IN MODERN SCIENCE. 



65 



series of species, each in its time distinct and 

 composed of individuals each going regularly 

 through a genetic circle of its own. 



The latter is a process not open to observa- 

 tion within the time at our command — purely 

 hypothetical, therefore, and of which the possi- 

 bility remains to be proved ; while the causes 

 on which it must depend are necessarily alto- 

 gether different from those at work in onto- 

 genesis, and the conditions of a long series of 

 different kinds of animals, each perfect in its 

 kind, are equally dissimilar from those of an 

 animal passing through the regular stages from 

 infancy to maturity. The similarity, in sortie 

 important respects, of ontogenesis to phylo- 

 genesis was inevitable, provided that animals 

 were to be of different grades of complexity, 

 since the development of the individual must 

 necessarily be from a more simple to a more 

 complex condition. On any hypothesis, the 

 parallelism between embryological facts and 

 the history of animals in geological time affords 

 many interesting and important coincidences. 

 Yet it is perfectly obvious that the causes and 

 the conditions of these two successions cannot 

 have been the same. Further, when we con- 

 sider that the embryo- il. which develops into 



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