§ 266. The problems of Physiology, in the wide sense above described, are, like the problems of Morphology, to be considered as problems to which answers must be given in terms of incident forces. On the hypothesis of Evolution these specializations of tissues and accompanying concentrations of functions, must, like the specializations of shape in an organism and its component divisions, be due to the actions and reactions which its intercourse with the environment involves; and the task before us is to explain how they are wrought—how they are to be comprehended as results of such actions and reactions.
Or, to define these problems still more specifically:—Those extremely unstable substances composing the protoplasm of which organisms are mainly built, have to be traced through the various modifications in their properties and powers, that are entailed on them by changes of relation to agencies of all kinds. Those organic colloids which pass from liquid to solid and from soluble to insoluble on the slightest molecular disturbance—those albuminoid matters which, as we see in clotted blood or the coagulable lymph poured out on abraded surfaces and causing adhesion between inflamed membranes, assume new forms with the greatest readiness—are to have their metamorphoses studied in connexion with the influences at work. Those compounds which, as we see in the quickly-acquired brownness of a bitten apple or in the dark stains produced by the milky juice of a Dandelion, immediately begin to alter when the surrounding actions alter, are to be everywhere considered as undergoing modifications by modified conditions. Organic bodies, consisting of substances that, as I here purposely remind the reader, are prone beyond all others to change when the incident forces are changed, we must contemplate as in all their parts differently changed in response to the different changes of the incident forces. And then we have to regard the concomitant differentiations of their reactions as being concomitant differentiations of their functions.
Here, as before, we must take into account two classes of factors. We have to bear in mind the inherited results of actions to which antecedent organisms were exposed, and to join with these the results of present actions. Each organism is to be considered as presenting a moving equilibrium of functions, and a correlative arrangement of structures, produced by the aggregate of actions and reactions that have taken place between all ancestral organisms and their environments. The tendency in each organism to repeat this adjusted arrangement of functions and structures, must be regarded as from time to time interfered with by actions to which its inherited equilibrium is not adjusted—actions to which, therefore, its equilibrium has to be re-adjusted. And in studying physiological development we have in all cases to contemplate the progressing compromise between the old and the new, ending in a restored balance or adaptation.
Manifestly our data are so scanty that nothing more than very general and approximate interpretations of this kind are possible. If the hypothesis of Evolution furnishes us with a rude conception of the way in which the more conspicuous and important differentiations of functions have arisen, it is as much as can be expected.
§ 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.
When we remember that this trait is one which these simple living bodies have in common with bodies that are not living—when we remember that each inorganic mass eventually has its outer part more or less differentiated from its inner part, here by oxidation, there by drying, and elsewhere by the actions of light, of moisture, of frost; we can scarcely resist the conclusion that, in the one case as in the other, the contrast is due to the unlike actions to which the parts are subject. Given an originally-homogeneous portion of protoplasm, and it follows from the general laws of Evolution (First Principles, §§ 149–155), first, that it must lose its homogeneity, and, second, that the leading dissimilarities must arise between the parts most-dissimilarly conditioned—that is, between the outside and the inside. The exterior must bear amounts and kinds of force unlike the amounts and kinds which the interior bears; and from the persistence of force it follows inevitably that unlike effects must be wrought on them—they must be differentiated.
What is the limit towards which the differentiation tends? We have seen that the re-distribution of matter and motion whence, under certain conditions, evolution results, can never cease until equilibrium is reached—proximately a moving equilibrium, and finally a complete equilibrium (First Principles, §§ 170–175). Hence, the differentiation must go on until it establishes such differences in the parts as shall balance the differences in the forces acting on them. When dealing with equilibration in general, we saw that this process is what is called adaptation (First Principles, § 173); and, in this work, we saw that by it the totality of functions of an organism is brought into correspondence with the totality of actions affecting it ([§§ 159–163]). Manifestly in this case, as in all others, either death or adjustment must eventually result. A force falling on one of these minute aggregates of protoplasm, must expend itself in working its equivalent of change. If this force is such that in expending itself it disturbs beyond rectification the balance of the organic processes, then the aggregate is disintegrated or decomposed. But if it does not overthrow that moving equilibrium constituting the life of the aggregate, then the aggregate continues in that modified form produced by the expenditure of the force. Thus, by direct equilibration, continually furthered by indirect equilibration, there must arise this distinction between the outer part adapted to meet outer forces, and the inner part adapted to meet inner forces. And their respective actions, as thus meeting outer and inner forces, must be what we call their respective functions.
§ 269. Aggregates of the second order exhibit parallel traits, admitting of parallel interpretations. Integrated masses of cells or units homologous with protophytes, habitually show us contrasts between the characters of the superficial tissues and the central tissues. Such among these aggregates of the second order as have their component units arranged into threads or laminæ, single or double, cannot, of course, furnish contrasts of this kind; for all their units are as much external as internal. We must turn to the more or less massive forms.