ON THE GENETIC VIEW OP NATURE. 305 



Animals differ according to the type of organisation to 

 which they belong. Thus the " embryo of the vertebrate 

 animal is from the very first a vertebrate animal, and at 

 no time agrees with an invertebrate animal." 1 Having, 

 however, once fixed the existence of special organic 

 forms, he asks whether within the limits of such form 

 no law can be discovered to formulate the development of 

 the individual. He believes there can, 2 and he proceeds 

 to explain it in terms which for the most part might 

 appear unaltered in the most modern work on evolution. 

 He states that the more special type is developed from 

 the more general, " and that the more different two 

 animal forms are, so much the further back must their 

 development be traced to find them similar." Indeed he 

 thinks it probable that " in the condition of the actual 

 germ all embryos which are developed from true ova 

 agree," and he anticipates the cellular theory of Schwann, 

 established by observation ten years later, by suggesting 

 that the simple vesicle is the common fundamental form 

 " from which all animals are developed, not only ideally 

 but actually and historically." 3 In further examining 

 the process of development, von Baer introduces the 

 very suggestive term 4 differentiation. " The higher and 

 lower development of the animal coincides perfectly with 

 that histological and morphological differentiation which 

 gradually arises in the course of the development of 

 the individual." 5 Development, in fact, is the estab- 



1 Loc. cit., p. 220 ; transl., p. 210. 



2 Ibid., p. 221. 



3 Loc. cit., p. 224 ; transl., p. 213. 

 On this anticipation see, however, 

 von Baer's later explanation in 



4 The German term is " Son- 

 derung," which Huxley renders by 

 the English term " Differentiation." 



6 Loc. cit., p. 229, 230; transl., 

 p. 219. 



' lieden, &c.,' vol. ii. p. 250. 



VOL. II. U 



