[105] THE EVOLUTION OF THE FINS OF FISHES. 1085 



These data might be used as objections to the conclusions reached in 

 this paper, were it not that we have the most conclusive grounds for 

 believing that the morphological differentiations or metamorphoses, so 

 rapidly passed over during the embryonic stages of many vertebrated 

 organisms, are the transitory expression of characters acquired during 

 the adult life of ancestral series, extending back into long ages past. 

 The embryo has obviously not had time during its brief career of de- 

 velopment to acquire the differentiations which we note in the adult. 

 Many embryonic traits are also the necessary complements of a mode 

 of development which is characteristic of all Vertebrates, namely, their 

 evolution from a spherical, more or less meroblastic egg. We may 

 therefore, I think it probable, look upon almost every character above 

 classical value as acquired by a type during the post-larval history of 

 the individuals constituting its ancestral series; and inasmuch as natural 

 selection cannot be looked upon as an originative force, but only as a 

 conservative law or principle, we are forced to conclude that advan- 

 tageous variations must have arisen as the direct results of the interac- 

 tion and retroaction constantly going on between an organism and its 

 environment. 



