58 THE DOCTRINE OF DESCENT. 



rapidly passes through. Although mammals arc never 

 actual fish, there is much that is fish-like in the em- 

 bryonic phases of their organs ; the embryonic fissures 

 in the thorax correspond with the germinal branchial 

 fissures; the formation of the brain may be traced to 

 the complete brain of the lampreys and the sharks, &c. 



In order to refute the doctrine that the embryo passes 

 through the whole animal kingdom, Von Baer was con- 

 tent to prove that it never changes from one type to 

 another. He repudiated the other, and more probable 

 part of this theory, that is, that, at least within the types, 

 the higher groups, in their embryonic phases, repeated 

 the permanent forms of the lower ones, by terming it a 

 question of mere analogies. The embryo, as it is gradually 

 perfected by progressive histological and morphological 

 differentiation, necessarily accords, in this respect, with 

 less developed animals in proportion to its youth. " It 

 is, therefore, very natural that the embryo of the mammal 

 should be more like that of the fish, than the embryo of 

 the fish is like the mammal. Now, if the fish be regarded 

 merely as a less perfect mammal (and this is an un- 

 founded hypothesis), the mammal must be considered 

 as a more highly developed fish ; and, in that case, it is 

 quite logical to say that the embryo of the vertebrate 

 animal is originally a fish." *" 



We have been somewhat faithless to our intention of 

 confining ourselves in this chapter to facts only. The 

 facts are too apt to provoke reflections, and we have, 

 moreover, repeated these reflections merely as historical 

 facts ; we must now inquire whether they are really 

 capable of satisfying us. I think not. It is by no 

 means a merely histological and morphological diffcrcn- 



