ilarity becomes greater. And what is true of the individual animal or plant is equally true of the whole organic world. Baer's law would lead us to suppose that the organic world has developed like the individual; that, starting from homogeneity, it has resulted in heterogeneity. In the early stages of their existence, all organisms are alike in most of their characters; somewhat later their structure resembles that found at the corresponding period in a smaller group; at each subsequent stage the organism acquires traits which distinguish the developing embryo from one after another of the groups which before it resembled; till finally the class of organisms which it resembles includes only the species to which the embryo belongs. Thus, in the process of differentiation, the embryo first acquires those characters which determine the sub-kingdom to which it belongs, then the class, then the genus, finally the species. In the series of organisms we should thus find a succession of states like those which constitute the history of the individual, with this difference, that in the individual we can make out the link which connects the primitive homogeneity with the final heterogeneity, while in the series of organisms all we can do is to connect, with a considerable degree of probability, the hypothetical starting-point with the positive goal.
Side by side with heterogeneity and distinction of parts in the structure, we have a correlative result of this same segregative operation, viz., differentiation, which tends to produce heterogeneity and distinction of functions. The expenditure of the force that is stored up in the shape of materials takes place through the parts of the organism, however little heterogeneous these may be supposed to be, and this force is in fact for the parts an incident force which, by the law of the multiplication of effects, must break up in the process of differentiation, when applied to heterogeneous parts. The functions are simply the variously-modified forms assumed by the forces disengaged by the organism as they traverse specialized parts; and, the more diversified the organs, the more diversified are the functions they manifest. Of these some may be denominated static, inasmuch as they serve only to withstand external forces by equilibrating them; such, for example, are the functions of the woody axis in plants and of the skeleton in the vertebrata; others may be called dynamic, as producing motion and giving it direction; such, for example, are the functions of the circulatory apparatus and its belongings in both kingdoms of the organic world, and of the muscular apparatus in animals.
Like structure, function obeys the law of evolution; it proceeds from the homogeneous, the undefined, the incoherent, to the heterogeneous, the definite, the coherent. Like structure, function proceeds from the simple to the composite, from the general to the special. An important corollary results from this law—one that settles the dispute which has so long divided physiologists upon the question as to which precedes the other, function or structure. If the starting-point
