§ 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.