§ 314a. That from the beginning of life there has been an ever-increasing heterogeneity in the Earth’s Flora and Fauna, is a truth recognized by all biologists who accept the doctrine of evolution. In discussing the origin of species Mr. Darwin and others have been mainly occupied in explaining the genesis of now this and now that form of organism, considered as a member of one or other series, and regarded as becoming differentiated from its allies. But by implication, if not avowedly, there has been simultaneously accepted the belief that the forms continually produced by divergences and re-divergences, have constituted an assemblage increasingly multiform in its included kinds. And this, which we are shown by the process of organic evolution as followed out in its details, is a corollary from the doctrine of evolution at large, as was pointed out in § 159 of First Principles.

Meanwhile there has been little if any recognition of an accompanying change, no less fundamental. In the general transformation which constitutes Evolution, differentiation and integration advance hand in hand; so that along with the production of unlike parts there progresses the union of these unlike parts into a whole. Examples of various kinds before given will recur to the reader, and an addition to them has just been set forth in the chapter on “Physiological Integration.” One more example, world-wide in its reach, has still to be named.

For here it remains to point out that along with the increasing multiplication of types of organisms covering the Earth’s surface, there has been ever going on an increasing mutual dependence of them—an increasing integration of the entire aggregate of living things.

Many facts which are obvious and many which are quite familiar will be named as evidence. But I must be excused for reminding the reader of things that he knows and things that he may easily observe, since, unless the evidence, trite as it may be, is gathered together and properly marshalled, the generalization enunciated will not be thought valid.

§ 314b. Respecting the physiological characters of the earliest forms there is an assumption from which no escape seems possible—the assumption that they united animal and vegetal characters. Even among existing microscopic types of the lowest classes, there is such community of plant-traits and animal-traits that doubts respecting their proper places in one or the other kingdom are continually raised—doubts, too, whether, if regarded as vegetal, they are to be grouped as algoid or fungoid.

Here, however, without entering on moot questions, we may draw the à priori conclusion that these earliest living things were double-natured, in so far that they must have had the ability to assimilate from the inorganic world all the materials of which protoplasm consists—must therefore, along with the power of appropriating carbon from its gaseous compound, also have had the power of appropriating nitrogen, either from one of its combined oxides or directly from the air with which water is more or less charged. For before organic substances existed there could have been none but inorganic sources from which nitrogen could be obtained.

This conclusion concerns us only because it implies homogeneity of nature in these primordial forms of life. There could not at first have existed among these minutest of Protozoa even such vague distinctions as are now presented in a shadowy way by their modern representatives. And the implication is that during the period throughout which these smallest, lowest, and simplest living things alone existed, there could have been, in the absence of kinds, no mutual dependence.

Since, among various of the lowest types now known to us, the same individual exhibits a life which is now predominantly vegetal and now predominantly animal, we cannot err in assuming that there eventually took place differentiations of this original plant-animal type into types permanently unlike: some in which the traits were more markedly vegetal and others in which they were more markedly animal. As fast as this differentiation arose, there came the beginnings of co-operation between the predominantly vegetal types which by the aid of light formed organic matter from the inorganic world, and the predominantly animal types which, in chief measure, utilized the matter so formed. Evidently with the rise of such a differentiation came an incipient mutual dependence. If to the implied algoid type and the animal type there be added the fungoid type, somewhat intermediate in character, which in a large proportion of cases lives on the decaying remnants of the other two, we are furnished with a rude conception of the primary differentiations and the accompanying vague mutual dependences.

Speculation aside, it suffices to say that early in the history of life there must have arisen the distinction between Protozoa and Protophyta, and that this distinction foreshadowed that widest contrast which the higher organic world presents—the contrast between plants and animals. It is needless to do more than name the mutual dependence between these two great divisions. That, as being respectively decomposers of carbon dioxide and exhalers of carbon dioxide, they act reciprocally, as also in some measure by interchange of nitrogenous matters; and that the implied general co-operation serves in an indirect way to unite their lives, and in that sense to integrate the two kingdoms; needs not to be insisted upon. Further complications of the mutual dependence will be mentioned by and by. For the present it suffices to recognize this division of organic functions as the first which arose and as continuing to be that fundamental one which more than all others binds organisms at large together.

§ 314c. It will be thought by many readers that in speaking of the contrasted vital activities of plants and animals as constituting a “division of organic functions,” I am straining words beyond their meanings; since the conception of organic functions postulates an organized whole in which they exist, and plants and animals constitute no such organized whole. But there is at hand an unexpected defence for this conception—a defence not forthcoming a generation ago, but which now all biologists will recognize as relevant. I refer to the phenomena of symbiosis. These present various cases in which the plant-function and the animal-function are carried on in the same body,—cases in which the co-operation is not between separate vegetal organisms which accumulate nutritive matters and separate animal organisms which consume them, but is a co-operation between vegetal elements and animal elements forming parts of the same organism.

As introductory to examples of these must first, however, be named an example of such co-operation between the two great classes of vegetal organisms—the fungoid and the algoid. Incredible as the statement once seemed, it is a statement now accepted, that what we know as lichens, and used to consider as plants forming a certain low class, are now found to be not plants in the ordinary sense at all, but compound growths formed of minute algæ and minute fungi, carrying on their lives together: the algæ furnishing to the fungi certain constituents they need but cannot directly obtain, and the fungi profiting by certain materials they obtain from the algæ, either while living or while individually decaying. Whence it would seem that after the microscopic vegetal type had become in a large degree differentiated into two main types, in adaptation to different conditions of life, and had acquired appropriate specialities of nature, there grew up this communistic arrangement between certain of them, enabling each to benefit by the powers which the other had acquired: evidently an exchange of services, a physiological division of labour, a mutual dependence of functions analogous to that which exists between functions in an ordinary plant or animal.

Not differing in principle but only in application, is that symbiosis above referred to as existing between Protophyta and many Protozoa, as well as between such Protophyta and the lowest kinds of Metazoa. A recent statement that certain amœbæ, made green by contained chlorophyll, continue to grow and multiply after they have consumed what nutritive matter may be at hand, is in harmony with various facts alleged of other Protozoa—various other kinds of Rhizopods, various Heliozoa, numerous ciliated and flagellated Infusoria. Among Metazoa the like association occurs in one of the sponges, in the Hydra viridis, in various turbellarians, in a rotifer, and even in two molluscs. In these cases the partnership between the vegetal cells and the animal cells (existing either as units or as an organized group such as a polype), is a partnership which, as before, profits each of the partners—an inference supported by the fact that Metazoa containing these algoid cells usually place themselves where the light falls upon them, and can therefore further the production of the carbo-hydrates which eventually become useful to the animal cells, while these in some way reciprocate the benefit.

Here, then, we have exchange of services between associated plant-elements and animal-elements—a performance by them of different organic functions for the benefit of the aggregate which they unite to form. Hence, when these vegetal elements and animal elements are separately embodied in plants and animals, which profit by one another, we may still properly regard their respective lives as mutually-dependent organic functions, as said in the preceding section. We are enabled the better to see how the Earth’s Flora and Fauna, which are respectively accumulators of motion and expenders of motion, form mutually-dependent parts of a whole, and are in that sense integrated. And we shall be prepared to see how all other relations between organisms which make them subservient one to another, similarly constitute elements in a general integration of the organic world.

§ 314d. Another form of mutual dependence and consequently of integration is conspicuous—that which accompanied the progressive increase of size in organisms of the higher classes. We have but to contemplate the possibilities to see that life must necessarily have commenced with minute forms, and that the progress to larger ones must have been by small steps.

For had creatures of appreciable sizes been the first to exist they would inevitably have disappeared from lack of food. Having no resource but to devour one another, they would quickly have brought life to an end. There must have been smaller types serving as prey for larger ones before these could continue to exist and to multiply: microbes affording food to infusoria, infusoria affording food to such sized creatures as the Entomostraca, these again supplying food to small fishes, such as loch-trout, and these last yielding to larger fishes masses sufficiently great for their needs: each higher grade requiring lower grades of appropriate bulk. It needs but to ask what would become of tigers if there were no mammals larger than mice, to see that the animal world is a linked assemblage, of which the connected members stand within certain ratios of mass; and that during the evolution of higher and larger types the linking of grades has become closer.

That among plants considered as an aggregate relations of like kind, though far less distinct ones, have all along been growing may be reasonably concluded. In a world peopled only by microscopic types there could not have existed the conditions needful for large trees. Gradual disintegration of rock-surfaces, partly effected by physical agencies and partly by low forms of plants, had to prepare the way for superior plants. The production of sufficient soil by mineralogical decay as well as by the decay of organisms, plant and animal, may be regarded as having been a preliminary to larger plant-growth; and though at present the dependence is far less close than that among animals, yet the benefits yielded to metaphytes by the decomposing actions carried on by protophytes, as well as those carried on by microbes permeating the soil, imply a continued general interdependence throughout the aggregate of plant-forms, apart from more special interdependences. And then along with this indebtedness of the greater plants to the smaller during the process of evolution, there must be named that indebtedness of plant-life to animal-life which Mr. Darwin has shown in his book on the agency of worms as producers of mould.

§ 314e. Services of one to another, and consequent unions, of more special kinds are infinitely varied, alike within each kingdom and between the two kingdoms. I refer to those seen in parasitism, commensalism, and other forms of association. While they do not conduce to unions of the kind thus far considered, these nevertheless constitute innumerable links whereby the lives of organisms, plant and animal, are tied together; sometimes for the advantage of both but in most cases for the benefit of one to the injury of the other.

Among plants the degrees of dependence are various. Unable to raise themselves into the air and light, some climb, like the ivy, by modified rootlets, or spirally coil themselves, or hang by tendrils. Others there are which gradually strangle the trees they embrace, or which, like lichens in damp climates, festooning the smaller trees, by and by cause their decay. Of higher types of epiphytes which use trees only to gain elevation, the orchids may be instanced. And then we have plants which, like the mistletoe, fix themselves on the bark of their hosts, utilizing them partly for purposes of elevation and partly by appropriation of their juices. After these may be named those extreme cases in which the parasitic plants, ceasing to have any chlorophyll-bearing leaves, live wholly on the juices of the invaded plants. At home the common dodder, and in the tropics the Rafflesiaceæ, belong to this group. There must be added the numerous forms of minute fungi which in like manner thrive at the expense of the plants they infest. In all these cases the interdependence is one-sided, though, as we shall presently see, while detrimental to one of the two concerned, it is not always detrimental to the organic world as a whole.

That utilization of one by another among animals which causes immediate death, is familiar enough in the relations between carnivores and herbivores. Almost as familiar are those seen in parasitism. Less familiar are those seen in commensalism; and the least familiar are those which show us exchange of services. Among these last—the mutually-beneficial relations—that between the crocodile and the bird which picks parasites out of its teeth is a striking one; and no less so is that of the pique-gouffe, an African bird which pierces the tumour on a buffalo’s back that incloses a parasite. Then of another kind we have the connexion between aphides and ants: the one profiting by being carried to better pastures and the other by increased saccharine excretion. Next comes the class of messmates, the connexions between some of which are relatively innocent, as witness the Sea-anemone which settles itself on the shell occupied by a Hermit-crab, or as witness the Remora fixed on a shark’s skin. Less innocent is the relation under which one of the two seizes a share of the food obtained by the other, like the annelid which insinuates itself between the Hermit-crab and the whelk-shell it inhabits, or like the small fishes inhabiting certain Medusæ, or those which nestle in the branchial sac of the Lophius. After these may be named the less injurious forms of parasites proper—those which, distinguished as Epizoa, fix themselves on the skins of their hosts, permanently or temporarily, such as, of the one kind, the Lernæa on fishes, and of the other kind the Tick on mammals and birds. Then there come the other class of parasites, most of them highly injurious, distinguished as Entozoa, living within the bodies of their hosts, now in parts of their alimentary canals, now on other of their mucous surfaces, and now in various of their organs: these last two groups being so numerous in their kinds that there are commonly more species than one proper to each larger animal. One stage further in the complication meets us in the parasites upon parasites.

But now the general fact, to which these brief indications are introductory, is that the use made of one organism by another has been ever widening and becoming more involved. Among plants utilization of the larger by the smaller—of trees by epiphytes and parasites—must have arisen since the times when the larger came into existence—times relatively late in the course of organic evolution. Moreover most of the plants which utilize others, either by climbing up them or settling themselves high up on their stems or sucking their juices, are phænogams, and the plants they utilize are also phænogams; so that these innumerable interdependences must have been established since the phænogamic type has become so predominant in respect of both size and kind. Similarly among animals. Though there are many parasites belonging, like the Trematodes, to very low classes, there are many which belong to the Arthropoda, and, being degraded forms of that class, must have come into existence after Arthropods of considerable structure had been evolved. Again, a large part of the animals infested by Epizoa and Entozoa are vertebrates—many of the highest types; and as these are relatively modern all this parasitism must be of late date. So, too, of much commensalism and many mutually-beneficial associations. The reciprocal services of ants and aphides must have originated since the Hymenoptera and Hemiptera became established types, and since the days when certain insects of the ant-type had become social, and since the days when aphides had become degraded members of their order: both dates being relatively recent. And still more recent must have been the commensalism between the ants and the many species of other insects which inhabit their nests.

Leaving out relations of the kinds just named, it seems that down from those between carnivores and their prey to those between lice and their hosts, such relations profit one of the two species concerned and injure the other, and that there the matter ends. But it does not end there; for that multiplication of effects to which people are usually blind, brings about changes which, as hinted above, though injurious to the individual are beneficial to the species, and which, when not beneficial to the species, are often beneficial to the aggregate of species.

Even where animals of one class live by devouring animals of another class, we see, on looking beyond the immediate results, certain remote results that are advantageous. In the first place the process is one by which inferior individuals—the least agile, swift, strong, or sagacious—are picked out and prevented from leaving posterity and lowering the average quality of their kind. At the same time individuals made feeble by injury or old age, are among those to be killed and saved from suffering prolonged pains: the evils of death by disease and starvation being thus limited to the predatory animals, relatively small in their numbers. Meanwhile a check is put on undue multiplication. Where a tract of country has been overrun by rabbits, weasels, thriving on the abundant supply of food, presently become numerous enough to bring the population of rabbits within moderate limits; and by doing this benefit not only all those kinds of plants which are being eaten down, and all those other animals which live on such plants, but also the rabbits themselves; since, increasing beyond the means of subsistence, a large part of them would, if not killed, die of hunger. Between aphides and lady-birds we see a connexion of like nature: great increase of the first yielding abundant food to larvæ of the second, ending after a season or so in swarms of lady-birds, and consequently of their larvæ, whereby the aphides, immensely diminished, cease so greatly to injure various plants and the animals dependent on them. Even minute parasites, by the evils they inflict on one species, profit others: instance the enormous destruction of flies which a microscopic fungus caused a few years ago—a destruction which relieved not only man but all the animals which flies irritate: often so much as to hinder them from feeding. Such instances remind us how numerous are the bonds by which the lives of organisms are tied together.

§ 314f. I have reserved to the last the clearest and most striking illustration of this progressing integration throughout the organic world. I refer to the mutually-beneficial relations established between plants and animals through the agency of flowers and insects.

Everyone nowadays has been made familiar with the process of plant-fertilization, and knows that (leaving out of consideration plants fertilized by wind-borne pollen) the ability to bear seed depends largely on the aid given by bees, butterflies, and moths. The exchange of services has been growing ever more various and complicated during long past periods. We have the acquirement by flowers of bright colours serving to guide these insects to places where honey is to be found; and we have their perfumes, also serving for guidance. Then we have the many different arrangements, often complicated, by which the visiting insects are obliged to carry away pollen and dust with it the stigmas of flowers on which they subsequently settle: thus effecting crossfertilization. Pari passu have gone on insect-developments made possible by these arrangements and furthering them. Especially must be named the modification of certain Hymenoptera into honey-storing bees: the implication being that the entire economy established by these social insects has been sequent on the growth of this system of reciprocal benefits. And then, just instancing the dependence between a particular flower having a long tubular corolla, and a particular moth having an appropriately long proboscis, it suffices to say that innumerable specialities of this general relation everywhere multiply the links by which the vegetal world and the animal world are here connected. That the effects of the connections tell largely on the prosperity of both, is suggested by some instances Mr. Darwin gives, and by a statement recently made in the United States, by Dr. L. O. Howard, that the greater fostering of bees would much increase certain of the crops.

But now observe the broad fact to which these few details concerning plant-fertilization are introductory. All these general and special relations between plants and animals have arisen since the phænogamic type came into existence—have, indeed, arisen since the higher members of that type, the Angiosperms, have appeared; for the Gymnosperms do not play any part in this intercommunion. But so far as we can judge of present results of geologic explorations, there were no Angiosperms during the Eozoic and Paleozoic periods. So that this class of connexions between animals and vegetals must have been established since carboniferous times—a period long, indeed, but far shorter than that which organic evolution at large has occupied.

§ 314g. I have but just touched on some salient parts of a subject, immense in extent and extremely involved, which it would take a volume to set forth adequately. Enough has been said, however, to indicate the truth which it is the purpose of the chapter to bring into view and emphasize— the truth that both of the two great laws of evolution are exemplified in the organic world as a whole, as they are exemplified in every organism, and in all other things.

The reader has long since become familiar with the generalization that while Evolution is a change from the homogeneous to the heterogeneous, it is also a change from the incoherent to the coherent; and this change from the incoherent to the coherent has been above exhibited as going on even throughout that vast assemblage of organisms, plant and animal, which cover the Earth’s surface. In what we are obliged to conceive as the earliest stage, when the most minute types of life alone existed, the aggregate of living things was at once homogeneous and incoherent. In the course of epochs immeasurable in duration, this uniform aggregate of beings has been becoming more multiform. And now we see that instead of forms of life everywhere without the slightest union caused by mutual dependence, there have slowly arisen forms of life among which mutual dependences have entailed vital connexions correspondingly marked. Along with progressing differentiation there has ever been progressing integration. So that we may recognize something like a growing life of the entire aggregate of organisms in addition to the lives of individual organisms—an exchange of services among parts enhancing the life of the whole.

In this final generalization the law of Evolution is manifested under its most transcendental form.