§ 286. Thus that which the general doctrine of evolution leads us to anticipate, we find implied by the facts. The physiological division of labour among parts, can go on only in proportion to the mutual dependence of parts; and the mutual dependence of parts can progress only as fast as there arise structures by which the parts are efficiently combined, and the mutual utilization of their actions made easy.
To say definitely by what process is brought about this co-ordination of functions which accompanies their specialization, is hardly practicable. Direct and indirect equilibration doubtless co-operate in establishing it. We may see, for example, that every increase of fitness for function produced in the aërial part of a plant by light, as well as every increase of fitness for function produced in its imbedded part by the direct action of the moist earth, must conduce to an increased current of the liquid evaporated from the one and supplied by the other—must serve, therefore, to aid the formation of sap-channels in the ways already described; that is—must serve to develop the structures through which mutual aid of the parts is given: the additional differentiation tends immediately to bring about the additional integration. Contrariwise, it is obvious that an interdependence such as we see between the secretion of honey and the fertilization of germs, or between the deposit of albumen in the cotyledons of an embryo-plant and its subsequent striking root, is a kind of integration in the actions of the individual or of the species, which no differentiation has a direct tendency to initiate. Hence we must regard the total results as due to a plexus of influences acting simultaneously on the individual and on the species: some chiefly affecting the one and some chiefly affecting the other.
[Note.—In Nature for June 11, 1896, Dr. Maxwell Masters, in an essay on “Plant Breeding,” names an instructive fact concerning the production of varieties by selection of slightly divergent forms. He says:—
“To the untrained eye, the primordial differences noted are often very slight; even the botanist, unless his attention be specially directed to the matter, fails to see minute differences which are perceptible enough to the raiser or his workmen. Nor must it be thought that these variations, difficult as they are to recognise in the beginning, are unimportant. On the contrary, they are interesting, physiologically, as the potential origin of new species, and very often they are commercially valuable also. These apparently trifling morphological differences are often associated with physiological variations which render some varieties, say of wheat, much better enabled to resist mildew and disease generally than others. Some, again, prove to be better adapted for certain soils or for some climates than others; some are less liable to injury from predatory birds than others, and so on.”
Thus we are shown that, to a much greater degree than might be supposed, minute changes of forms and functions in one part of a plant are correlated with changes of forms and functions throughout it. The interdependence—that is to say, the physiological integration—is very close at the same time that it is very complex.
Here while naming these facts in illustration of physiological integration in plants I name them because they illustrate an important truth bearing upon the general question of heredity which I have dealt with in Appendix G, and to which I now especially draw attention.]
CHAPTER VI.
DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF ANIMALS.
§ 287. What was said respecting the primary physiological differentiation in plants, applies with little beyond change of terms to animals. Among Protozoa, as among Protophyta, the first definite contrast of parts is that between outside and inside. The speck of jelly or sarcode which appears to constitute the simplest animal, proves, on closer examination, to be a mass of substance containing a nucleus—a periplast in the midst of which there is a minute endoplast, consisting of a spherical membrane and its contents.
This parallel, only just traceable among these Rhizopods, which are perpetually changing the distribution of their outer substance, becomes at once marked in those higher Protozoa which have fixed shapes, and maintain constant relations between their surfaces and their environments. Indeed the Rhizopods themselves, on passing into a state of quiescence in which the relations of outer and inner parts are fixed, become encysted: there is formed a hardened outer coat different from the matter which it contains. And what is here a temporary character answering to a temporary definiteness of conditions, is in the Infusoria a constant character, answering to definite conditions that are constant. Each of these minute creatures, though not coated by a distinct membrane, has an outer layer of excreted substance forming a delicate cuticle.