§ 291. The outer tissues of animals, originally homogeneous over their whole surfaces, pass into a heterogeneity which fits their respective parts to their respective conditions. So numerous and varied are the implied differentiations, that it is impracticable here to deal with them all even in outline. To trace them up through classes of animals of increasing degrees of aggregation, would carry us into undue detail.
Did space permit, it would be possible to point out among the Protozoa, various cases analogous to that of the Arcella; which may be described as like a microscopic Limpet, having a sarcode body of which the upper surface has become horny, while the lower surface with its protruding pseudopodia, retains the primitive jelly-like character. That differentiations of this kind have been gradually established among these minute creatures through the unlike relations of their parts to the environment, is an inference supported by a form which, while the rest of the body has a scarcely distinguishable coating, “agrees with Arcella and Difflugia in having the pseudopodia protrusible from one extremity only of the body.”
Many parallel specializations of surface among aggregates of the second order might be instanced from the Cœlenterata. In the Hydra, the ectoderm presents over its whole area no conspicuous unlikenesses; but there usually exist in the hydroid polypes of superior types, decided contrasts between the higher and lower parts. While the higher parts retain their original characters, the lower parts excrete hard outer layers yielding support and protection. Various stages of the differentiation might be followed. “In Hydractinia,” says Prof. Green, this horny layer “becomes elevated at intervals to form numerous rough processes or spines, while over the general surface of the ectoderm its presence is almost imperceptible.” In other types, as in Cordylophora, it spreads part way up the animal’s sides, ending indefinitely. In Bimeria it “extends itself so as to enclose the entire body of each polypite, leaving bare only the mouth and tips of the tentacles.” While in Campanularia it has become a partially-detached outer cell, into which the creature can retract its exposed parts.
But it is as needless as it would be wearisome to trace through the several sub-kingdoms the rise of these multiform contrasts, with the view of seeking interpretations of them. It will suffice if we take a few groups of the illustrations furnished by the higher animals.
§ 292. We may begin with those modifications of surface which subserve respiration. Though we ordinarily think of respiration as the quite special function of a quite special organ, yet originally it is not so. Little-developed animals part with their carbonic acid and absorb oxygen, through the general surface of the body. Even in the lower types of the higher classes, the general surface of the body aids largely in aërating the blood; and the parts which discharge the greater part of this function are substantially nothing more than slightly altered and extended portions of the skin.
Such differentiations, marked in various degrees, are to be seen among Mollusca. In the Pteropoda the only modification which appears to facilitate respiration, is the minute vascularity of one part of the skin. Higher types possess special skin-developments. The Doris has appendages developed into elaborately-branched forms—small trees of blood-vessels covered by slightly-changed dermal tissues. And these arborescent branchiæ are gathered together into a single cluster. Thus there is evidence that large external respiratory organs have arisen by degrees from simple skin: as, indeed, they do arise during the development of each individual having them. Just as gradually as in the embryo a simple bud on the integument, with its contained vascular loop, passes by secondary buddings into a tree-like growth penetrated everywhere by dividing and subdividing blood-vessels; so gradually has there probably proceeded the differentiation which has turned part of the outer surface into an organ for excreting carbonic acid and absorbing oxygen.
Certain inferior vertebrate animals present us with a like metamorphosis of tissues. These are the Amphibia. The branchiæ here developed from the skin, are covered with cellular epidermis, not much thinner than that covering the rest of the body. Like it they have their surfaces speckled with pigment-cells; and are not even conspicuous by their extra vascularity—where they are temporary at least. They facilitate the exchange of gases in scarcely any other way than by affording a larger area of contact with the water, and interposing a rather thinner layer of tissue between the water and the blood-vessels. Those very simple branchiæ of the larval Amphibia that have them but for a short time, graduate into the more complex ones of those that have them for a long time or permanently; showing, as before, the small stages by which this heterogeneity of surface accompanying heterogeneity of function may arise.
In what way are such differentiations established? Mainly, no doubt, by natural selection; but also to some degree, I think, by the inheritance of direct adaptations. That a portion of the integument at which aëration is favoured by local conditions, should thereby be led to grow into a larger surface of aëration, appears improbable. Survival of those individuals which happen to have this portion of the integument somewhat more-developed, seems here the only likely cause.
§ 293. Among the conspicuous modifications by which the originally-uniform outer layer is rendered multiform, are the protective structures. Let us look first at the few cases in which the formation of these is ascribable mainly to direct equilibration.
Already reference has been more than once made to those thickenings that occur where the skin is exposed to unusual pressure and friction. Are these adaptations inheritable? and may they, by accumulation through many generations, produce permanent dermal structures fitted to permanent or frequently-recurring stress? Take, for instance, the callosities on the knuckles of the Gorilla, which are adapted to its habit of partially supporting itself on its closed hands when moving along the ground. Shall we suppose that these defensive thickenings are produced afresh in each individual by the direct actions; or that they are inherited modifications caused by such direct actions; or that they are wholly due to the natural selection of spontaneous variations? The last supposition does not seem a probable one. Such thickenings, if spontaneous, would be no more likely to occur on the knuckles than on any other of the hundred equal areas forming the skin-surface at large; and the chances against their simultaneous occurrence on all eight knuckles would be incalculable. Moreover, the implication would be that those slight extra thicknesses of skin on the knuckles, with which we must suppose the selection to have commenced, were so advantageous as to cause survivals of the individuals having them, in presence of other superiorities possessed by other individuals. Then that survivals so caused, if they ever occurred at all, should have occurred with the frequency requisite to establish and increase the variation, is hardly supposable. And if we reject, as also unlikely, the reproduction of these callosities de novo in each individual (for this would imply that after a thousand generations each young gorilla began with knuckles having skin no thicker than elsewhere), there remains only the inference that they have arisen by the transmission and accumulation of functional adaptations. Another case which seems interpretable only in an analogous way, is that of the spurs that are developed on the wings of certain birds—on those of the Chaja screamer for example. These are weapons of offence and defence. It is a familiar fact that some birds strike with their wings, often giving severe blows; and in the birds named, the blows are made more formidable by the horny, dagger-shaped growths standing out from those points on the wings which deliver them. Are these spurs directly or indirectly adaptive? To conclude that natural selection of spontaneous variations has caused them, is to conclude that, without any local stimulus, thickenings of the skin occurred symmetrically on the two wings at the places required; that such thickenings, so localized, happened to arise in birds given to using their wings in fight; and that on their first appearance the thickenings were decided enough to give appreciable advantages to the individuals distinguished by them—advantages in bearing the reactions of the blows if not in inflicting the blows. But to conclude this is, I think, to conclude against probability. Contrariwise, if we assume that the thickening of the epidermis produced by habitual rough usage is inheritable, the development of these structures presents no difficulty. The points of impact would become indurated in wings used for striking with unusual frequency. The callosities of surface thus generated, rendering the parts less sensitive, would enable the bird in which they arose to give, without injury to itself, more violent blows and a greater number of them: so, in some cases, helping it to conquer and multiply. Among its descendants, inheriting the modification and the accompanying habit, the thickening would be further increased in the same way: survival of the fittest tending ever to accelerate the process. Presently the horny nodes so formed, hitherto defensive only in their effects, would, by their prominence, become offensive—would make the blows given more hurtful. And now natural selection, aiding more actively, would mould the nodes into spurs: the individuals in which the nodes were most pointed would be apt to survive and propagate; and the pointedness generation after generation thus increased, would end in the well-adapted shape we see.