GRADATION AMONG ANIMALS. 91 



pass through stages of development in which they 

 transiently resemble lower orders of the same 

 class. This gradation of growth corresponds to 

 the gradation of rank in adult animals, as estab- 

 lished upon comparative complication of struct- 

 ure. For instance, according to their structural 

 character, all naturalists have placed Fishes low- 

 est in the scale of Vertebrates. Now all the 

 higher Vertebrates have a Fish-like character at 

 first, and pass successively through phases in 

 which they vaguely resemble other lower forms 

 of the same type before they assume their own 

 characteristic form ; and this is equally true of 

 the other great divisions, so that the history of 

 the individual is, in some sort, the history of its 

 type. 



There is still another aspect of this question, 

 that of time. If neither the gradation of 

 structural rank among adult animals nor the 

 gradation of growth in their embryological de- 

 velopment gives us any evidence of a transition 

 between types, does not the sequence of animals 

 in their successive introduction upon the globe 

 afford any proof of such a connection ? In this 

 relation, I must briefly allude to the succession 

 of geological formations that compose the crust 

 of our globe. The limits of this article will not 

 allow me to enter at any length into the geologi- 

 cal details connected with this question; but I 



