§ 267. It will be best, for brevity and clearness, to deal with these physiological problems as we dealt with the morphological ones--to carry on the inductive statement and the deductive interpretation hand in hand: so disposing of each general truth before passing to the next. Treating separately vegetal organisms and animal organisms, we will in each kingdom consider:--first, the physiological differentiations and accompanying changes of structure which arise between outer tissues and inner tissues; next, those which arise between different parts of the outer tissues; and, finally, those which arise between different parts of the inner tissues. What little has to be said concerning physiological integration must come last. For though, in tracing up Morphological Evolution, we have to study those processes of integration by which organic aggregates are formed, before studying the differentiations that arise among their parts; we must, contrariwise, in tracing up Physiological Evolution, study the genesis of the different functions before we study the interdependence that eventually arises among them and constitutes physiological unity. CHAPTER II. DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES OF PLANTS. § 268. The simplest plant presents a contrast between its peripheral substance and its central substance. In each protophyte, be it a spherical cell or a branched tube, or such a more-specialized form as a Desmid, a marked unlikeness exists between the limiting layer and that which it limits. These vegetal aggregates of the first order may differ widely from one another in the natures of their outer coats and in the natures of their contents. As in the Palmella-form of one of the lower _Algæ_, there may exist a clothing of jelly; or, as in Diatom, the walls may take the form of silicious valves variously sculptured. The contained matter may be partly or wholly here green, there red, and in other cases brown. But amid all these diversities there is this one uniformity--a strong distinction between the parts in contact with the environment and the parts not in contact with the environment.