338 GEOLOGICAL SUCCESSION. Chap. X. 



species of the two countries could not have foreseen 

 this result. 



Agassiz insists that ancient animals resemble to a 

 certain extent the embryos of recent animals of the 

 same classes ; or that the geological succession of 

 extinct forms is in some degree parallel to the embryo- 

 logical development of recent forms. I must follow 

 Pictet and Huxley in thinking that the truth of this 

 doctrine is very far from proved. Yet I fully expect to 

 see it hereafter confirmed, at least in regard to subordi- 

 nate groups, which have branched off from each other 

 within comparatively recent times. For this doctrine 

 of Agassiz accords well with the theory of natural 

 selection. In a future chapter I shall attempt to show 

 that the adult differs from its embryo, owing to varia- 

 tions supervening at a not early age, and being inhe- 

 rited at a corresponding age. This process, whilst it 

 leaves the embryo almost unaltered, continually adds, in 

 the course of successive generations, more and more 

 difference to the adult. 



Thus the embryo comes to be left as a sort of picture, 

 preserved by nature, of the ancient and less modified 

 condition of each animal. This view may be true, and 

 yet it may never be capable of full proof. Seeing, for 

 instance, that the oldest known mammals, reptiles, and 

 fish strictly belong to their own proper classes, though 

 some of these old forms are in a slight degree less dis- 

 tinct from each other than are the typical members of 

 the same groups at the present day, it would be vain to 

 look for animals having the common embryological 

 character of the Yertebrata, until beds far beneath the 

 lowest Silurian strata are discovered — a discovery of 

 which the chance is very small. 



On the Succession of the same Types within the same 



