Even aggregates of the first order illustrate this truth. Such so-called unicellular plants as those delineated in Figs. 4, 5, and 6, show us, on comparing the contents of their fixed ends and their loose ends, that different processes are going on in them, and that different functions are being performed by their limiting membranes. _Caulerpa prolifera_, which “consists of a little creeping stem with roots below and leaves above,” originating “in the growth of a body which may be regarded as an individual cell,” supplies a still-better example. Among aggregates of the second order a like connexion is displayed in more various modes but with equal consistency. As before, the Puff-ball served to exemplify the primary physiological differentiation of outer parts from inner parts; so, here, it supplies a simple illustration of the way in which the differentiated outer part is re-differentiated, in correspondence with the chief contrast in its relations to the environment. The only marked unlikeness which the cortical layer of the Puff-ball presents, is that between the portion next the ground and the opposite portion. The better-developed _Fungi_ exhibit a more decided heterogeneity of parallel kind. Such incrusting _Algæ_ as _Ralfsia verrucosa_ furnish a kindred contrast; and in the higher _Algæ_ it is uniformly repeated. Phænogams display this physiological differentiation very conspicuously. That earth and air are unlike portions of the environment, subjecting roots and leaves to unlike physical forces, which entail on them unlike reactions, and that the unlike functions and structures of their respective surfaces are fitted to these unlike physical forces, are familiar facts which it would be needless here to name, were it not that they must be counted as coming within a wider group of facts.