MODERN PHILOSOPHICAL BIOLOGY. 717 



ilarity becoraee 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 tlie organic world has developed like the indi- 

 vidual ; tliat, starting from homogeneity, it has resulted in heteroge- 

 neity. 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 diffei'entiation, the embryo first acquires those characters 

 which determine the suh-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 tliis 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 M'hich, by 

 the law of the multiplication of effects, must break up in the process 

 of differentiation, when applied to heterogeneous parts. The functions 

 are simjDly the variously-modified forms assumed by the forces disen- 

 gaged 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 king- 

 doms 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 hetero- 

 geneous, 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 dis- 

 pute which has so long divided physiologists \ipon the question as to 

 which precedes the other, function or structure. If the starting-point 



