterwards studied similar unnucleated cells (Gruber, Cienkowski, and others), were no more successful. On the ground of these observations, which were often repeated afterwards, I formed the class of themonera—the simplest unnucleated organisms—in myGeneral Morphologyin 1866, and pointed out their great importance in solving some of the chief problems of biology. This importance has been much enhanced of late, since the chromacea and bacteria have also been recognized as unnucleated cells. Bütschli has, it is true, raised the objection that their homogeneous plasma-body behaves, not as cytoplasm, but as caryoplasm (or nuclein), and so that these simplest plastids correspond, not to the cell-body, but to the nucleus of other cells. On this view the bacteria and chromacea are not cells without nuclei, but nuclei without cell-bodies. This idea agrees with my own in conceiving the plasma-body of the monera (apart from its molecular structure) as homogeneous and not yet advanced as far as the characteristic differentiation of inner nucleus and outer cell-body. Bearing in mind that these essential parts of the cell (in the view of most cytologists) are chemically related yet different from each other, we have three possible cases of the original formation of the nucleated cell from the unnucleated cytode: (i) The nucleus and cell-body have arisen by differentiation of a homogeneous plasm (monera); (2) the cell-body is a secondary growth from the primary nucleus; (3) the nucleus is a secondary development from the cell-body.
On the first view, which I hold, the plasm, or living matter, of the earliest organisms on the earth (which can only be conceived as archigonous monera) was ahomogeneousplassonor archiplasm—that is to say, a plasma-compound that was not yet differentiated into outer cytoplasm and inner caryoplasm. The rise of this chemical distinction—and the accompanying morphological division of cell-body and nucleus—was due to a phyletic differentiation; it was the outcome of a very early and most important division of labor. The hereditary matter gathered in the nucleus, the outer cell-matter controlling the intercourse with the external world. Thus, by this first ergonomy, the nucleus became the vehicle of heredity and the cell-body the organ of adaptation. Opposed to this view is the second, the hypothesis which the founder of the cell-theory, Schleiden, had put forward—that the nucleus is the original base of the cell, and the cell-body a secondary development from it. This opinion (which, in the main, corresponds to that of Bütschli) raises a number of difficulties; as does also the third hypothesis, that the unnucleated "protoplasm-body" (the outer cytoplasm-body) is the original formation, and that the nucleus arose secondarily by condensation and chemical modification of it. At the bottom, however, the difference between the three hypotheses on the primary cytogenesis is not as great as it seems at first sight. However, I am more inclined to adhere to the first; it supposes that the physiological and chemical differences between nucleus and cell-body, which afterwards became so important, were not originally present. The phenomena of caryolysis in indirect cell-division show us still how close are the relations of the two substances.
If the organic population of our planet has arisen naturally, and not by a miracle, as Reinke and other vitalists suppose, the earliest elementary organisms, produced by the chemical process of archigony (spontaneous generation), could not be real nucleated cells, but unnucleated cytodes of the type of the chromacea(cf.chapter ii.). The nucleated real cell, as Oscar Hertwig and others define it to-day, can only have arisen by phylogenetic differentiation of nucleus and cell-body from the simple cytode of the monera. In that case it is a matter of simple logic to distinguish the older cytode from the later cell. The two may then best be comprised (as I proposed in vain in 1866) under the name of "plastids" (formative principles)—that is, the elementary organism in the broader sense. But if it is preferred to call the lattercells(in the broader sense), the wrong modern idea of the cell must be altered, and the nucleus-feature omitted from it. The cell is then simply the living particle of plasm, and its two stages of development must be described by other names. The unnucleated plastid might be calledprimitive cell(protocytos), and the ordinary nucleated one the nuclear cell (caryocytos).
A long gradation of cellular organization leads from the simplest primitive cells (monera) to the highest developed protists. While no morphological organization whatever is discoverable in the homogeneous plasma-body of the chromacea and bacteria, we find a composition from different parts in the highly differentiated body of the advanced protophyta (diatomes, siphonea) and protozoa (radiolaria, infusoria). The manifold parts of the unicellular organism, developed by division of work in the plasm, discharge various functions, and behave physiologically like the organs of the multicellular histona. But as the idea of "organ" in the latter is morphologically fixed as a multicellular part of the body, made up of numerous tissues, we cannot call these similarly functioning parts "organs of the cell," and had better describe them as organella (or organoids).
The great majority of the protists are, in the developed condition, as actual individuals, equivalent morphologicallyto real nucleated cells. By means of adaptation to the most varied conditions and the inheritance of the properties thus acquired such a variety of unicellular forms has been evolved in the course of millions of years that we can distinguish thousands of living species, both of plasmodomous protophyta and plasmophagous protozoa. The number of known and named species is already as high as this in several distinct classes, as, for instance, in the diatomes of the primitive plants and the radiolaria of the primitive animals. These solitary living unicellulars, or "hermit-cells," may be calledmonobia.
Many other protists have abandoned this original solitary life; they follow their social instincts and form communities or colonies of cells (cœnobia). These are usually formed by the daughter-cells which arise from the cleavage of a mother-cell remaining united after the division, and so on with the succeeding generations which come from their repeated segmentation. The following are the chief forms of these cœnobia:
1.Gelatinous Cœnobia.—The social cells secrete a structureless mass of jelly, and remain associated in the common gelatinous mass, without actual contact. Sometimes they are regularly, at other times irregularly, distributed in it. We find cœnobia of this kind even among the monera, such as thezooglœaof many bacteria and chromacea. They are common among the protophyta and protozoa.
2.Spherical Cœnobia.—The cell-community forms a sort of ball, the cells lying close together at its surface, touching each other or even forming a continuous layer; such areholosphæraandvolvoxamong the protophyta,magosphæraandsynuraamong the protozoa. The latter are particularly interesting because they resemble theblastula, an important embryological stage of the metazoa, of which the simple, epithelial cell-layer atthe surface of the hollow sphere is called theblastoderm(or germinal membrane).
3.Arboreal Cœnobia.—The cell-community takes the form of a small tree or shrub, the fixed cells secreting jelly-like stalks at their base and these forming branches. At the top of each stalk or branch is an independent cell; so in the case of thegomphonemaand many other diatomes, thecodonocladiumamong the flagellata, and thecarchesiumamong the ciliata.
4.Catenal Cœnobia.—The cell-community forms a chain, the links of which (the individual cells) are joined in a row. We find chainlike cell-communities of this sort, or "articulated threads," even among the monera (oscillariaandnosticamong the chromacea,leptothrixamong the bacteria). Among the diatomes we have thebacillaria, among the thalamophoranodosaria, as examples. Many of the lower protophyta (algaria and algetta) form the direct transition to the true algæ among the metaphyta, as the threadlike layer of the latter (for instance,cladophora) is only a higher development of the catenal cœnobium, with polymorphism of the co-ordinated cells. We may also regard these articulated multicellular threads as the first sketch for the formation of tissues in the metaphyta.
The stable communities of cells which make up the body of the histona, or multicellular plants and animals, are called tissues (telaorhista). They differ from the cœnobia of the protists in that the social cells give up their independence, assume different forms in the division of labor, and subordinate themselves to the higher unity of the organ. However, it would be just as difficult to lay down a sharp limit between the cœnobia and the tissues as between the protists and the histona which possess them; the latter have been developed phylogenetically from the former. The original physiological independence of the cells which have combined to formtissues is more completely lost in proportion to the closeness of their combination, the complexity of their division of labor, and the differentiation and centralization of the tissue-organism. Hence the various kinds of tissue in the body of the histona behave like the various classes and professions in a state. The higher the civilization and the more varied the classes of workers, the more they are dependent on each other, and the state is centralized.
In the lower tissue-forming plants, the algæ and fungi, the plant-body has the appearance of a layer of cells, the tissues of which show little or no division of labor. In thesethallophytathere are none of the conducting or vascular fibres, the formation of which is of great importance in the higher plants in connection with their physiological function of circulation of the sap. These more advanced vascular plants comprehend the two great groups of ferns (pteridophyta) and flowering plants (anthophyta, or phanerogams). Their body is always composed of two chief organs, the axial stem and the lateral leaves. This is also the case with the mosses (bryophyta), which have no vascular fibres; they lie between the two chief groups of the non-vascular thallophyta and the vascular cormophyta. However, this histological and organological division of the two great groups of tissue-plants must not be pressed; there are many exceptions and intermediate forms. In general their manifold tissue-forms may be brought under two chief groups, which we may call primary and secondary. The primary tissues are the phylogenetically older and histologically simple "cell-tissues," such as we have in the thallophyta (algæ, fungi, and mosses); in these there are no conducting fibres, or, at least, only rudimentary ones. The secondary tissues are a later development from these; they form conducting and vascular fibres and other highly differentiated forms of tissue (cambium,wood, etc.). They make up the bodies of the more complex vascular plants, the ferns and flowering plants.
In the bodies of the tissue-animals we may similarly distinguish two chief groups of tissues, the primary and secondary. The former are phylogenetically and ontogenetically older than the latter. The primary tissues of the metazoa are theepitelia, simple layers of cells or forms of tissue directly derived from such (glands, etc.). Secondary tissues, evolved from the former by physiological change of work and morphological differentiation, are theapotelia; of these "derivative tissues" we may distinguish the three leading groups of connective tissue, muscular tissue, and nerve tissue. These three great groups of tissue in the animal world may be subdivided, like the plant groups, into lower and higher sub-sections. The cœlenteria (gastræads, sponges, cnidaria) are predominantly built up of epitelia, as are also the phyletically older group of the cœlomaria; in the vast majority of the latter, however, the great mass of the body is formed of apotelia, and they are subject to the most extensive differentiation. The embryo of all the metazoa consists solely of epitelia (the germ-layers) at first; apotelia are developed from these afterwards by differentiation of the tissues.
Comparative anatomy distinguishes in the multicellular body of the tissue-forming organisms a great number of different parts, which are regularly adapted to discharge definite vital functions, and have been most intricately developed in virtue of the division of labor. They are called "organs" in the stricter sense in opposition to the organella (or organoids) of the protists; the latter have, it is true, a similar physiological purport, but are not (being parts of a cell) equal to the former morphologically. The remarkable efficiency that we find in the structure of the various organs inview of the functions they have to discharge, and the regularity of their construction in the unity of the histon—in other words, their adaptive organization—is explained mechanically by the theory of selection, while the teleological hypotheses of dualistic biology (for instance, the "intelligent dominants" of Reinke) completely fail to account for their origin. The gradual advance of the organs and their physiological division of labor have many analogies in the two kingdoms of the histona. While at the lowest stages the simple organ represents only a separate individual piece of primitive tissue, we find special systems of organs and organic apparatus in the higher stages.
The idea of a particular system of organs is determined by the unity of one tissue which forms the characteristic element in the totality of the organs that belong to it. Of such systems in the kingdom of the metaphyta we have: the skin-system (with the tissue of the epidermis), the vascular system (with its conducting and vascular fibres), and the complementary tissue system (with the basic tissue). In the kingdom of the metazoa we may similarly distinguish: the skin-system (integument of the epidermis), the vascular system (with the mesenchyma-tissue of the blood and blood-vessels), the muscular system (with the muscle-tissue), and the nervous system (with the neurona of the nerve-tissue).
In contrast with the histological idea of a system of organs, we have the physiological conception of an apparatus of organs. This is not determined by the unity of the constituent tissue, but by the unity of the lifework that is accomplished by the particular group of organs in the histona. Such an apparatus of organs is, for instance, the flowers and the fruit developing therefrom in the phanerogams, or the eye or the gut of an animal. In these apparatus the most diverse organsand systems of organs may be associated for the fulfilment of a definite physiological task.
In the higher animals and plants we usually regard as the "real individual" (in the wider sense of the word) the tissue-forming organism made up of various organs; and we may here briefly and instructively call this the histonal individual (or, more briefly, the "histonal"). Botanists call this individual phenomenon among the metaphyta a sprout (blastus). Zoologists give the title of "person" (prosopon) to the corresponding unity among the animals. The two forms agree very much in their general features, and may be called "individuals of the second order," if we take the cells to be the first and the stock the third stage in the hierarchy of organic individuality. In comprising them here under the general head of histonals, or histonal individuals, I mean by this to designate the definite physiological unity of the multicellular and tissue-forming organism, as contrasted with the unicellular protist on the one hand, and the higher stem, made up of several histonals, on the other.
The plant-histonal, which Alexander Braun especially clearly marked out and described as the sprout, is found in two principal forms in the kingdom of the metaphyta—the lower form of the layer-sprout (thallus) and the higher form of the stalk-sprout (culmus). The thallus predominates in the lower and older sub-kingdom of the layer-plants (thallophyta), in the classes of the algæ and fungi; the culmus in the higher and younger sub-kingdom of the stalk-plants (cormophyta), in the classes of the mosses, ferns, and flowering plants. The culmus presents in general the characteristic form of an axial central organ, the stalk, with lateral organs, the leaves, attached to this at the sides, the former having an unlimited vertical growth and the latter an unlimited basal growth. The thallus does not yet show this importantmorphological division. There are, however, exceptions in both groups of the metaphyta. The large and highly developedfucoideaamong the algæ exhibit similar differentiations of organs to those we distinguish as stalk and leaves in the higher cormophyta. On the other hand, they are wanting in the lower liverworts, which form a thallus like many of the algæ; thus, for instance, the liverwortriccia fluitansis just like the brown algadictyota dichotoma. Other primitive liverworts (such as theanthoceros) have also a very simple thallus; but most of them have a separation of the thallus into an axial organ (stalk) and several lateral organs (leaves). In the distribution of labor among the leaves there then emerge the differences between the lower leaves, foliage leaves, higher leaves, and flower leaves. A simple poppy-plant (papaver) or a single-floweredgentiana ciliata, which has only one bloom at the top of its branchless stalk, is a good example of a highly developed culmus.
To the plant-sprout corresponds in the animal world theperson. All the tissue-animals pass in the course of their embryonic development through the important stage of thegastrula, or "cup-shaped embryo." The whole body of the tissue-animal at this stage forms at first a simple gut-sac or gastric sac (the primitive gut), the cavity of which opens outward by a primitive mouth. The thin wall of the sac is formed by two superimposed layers of cells, the two primary germinal layers. This gastrula is the simplest form of the "person," and the two germinal layers are its sole organs.
The diverse animal forms which develop along different lines from this common embryonic form of the gastrula may be grouped into two sub-kingdoms, the lower (cœlenteria) and the upper (cœlomaria) animals. The former correspond in the simplicity of their structure in many respects to the thallophyta, and the latter to thecormophyta. Of the four stems of the cœlenteria (which have only a ventral opening and no gut-cavity) the gastræads remain at the gastrula stage, and the sponges are formed by multiplication of the same stems of gastræads. On the other hand, the cnidaria develop into higher radial (star-shaped) persons, and the platodes into lower bilateral persons. From the latter are derived the worms (vermalia), the common stem-groups of the five higher animal stems, the unarticulated mollusks, echinoderms, and tunicates, and the limb-forming articulates and vertebrates.
A large part of the physiological advantages and morphological perfection which the higher histona have, as contrasted with the lower, may be traced to the circumstance that the tissue-forming organism articulates—that is to say, divides on its long axis into several sections. With this multiplication of groups of organs there goes, as a rule, a more or less extensive division of work among them, a leading factor of higher development. In this point also we see the biogenetic parallelism between the two great groups of the tissue-plants and tissue-animals.
In the kingdom of the tissue-plants the articulated cormophyta rise high above the unarticulated thallophyta. While the articulation of the stem of the former proceeds and leaves are developed at the knots (nodi) between each two sections of the stalk, far greater play is offered to polymorphic differentiation than in the thallophyta, which are generally without this metamerism. The formation of the bloom in the flowering plants or phanerogams consists in a sexual division of labor among the thickly gathered leaves in a short section of a stem.
To the two groups of unarticulated and articulated sprouts in the kingdom of the tissue-plants correspond, in many respects, the two sections of the tissue-animals, the unarticulated and the articulated. The two stems ofthe articulates and vertebrates rise above all the other metazoa by the perfection of their organism and the variety of their functions. In the articulates the metamerism is chiefly external—an articulation of the body wall. In the vertebrates it mainly affects the internal organs, the skeleton, and the muscular system. The vertebration (articulation) of the vertebrates is not outwardly visible like that of the articulates. In both stems the articulation is similar in the lower and upper forms, as we find in the annelids and myriapods, the acrania and cyclostoma. On the other hand, the higher the organization the greater is the unlikeness of the members or articulated parts, as in the arachnida and insects, the amphibia and amniotes. The same antithesis is found in the lower and higher crustacea. This metamerism of the higher metazoa is of a motor character, having been acquired through the manner of movement of the lengthened body; but we find in some groups of the lower, and usually unarticulated, metazoa a propagative metamerism, determined by budding at the end; such is the strobilation of the chain-worms and the scyphostoma polyps. The individual metamera (parts) that are released from the end of the chain in these cases immediately show their individuality. This is also the case with many of the annelids, in which every member that is separated has the power to reproduce the whole chain of metamera.
The third and highest stage of individuality to which the multicellular organism attains is the stock or colony (cormus). It is usually formed by a permanent association of histonals that are produced by cleavage (imperfect segmentation or budding) from one histonal individual. The great majority of the metaphyta form complex plants in this sense. But among the metazoa we find this form of individuality only in the lower (and generally stationary) stages of development. Here alsothere is a striking parallelism of development between the two chief groups of the histona. At the lower stages of stock-formation there is equality of the social histonals. But in the higher grades they become unequally developed in the division of labor; and the greater the differences between them become, the greater is the centralization of the whole stock (as in the case of the siphonophora). We may therefore distinguish two principal forms of stocks—the homonomous and heteronomous, the one without, and the other with, division of labor among the histonals.
The history of civilization teaches us that its gradual evolution is bound up with three different processes: (1) Association of individuals in a community; (2) division of labor (ergonomy) among the social elements, and a consequent differentiation of structure (polymorphism); (3) centralization or integration of the unified whole, or rigid organization of the community. The same fundamental laws of sociology hold good for association throughout the entire organic world; and also for the gradual evolution of the several organs out of the tissues and cell-communities. The formation of human societies is directly connected with the gregariousness of the nearest related mammals. The herds of apes and ungulates, the packs of wolves, the flocks of birds, often controlled by a single leader, exhibit various stages of social formation; as also the swarms of the higher articulates (insects, crustacea), especially communities of ants and termites, swarms of bees, etc. These organized communities of free individuals are distinguished from the stationary colonies of the lower animals chiefly by the circumstance that the social elements are not bodily connected, but held together by the ideal link of common interest.
VIII
Morphology—Laws of symmetry—Fundamental forms of animals and plants—Fundamental forms of protists and histona—Four chief classes of fundamental forms: (1) Centrostigma: vesicles (smooth vesicle and tabular vesicle); (2) Centraxonia: typical forms with central axis—Uniaxial (monaxonia, equipolar and unequipolar)—Transverse-axial (stauraxonia, double-pyramidal and pyramidal); (3) Centroplana: fundamental forms with central plane—Bilateral symmetry—Bilateral-radial and bilateral-symmetrical fundamental forms—Asymmetrical fundamental forms; (4) Anaxonia: irregular fundamental forms—- Causes of form-construction—Fundamental forms of monera, protists, and histona—Fundamental form and mode of life—Beauty of natural forms—Æsthetics of organic forms—Art forms in nature.
The infinite variety of forms which we observe in the realm of organic life not only delight our senses with their beauty and diversity, but also excite our curiosity, in suggesting the problem of their origin and connection. While the æsthetic study of the forms of life provides inexhaustible material for the plastic arts, the scientific study of their relations, their structures, their origin and evolution, forms a special branch of biology, the science of forms or morphology. I expounded the principles of this science in myGeneral Morphologythirty-eight years ago. They are so remote from the ordinary curriculum of education, and are so difficult to explain without the aid of numerous illustrations, that I cannot think ofgoing fully into them here. In the present chapter I will only briefly describe those features of living things which relate to the difficult question of their ideal fundamental forms, the laws of their symmetry, and their relation to crystal-formation. I have treated these intricate questions somewhat fully in the last (eleventh) part ofArt-forms in Nature. The hundred plates contained in this work may serve as illustrations of morphological relations. In the following pages the respective plates are indicated by the letters A-f, with the number of each.
The unity of the organic structure, which expresses itself everywhere in the fundamental features of living things and in the chemical composition and constructive power of their plasm, is also seen in the laws of symmetry in their typical forms. The infinite variety of the species may, both in the animal and plant worlds, be reduced to a few principal groups or classes of fundamental forms, and these show no difference in the two kingdoms (cf.plate 6). The lily has the same regular typical form as the hexaradial coral or anemone (A-f, 9, 49), and the bilateral-radial form is the same in the violet and the sea-urchin (clypeaster, A-f, 30). The dorsiventral or bilateral-symmetrical form of most green leaves is repeated in the frame of most of the higher animals (the cœlomaria); the distinction of right and left determines in each the characteristic antithesis of back and belly.
The distinction between protists and histons is much more important than the familiar division of organisms into plants and animals, in respect of their fundamental forms and their configuration. For the protists, the unicellular organisms (without tissue) exhibit a much greater freedom and variety in the development of their fundamental forms than the histons, the multicellular tissue-forming organisms. In the protists (both protophytaand protozoa) the constructive force of the elementary organism, the individual cell, determines the symmetry of the typical form and the special form of its supplementation; but in the histons (both metaphyta and metazoa) it is the plasticity of the tissue, made up of a number of socially combined cells, that determines this. On the ground of this tectological distinction we may divide the whole organic world into four kingdoms (or sub-kingdoms), as the morphological system in the seventh table shows.
In respect of the general science of fundamental forms (promorphology), the most interesting and varied group of living things is the class of the radiolaria. All the various fundamental forms that can be distinguished and defined mathematically are found realized in the graceful flinty skeletons of these unicellular sea-dwelling protozoa. I have distinguished more than four thousand forms of them, and illustrated by one hundred and forty plates, in my monograph on theChallengerradiolaria [translated].
Only a very few organic forms seem to be quite irregular, without any trace of symmetry, or constantly changing their formless shape, as we find, for instance, in the amœbæ and the similar amœboid cells of the plasmodia. The great majority of organic bodies show a certain regularity both in their outer configuration and the construction of their various parts, which we may call "symmetry" in the wider sense of the word. The regularity of this symmetrical construction often expresses itself at first sight in the arrangement side by side of similar parts in a certain number and of a certain size, and in the possibility of distinguishing certain ideal axes and planes cutting each other at measurable angles. In this respect many organic forms are like inorganic crystals. The important branch of mineralogy that describes these crystalline forms, and gives themmathematical formulæ, is called crystallography. There is a parallel branch of the science of biological forms, promorphology, which has been greatly neglected. These two branches of investigation have the common aim of detecting an ideal law of symmetry in the bodies they deal with and expressing this in a definite mathematical formula.
The number of ideal fundamental forms, to which we may reduce the symmetries of the innumerable living organisms, is comparatively small. Formerly it was thought sufficient to distinguish two or three chief groups: (1) radial (or actinomorphic) types, (2) bilateral (or zygomorphic) types, and (3) irregular (or amorphic) types. But when we study the distinctive marks and differences of these types more closely, and take due account of the relations of the ideal axes and their poles, we are led to distinguish the nine groups or types which are found in the sixth table. In this promorphological system the determining factor is the disposition of the parts to the natural middle of the body. On this basis we make a first distinction into four classes or types: (1) the centrostigma have apointas the natural middle of the body; (2) the centraxonia a straight line (axis); (3) the centroplana a plane (median plane); and (4) the centraporia (acentra or anaxonia), the wholly irregular forms, have no distinguishable middle or symmetry.