Plants are eitherFlowerless, orFlowering; and these areDicotyledones, orMonocotyledones.
Plants are either
Flowerless, or
Flowering; and these are
Dicotyledones, or
Monocotyledones.
Among the minor groups which he placed under these general heads, "were Fungi, Mosses, Ferns, Composites, Cichoraceæ, Umbellifers, Papilionaceous plants, Conifers, Labiates, &c., under other names, but with limits not very different from those now assigned to them." Being much in advance of his age, Ray's ideas remained dormant until the time of Jussieu; by whom they were developed into what has become known as the Natural System: a system subsequently improved by De Candolle. Passing through various modifications in the hands of successive botanists, the Natural System is now represented by the following form, which is based upon the table of contents prefixed to Vol. II. of Prof. Oliver's translation of theNatural History of Plants, by Prof. Kerner. His first division, Myxothallophyta (= Myxomycetes), I have ventured to omit. The territory it occupies is in dispute between zoologists and botanists, and as I have included the group in the zoological classification, agreeing that its traits are more animal than vegetal, I cannot also include it in the botanical classification.
Here, linear arrangement has disappeared: there is a breaking up into groups and sub-groups and sub-sub-groups, which do not admit of being placed in serial order, but only in divergent and re-divergent order. Were there space to exhibit the way in which the Alliances are subdivided into Orders, and these into Genera, and these into Species, the same principle of co-ordination would be still further manifested.
On studying the definitions of these primary, secondary, and tertiary classes, it will be found that the largest are marked off from one another by some attribute which connotes sundry other attributes; that each of the smaller classes comprehended in one of these largest classes, is marked off in a similar way from the other smaller classes bound up with it; and that so, each successively smaller class has an increased number of co-existing attributes.
§ 100. Zoological classification has had a parallel history. The first attempt which we need notice, to arrange animals in such a way as to display their affinities, is that of Linnæus. He grouped them thus:[42]—
Cl. 1.MAMMALIA.Ord.Primates, Bruta, Feræ, Glires, Pecora, Belluæ, Cete.Cl. 2.AVES.Ord.Accipitres, Picæ, Anseres, Grallæ, Gallinæ, Passeres.Cl. 3.AMPHIBIA.Ord.Reptiles, Serpentes, Nantes.Cl. 4.PISCES.Ord.Apodes, Jugulares, Thoracici, Abdominales.Cl. 5.INSECTA.Ord.Coleoptera, Hemiptera, Lepidoptera, Neuroptera, Diptera, Aptera.Cl. 6.VERMES.Ord.Intestina, Mollusca, Testacea, Lithophyta, Zoophyta.
Cl. 1.MAMMALIA.Ord.Primates, Bruta, Feræ, Glires, Pecora, Belluæ, Cete.
Cl. 2.AVES.Ord.Accipitres, Picæ, Anseres, Grallæ, Gallinæ, Passeres.
Cl. 3.AMPHIBIA.Ord.Reptiles, Serpentes, Nantes.
Cl. 4.PISCES.Ord.Apodes, Jugulares, Thoracici, Abdominales.
Cl. 5.INSECTA.Ord.Coleoptera, Hemiptera, Lepidoptera, Neuroptera, Diptera, Aptera.
Cl. 6.VERMES.Ord.Intestina, Mollusca, Testacea, Lithophyta, Zoophyta.
This arrangement of classes is obviously based on apparent gradations of rank; and the placing of the orders similarly betrays an endeavour to make successions, beginning with the most superior forms and ending with the most inferior forms. While the general and vague idea of perfection determines the leading character of the classification, its detailed groupings are determined by the most conspicuous external attributes. Not only Linnæus but his opponents, who proposed other systems, were "under the impression that animals were to be arranged together into classes, orders, genera, and species, according to theirmore or less close external resemblance." This conception survived until the time of Cuvier. "Naturalists," says Agassiz, "were bent upon establishing one continual uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals. The watchword of their school was:Natura non facit saltum. They called their systemla chaine des êtres."
The classification of Cuvier, based on internal organization instead of external appearance, was a great advance. He asserted that there are four principal forms, or four general plans, on which animals are constructed; and, in pursuance of this assertion, he drew out the following scheme.
First Branch. ANIMALIA VERTEBRATA.Cl. 1. Mammalia.Cl. 2. Birds.Cl. 3. Reptilia.Cl. 4. Fishes.Second Branch. ANIMALIA MOLLUSCA.Cl. 1. Cephalapoda.Cl. 2. Pteropoda.Cl. 3. Gasteropoda.Cl. 4. Acephala.Cl. 5. Brachiopoda.Cl. 6. Cirrhopoda.Third Branch. ANIMALIA ARTICULATA.Cl. 1. Annelides.Cl. 2. Crustacea.Cl. 3. Arachnides.Cl. 4. Insects.Fourth Branch. ANIMALIA RADIATA.Cl. 1. Echinoderms.Cl. 2. Intestinal Worms.Cl. 3. Acalephæ.Cl. 4. Polypi.Cl. 5. Infusoria.
First Branch. ANIMALIA VERTEBRATA.
Cl. 1. Mammalia.
Cl. 2. Birds.
Cl. 3. Reptilia.
Cl. 4. Fishes.
Second Branch. ANIMALIA MOLLUSCA.
Cl. 1. Cephalapoda.
Cl. 2. Pteropoda.
Cl. 3. Gasteropoda.
Cl. 4. Acephala.
Cl. 5. Brachiopoda.
Cl. 6. Cirrhopoda.
Third Branch. ANIMALIA ARTICULATA.
Cl. 1. Annelides.
Cl. 2. Crustacea.
Cl. 3. Arachnides.
Cl. 4. Insects.
Fourth Branch. ANIMALIA RADIATA.
Cl. 1. Echinoderms.
Cl. 2. Intestinal Worms.
Cl. 3. Acalephæ.
Cl. 4. Polypi.
Cl. 5. Infusoria.
But though Cuvier emancipated himself from the conception of a serial progression throughout the Animal Kingdom, sundry of his contemporaries and successors remained fettered by the old error. Less regardful of the differently-combined sets of attributes distinguishing the different sub-kingdoms, and swayed by the belief in a progressive development which was erroneously supposed to imply a linear arrangement of animals, they persisted in thrusting organic forms into a quite unnatural order. The following classification of Lamarck illustrates this.
Passing over sundry classifications in which the serial arrangement dictated by the notion of ascending complexity, is variously modified by the recognition of conspicuous anatomical facts, we come to classifications which recognizeanother order of facts—those of development. The embryological inquiries of Von Baer led him to arrange animals as follows:—
III. Peripheric Type. (Radiata.)Evolutio radiata.The development proceeds from a centre, producing identical parts in a radiating order.III. Massive Type. (Mollusca.)Evolutio contorta.The development produces identical parts curved around a conical or other space.III. Longitudinal Type. (Articulata.)Evolutio gemina.The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis.IV. Doubly Symmetrical Type. (Vertebrata.)Evolutio bigemina.The development produces identical parts arising on both sides of an axis, growing upwards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. The embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures.
III. Peripheric Type. (Radiata.)Evolutio radiata.The development proceeds from a centre, producing identical parts in a radiating order.
III. Massive Type. (Mollusca.)Evolutio contorta.The development produces identical parts curved around a conical or other space.
III. Longitudinal Type. (Articulata.)Evolutio gemina.The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis.
IV. Doubly Symmetrical Type. (Vertebrata.)Evolutio bigemina.The development produces identical parts arising on both sides of an axis, growing upwards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. The embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures.
Recognizing these fundamental differences in the modes of development, as answering to fundamental divisions in the animal kingdom, Von Baer shows (among theVertebrataat least) how the minor differences which arise at successively later embryonic stages, correspond with the minor divisions.
Like the modern classification of plants, the modern classification of animals shows us the assumed linear order completely broken up. In his lectures at the Royal Institution, in 1857, Prof. Huxley expressed the relations existing among the several great groups of the animal kingdom, by placing them at the ends of four or five radii, diverging from a centre. The diagram I cannot obtain; but in the published reports of his lectures at the School of Mines the groups were arranged as on the following page. What remnant there may seem to be of linear succession in some of the sub-groups contained in it, is merely an accident of typographical convenience. Each of them is to be regarded simply as a cluster. And if Prof. Huxley had further developed the arrangement, by dispersing the sub-groupsand sub-sub-groups on the same principle, there would result an arrangement perhaps not much unlike that shown on the page succeeding this.
In the woodcut, the dots represent orders, the names of which it is impracticable to insert. If it be supposed that when magnified, each of these dots resolves itself into a cluster of clusters, representing genera and species, an approximate idea will be formed of the relations among the successively-subordinate groups constituting the animal kingdom. Besides the subordination of groups and their general distribution, some other facts are indicated. By the distances of the great divisions from the general centre, are rudelysymbolized their respective degrees of divergence from the form of simple, undifferentiated organic matter; which we may regard as their common source. Within each group, the remoteness from the local centre represents, in a rough way, the degree of departure from the general plan of the group. And the distribution of the sub-groups within each group, is in most cases such that those which come nearest to neighbouring groups, are those which show the nearest resemblances to them—in their analogies though not in their homologies. No such scheme, however, can give a correct conception. Even supposing the above diagram expressed the relations of animals to one another as truly as they can be expressed on a plane surface (which of course it does not), it would still be inadequate. Such relations cannot be represented in space of two dimensions, but only in space of three dimensions.
Relations in the Animal Kingdom (after Huxley)
§ 100a. Two motives have prompted me to include in its original form the foregoing sketch: the one being that in conformity with the course previously pursued, of giving the successive forms of classifications, it seems desirable to give this form which was approved thirty-odd years ago; and the other being that the explanatory comments remain now as applicable as they were then. Replacement of the diagram by one expressing the relations of classes as they are now conceived, is by no means an easy task; for the conceptions formed of them are unsettled. Concerning the present attitude of zoologists, Prof. MacBride writes:—
"They all recognize a certain number of phyla. Each phylum includes a group of animals about whose relation to each other no one entertains a doubt. Each zoologist, however, has his own idea as to the relationship which the various phyla bear to each other."The phyla recognized at present are:—(1) Protozoa.(2) Porifera (Sponges).(3) Cœlenterata.(4) Echinodermata.(5) PlatyhelminthesbraceCestodes.Trematodes.Turbellaria.(6) Nemertea.(7) Nematoda.(8) Acanthocephala (Echinorhyncus).(9) Chætognatha (Sagitta).(10) Rotifera.(11) Annelida (Includes Leeches and Gephyrea, Chætifera).(12) Gephyrea, Achæta.(13) ArthropodsbraceTracheata (Peripatus, Myriapods, Insects).Arachnids.Crustacea.Pycnogonida.(14) Mollusca.(15) Polyzoa (Including Phoronis).(16) Brachiopoda.(17) Chordata (Includes Balanoglossus and Tunicates. Somecontinental zoologists do not admit Balanoglossus)."[This last phylum of course includes theVertebrata.]
"They all recognize a certain number of phyla. Each phylum includes a group of animals about whose relation to each other no one entertains a doubt. Each zoologist, however, has his own idea as to the relationship which the various phyla bear to each other.
"The phyla recognized at present are:—
(13) Arthropods
[This last phylum of course includes theVertebrata.]
Though under present conditions, as above implied, it would be absurd to attempt a definite scheme of relationships, yet it has seemed to me that the adumbration of a scheme, presenting in a vague way such relationships as are generally agreed upon and leaving others indeterminate, may be ventured; and that a general impression hence resulting may be useful. On the adjacent page I have tried to make a tentative arrangement of this kind.
At the bottom of the table I have placed together, under the name "CompoundProtozoa," those kinds of aggregatedProtozoawhich show no differentiations among the members of groups, and are thus distinguished fromMetazoa; and I have further marked the distinction by their position, which implies that from them no evolution of higher types has taken place. Respecting the naming of the sub-kingdoms, phyla, classes, orders, &c., I have not maintained entire consistency. The relative values of groups cannot be typographically expressed in a small space with a limited variety of letters. The sizes of the letters mark the classificatory ranks, and by the thickness I have rudely indicated their zoological importance. In fixing the order of subordination of groups I have been aided by the table of contents prefixed to Mr. Adam Sedgwick'sStudent's Text Book of Zoologyand have also made use of Prof. Ray Lankester's classifications of several sub-kingdoms.
Relations in the Animal Kingdom (updated)
Let me again emphasize the fact that the relationships of these diverging and re-diverging groups cannot be expressed on a flat surface. If we imagine a laurel-bush to be squashed flat by a horizontal plane descending upon it, we shall see that sundry of the upper branches and twigs which were previously close together will become remote, and that the relative positions of parts can remain partially true only with the minor branches. The reader must therefore expect to find some of the zoological divisions which in the order of nature are near one another, shown in the table as quite distant.
§ 101. While the classifications of botanists and zoologists have become more and more natural in their arrangements, there has grown up a certain artificiality in their abstract nomenclature. When aggregating the smallest groups into larger groups and these into groups still larger, they have adopted certain general terms expressive of the successively more comprehensive divisions; and the habitual use of these terms, needful for purposes of convenience, has led to the tacit assumption that they answer to actualities in Nature. It has been taken for granted that species, genera, orders, and classes, are assemblages of definite values—that every genus is the equivalent of every other genus in respect of its degree of distinctness; and that orders are separated by lines ofdemarcationwhich are as broad in one place as another. Though this conviction is not a formulated one, the disputes continually occurring among naturalists on the questions, whether such and such organisms are specifically or generically distinct, and whether this or that peculiarity is or is not of ordinal importance, imply that the conviction is entertained even where not avowed. Yet that differences of opinion like these arise and remain unsettled, except when they end in the establishment of sub-species, sub-genera, sub-orders, and sub-classes, sufficiently shows that the conviction is ill-based. And this is equally shown by the impossibility of obtaining any definition of the degree of difference which warrants each further elevation in the hierarchy of classes.
It is, indeed, a wholly gratuitous assumption that organisms admit of being placed in groups of equivalent values; and that these may be united into larger groups which are also of equivalent values; and so on. There is noà priorireason for expecting this; and there is noà posteriorievidence implying it, save that which begs the question—that which asserts one distinction to be generic and another to be ordinal, because it is assumed that such distinctions must be either generic or ordinal. The endeavour to thrust plants and animals into these definite partitions is of the same nature as the endeavour to thrust them into linear series. Not that it does violence to the facts in anything like the same degree; but still, it does violence to the facts. Doubtless the making of divisions and sub-divisions, is extremely useful; or rather, it is necessary. Doubtless, too, in reducing the facts to something like order they must be partially distorted. So long as the distorted form is not mistaken for the actual form, no harm results. But it is needful for us to remember that while our successively subordinate groups have a certain general correspondence with the realities, they tacitly ascribe to the realities a regularity which does not exist.
§ 102. A general truth of much significance is exhibited in these classifications. On observing the natures of the attributes which are common to the members of any group of the first, second, third, or fourth rank, we see that groups of the widest generality are based on characters of the greatest importance, physiologically considered; and that the characters of the successively-subordinate groups, are characters of successively-subordinate importance. The structural peculiarity in which all members of one sub-kingdom differ from all members of another sub-kingdom, is a peculiarity that affects the vital actions more profoundly than does the structural peculiarity which distinguishes all members of one class from all members of another class. Let us look at a few cases.
We saw (§ 56), that the broadest division among the functions is the division into "theaccumulation of energy(latent in food); theexpenditure of energy(latent in the tissues and certain matters absorbed by them); and thetransfer of energy(latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." Now in the lowest animals, united under the general nameProtozoa, there is either no separation of the parts performing these functions or very indistinct separation: in theRhizopoda, all parts are alike accumulators of energy, expenders of energy and transferers of energy; and though in the higher members of the group, theInfusoria, there are some specializations corresponding to these functions, yet there are no distinct tissues appropriated to them. Similarly when we pass from simple types to compound types—fromProtozoatoMetazoa. The animals known asCœlenterataare characterized in common by the possession of a part which accumulates energy more or less marked off from the part which does not accumulate energy, but only expends it; and theHydrozoaandActinozoa, which are sub-divisions of theCœlenterata, are contrasted in this, that in the second these parts are much more differentiated from one another, as well as more complicated. Besides a completer differentiation of the organs respectively devoted to the accumulation of energy and the expenditure of energy, animals next above theCœlenteratapossess rude appliances for the transfer of energy: the peri-visceral sac, or closed cavity between the intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. And then out of this sac originates a more efficient appliance for the transfer of energy: the more highly-organized animals, belonging to whichever sub-kingdom, all of them possess definitely-constructed channels for distributing the matters containing energy. In all of them, too, the function of expenditure is divided between a directive apparatus and an executiveapparatus—a nervous system and a muscular system. But these higher sub-kingdoms are clearly separated from one another by differences in the relative positions of their component sets of organs. The habitual attitudes of annulose and molluscous creatures, is such that the neural centres are below the alimentary canal and the hæmal centres above. And while by these traits the annulose and molluscous types are separated from the vertebrate, they are separated from each other by this, that in the one the body is "composed of successive segments, usually provided with limbs," but in the other, the body is not segmented, "and no true articulated limbs are ever developed."
The sub-kingdoms being thus distinguished from one another, by the presence or absence of specialized parts devoted to fundamental functions, or else by differences in the distributions of such parts, we find, on descending to the classes, that these are distinguished from one another, either by modifications in the structures of fundamental parts, or by the presence or absence of subsidiary parts, or by both. Fishes andAmphibiaare unlike higher vertebrates in possessing branchiæ, either throughout life or early in life. And every higher vertebrate, besides having lungs, is characterized by having, during development, an amnion and an allantois. Mammals, again, are marked off from Birds and Reptiles by the presence of mammæ, as well as by the form of the occipital condyles. Among Mammals, the next division is based on the presence or absence of a placenta. And divisions of thePlacentaliaare mainly determined by the characters of the organs of external action.
Thus, without multiplying illustrations and without descending to genera and species, we see that, speaking generally, the successively smaller groups are distinguished from one another by traits of successively less importance, physiologically considered. The attributes possessed in common by the largest assemblages of organisms, are few in number but all-essential in kind. Each secondary assemblage,included in one of the primary assemblages, is characterized by further common attributes that influence the functions less profoundly. And so on with each lower grade.
§ 103. What interpretation is to be put on these truths of classification? We find that organic forms admit of an arrangement everywhere indicating the fact, that along with certain attributes, certain other attributes, which are not directly connected with them, always exist. How are we to account for this fact? And how are we to account for the fact that the attributes possessed in common by the largest assemblages of forms, are the most vitally-important attributes?
No one can believe that combinations of this kind have arisen fortuitously. Even supposing fortuitous combinations of attributes might produce organisms that would work, we should still be without a clue to this special mode of combination. The chances would be infinity to one against organisms which possessed in common certain fundamental attributes, having also in common numerous non-essential attributes.
Nor, again, can any one allege that such combinations are necessary, in the sense that all other combinations are impracticable. There is not, in the nature of things, a reason why creatures covered with feathers should always have beaks: jaws carrying teeth would, in many cases, have served them equally well or better. The most general characteristic of an entire sub-kingdom, equal in extent to theVertebrata, might have been the possession of nictitating membranes; while the internal organizations throughout this sub-kingdom might have been on many different plans.
If, as an alternative, this peculiar subordination of traits which organic forms display be ascribed to design, other difficulties suggest themselves. To suppose that a certain plan of organization was fixed on by a Creator for each vastand varied group, the members of which were to have many different modes of life, and that he bound himself to adhere rigidly to this plan, even in the most aberrant forms of the group where some other plan would have been more appropriate, is to ascribe a very strange motive. When we discover that the possession of seven cervical vertebræ is a general characteristic of mammals, whether the neck be immensely long as in the giraffe, or quite rudimentary as in the whale, shall we say that though, for the whale's neck, one vertebra would have been equally good, and though, for the giraffe's neck, a dozen would probably have been better than seven, yet seven was the number adhered to in both cases, because seven was fixed upon for the mammalian type? And then, when it turns out that this possession of seven cervical vertebræ is not an absolutely-universal characteristic of mammals (there is one which has eight), shall we conclude that while, in a host of cases, there was a needless adherence to a plan for the sake of consistency, there was yet, in some cases, an inconsistent abandonment of the plan? I think we may properly refuse to draw any such conclusion.
What, then, is the meaning of these peculiar relations of organic forms? The answer to this question must be postponed. Having here contemplated the problem as presented in these wide inductions which naturalists have reached; and having seen what proposed solutions of it are inadmissible; we shall see, in the next division of this work, what is the only possible solution.
DISTRIBUTION.
§ 104. There is a distribution of organisms in Space, and there is a distribution of organisms in Time. Looking first at their distribution in Space, we observe in it two different classes of facts. On the one hand, the plants and animals of each species have their habitats limited by external conditions: they are necessarily restricted to spaces in which their vital actions can be performed. On the other hand, the existence of certain conditions does not determine the presence of organisms that are fit for them. There are many spaces perfectly adapted for life of a high order in which only life of a much lower order is found.
While, in the inevitable restriction of organisms to environments with which their natures correspond we find anegativecause of distribution, there remains to be found thatpositivecause whence results the presence of organisms in some places appropriate to them and their absence from other places equally appropriate or more appropriate. Let us consider the phenomena as thus classed.
§ 105. Facts which illustrate the limiting influence of surrounding conditions are abundant, and familiar to all readers. It will be needful, however, here to cite a few typical ones of each order.
The confinement of different kinds of plants and differentkinds of animals, to the media for which they are severally adapted, is the broadest fact of distribution. We have extensive groups of plants that are respectively sub-aerial and sub-aqueous; and of the sub-aqueous some are exclusively marine, while others existonlyin rivers and lakes. Among animals we similarly find some classes confined to the air and others to the water; and of the water-breathers some are restricted to salt water and others to fresh water. Less conspicuous is the fact that within each of these contrasted media there are further widespread limitations. In the sea, certain organisms exist only between certain depths, and others only between other depths—the limpet and the mussel within the littoral zone, and numerous kinds at the bottom of the ocean; and on the land, there are Floras and Faunas peculiar to low regions and others peculiar to high regions. Next we have the familiar geographical limitations made by climate. There are temperatures which restrict each kind of organism between certain isothermal lines, and hygrometric states which prevent the spread of each kind of organism beyond areas having a certain humidity or a certain dryness. Besides such general limitations we find much more special limitations. Some minute vegetal forms occur only in snow. Hot springs have their peculiarInfusoria. The habitats of certain Fungi are mines or other dark places. And there are creatures unknown beyond the water contained in particular caves. After these limits to distribution imposed by physical conditions, come limits imposed by the presence or absence of other organisms. Obviously, graminivorous animals are confined within tracts which produce plants fit for them to feed on. The great carnivores cannot exist out of regions where there are creatures large enough and numerous enough to serve for prey. The needs of the sloth limit it to certain forest-covered spaces; and there can be no insectivorous bats where there are no night-flying insects. To these dependences of the relatively-superior organisms on the relatively-inferior organisms which they consume, must be added certainreciprocal dependences of the inferior on the superior. Mr. Darwin's inquiries have shown how generally the fertilization of plants is due to the agency of insects, and how certain plants, being fertilizable only by insects of certain structures, are limited to regions inhabited by insects of such structures. Conversely, the spread of organisms is often bounded by the presence of particular organisms beyond the bounds—either competing organisms or organisms directly inimical. A plant fit for some territory adjacent to its own, fails to overrun it because the territory is pre-occupied by some plant which is its superior, either in fertility or power of resisting destructive agencies; or else fails because there lives in the territory some mammal which browses on its foliage or bird which devours nearly all its seeds. Similarly, an area in which animals of a particular species might thrive, is not colonized by them because they are not fleet enough to escape some beast of prey inhabiting this area, or because the area is infested by some insect which destroys them, as the tsetse destroys the cattle in parts of Africa. Yet another more special series of limitations accompanies parasitism. There are parasitic plants that flourish only on trees of some few species, and others that have particular animals for their habitats—as the fungus which is fatal to the silk-worm, or that which so strangely grows out of a New Zealand caterpillar. Of animal-parasites various kinds lead lives involving specialities of distribution. We have kinds which use other creatures for purposes of locomotion, as theChelonobiauses the turtle, and as a certainActiniauses the shell inhabited by a hermit-crab. We have the parasitism in which one creature habitually accompanies another to share its prey, like the annelid which takes up its abode in a hermit-crab's shell, and snatches from the hermit-crab the morsels of food it is eating. We have again the commoner parasitism of theEpizoa—animals which attach themselves to the surfaces of other animals, and feed on their juices or on their secretions. And once more, we have theequally common parasitism of theEntozoa—creatures which live within other creatures. Besides being restricted to the bodies of the organisms it infests, each species has usually still narrower limits of distribution; in some cases the infested organisms furnish fit habitats for the parasites only in certain regions, and in other cases only when in certain constitutional states. There are more indirect modes in which the distributions of organisms affect one another. Plants of some kinds are eaten by animals only in the absence of kinds that are preferred to them; and hence the prosperity of such plants partly depends on the presence of the preferred plants. Mr. Bates has shown that some South American butterflies thrive in regions where insectivorous birds would destroy them, did they not closely resemble butterflies of another genus which are disliked by those birds. And Mr. Darwin gives cases of dependence still more remote and involved.
Such are the chief negative causes of distribution—the inorganic and organic agencies that set bounds to the spaces which organisms of each species inhabit. Fully to understand their actions we must contemplate them as working not separately but in concert. We have to regard the physical influences, varying from year to year, as now producing an extension or restriction of the habitat in this direction and now in that, and as producing secondary extensions and restrictions by their effects on other kinds of organisms. We have to regard the distribution of each species as affected not only by causes which favour multiplication of prey or of enemies within its own area, but also by causes which produce such results in neighbouring areas. We have to conceive the forces by which the limit is maintained, as including all meteorologic influences, united with the influences, direct or remote, of numerous co-existing species.
One general truth, indicated by sundry of the above illustrations, calls for special notice—the truth that all kinds of organisms intrude on one another's spheres of existence. Ofthe ways in which they do this the commonest is invasion of territory. That tendency which we see in the human races, to overrun and occupy one another's lands, as well as the lands inhabited by inferior creatures, is a tendency exhibited by all classes of organisms in various ways. Among them, as among mankind, there are permanent conquests, temporary occupations, and occasional raids. Every spring an inroad is made into the area which our own birds occupy, by birds from the South; and every winter the fieldfares of the North come to share the hips and haws of our hedges, and thus entail on our native birds some mortality. Besides these regularly-recurring incursions there are irregular ones; as of locusts into countries not usually visited by them, or of certain rodents which from time to time swarm into areas adjacent to their own. Every now and then an incursion ends in permanent settlement—perhaps in conquest over indigenous species. Within these few years an American water-weed has taken possession of our ponds and rivers, and to some extent supplanted native water-weeds. Of animals may be named a small kind of red ant, having habits allied to those of tropical ants, which has of late overrun many houses in London. The rat, which must have taken to infesting ships within these few centuries, furnishes a good illustration of the readiness of animals to occupy new places that are available. And the way in which vessels visiting India are cleared of the European cockroach by the kindredBlatta orientalis, shows us how these successful invasions last only until there come more powerful invaders. Animals encroach on one another's spheres of existence in further ways than by trespassing on one another's areas: they adopt one another's modes of life. There are cases in which this usurpation of habits is slight and temporary; and there are cases where it is marked and permanent. Grey crows often join gulls in picking up food between tide-marks; and gulls may occasionally be seen many miles inland, feeding in ploughed fields and on moors. Mr. Darwin has watched afly-catcher catching fish. He says that the greater titmouse sometimes adopts the practices of the shrike, and sometimes of the nuthatch, and that some South American woodpeckers are frugivorous while others chase insects on the wing. Of habitual intrusions on the occupations of other creatures, one case is furnished by the sea-eagle, which, besides hunting the surface of the land for prey, like the rest of the hawk-tribe, often swoops down upon fish. And Mr. Darwin names a species of petrel that has taken to diving, and has a considerably modified organization. The last cases introduce a still more remarkable class of facts of kindred meaning. This intrusion of organisms on one another's modes of life goes to the extent of intruding on one another's media. The great mass of flowering plants are terrestrial, and (aside from other needs) are required to be so by their process of fructification. But there are some which live in the water, and protrude their flowers above the surface. Nay, there is a still more striking instance. At the sea-side may be found an alga a hundred yards inland, and a phænogam rooted in salt water. Among animals these interchanges of media are numerous. Nearly all coleopterous insects are terrestrial; but the water-beetle, which like the rest of its order is an air-breather, has aquatic habits. Water appears to be an extremely unfit medium for a fly; and yet Mr. [now Sir John] Lubbock has discovered more than one species of fly living beneath the surface of the water and coming up occasionally for air. Birds, as a class, are specially fitted for an aerial existence; but certain tribes of them have taken to an aquatic existence—swimming on the surface of the water and making continual incursions beneath it, and some kinds have wholly lost the power of flight. Among mammals, too, which have limbs and lungs implying an organization for terrestrial life, may be named kinds living more or less in the water and are more or less adapted to it. We have water-rats and otters which unite the two kinds of life, and show but little modification; hippopotami passing the greater part of their time in thewater, and somewhat more fitted to it; seals living almost exclusively in the sea, and having the mammalian form greatly obscured; whales wholly confined to the sea, and having so little the aspect of mammals as to be mistaken for fish. Conversely, sundry inhabitants of the water make excursions on the land. Eels migrate at night from one pool to another. There are fish with specially-modified gills and fin-rays serving as stilts, which, when the rivers they inhabit are partially dried-up, travel in search of better quarters. And while some kinds of crabs do not make land-excursions beyond high-water mark, other kinds pursue lives almost wholly terrestrial.
Guided by these two classes of facts, we must regard the bounds to each species' sphere of existence as determined by the balancing of two antagonist sets of forces. The tendency which every species has to intrude on other areas, other modes of life, and other media, is restrained by the direct and indirect resistance of conditions, organic and inorganic. And these expansive and repressive energies, varying continually in their respective intensities, rhythmically equilibrate each other—maintain a limit that perpetually oscillates from side to side of a certain mean.
§ 106. As implied at the outset, the character of a region, when unfavourable to any species, sufficiently accounts for the absence of this species; and thus its absence is not inconsistent with the hypothesis that each species was originally placed in the regions most favourable to it. But the absence of a species from regions thatarefavourable to it cannot be thus accounted for. Were plants and animals localized wholly with reference to the fitness of their constitutions to surrounding conditions, we might expect Floras to be similar, and Faunas to be similar, where the conditions are similar; and we might expect dissimilarities among Floras and among Faunas, proportionate to the dissimilarities of their conditions. But we do not find such anticipations verified.