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. § 288. The early establishment of this primary contrast of tissues answering to this primary contrast of conditions, is no less conspicuous in aggregates of the second order. The feebly-integrated units of a Sponge, with individualities so little merged in that of the whole they form that most of them still retain their separate activities, nevertheless show us, in the unlikeness that arises between the outermost layer and the contained mass, the effect of converse with unlike conditions. This outermost layer is composed of units somewhat flattened and united into a continuous membrane--a kind of rudimentary skin.