Much more than this is obvious. The study of functions, considered from our present point of view as arising by Evolution, must be carried on _mainly_ by the study of the correlative structures. Doubtless, by experimenting on the organisms which are growing and moving around us, we may ascertain the connexions existing among certain of their actions, while we have little or no knowledge of the special parts concerned in those actions. In a living animal that can be conveniently kept under observation, we may learn the way in which conspicuous functions vary together--how the rate of a man’s pulse increases with the amount of muscular exertion he is undergoing; or how a horse’s rapidity of breathing is in part dependent on his speed. But though observations of this order are indispensable--though by accumulation and comparison of such observations we learn which parts perform which functions--though such observations, prosecuted so as to disclose the actions of all parts under all circumstances, constitute, when properly generalized and co-ordinated, what is commonly understood as Physiology; yet such observations help us but a little way towards learning how functions came to be established and specialized. We have next to no power of tracing up the genesis of a function considered purely as a function--no opportunity of observing the progressively-increasing quantities of a given action that have arisen in any order of organisms. In nearly all cases we are able only to show the greater growth of the part which we have found performs the action, and to infer that greater action of the part has accompanied greater growth of it. The tracing out of Physiological Development, then, becomes substantially a tracing out of the development of the organs by which the functions are known to be discharged--the differentiation and integration of the functions being presumed to have progressed hand in hand with the differentiation and integration of the organs. Between the inquiry pursued in Part IV, and the inquiry to be pursued in this Part, the contrast is that, in the first place, facts of structure are now to be used to interpret facts of function, instead of conversely; and, in the second place, the facts of structure to be so used are not those of conspicuous shape so much as those of minute texture and chemical composition. § 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.