58 THE DOCTRINE OF DESCENT. 



passes through. Although mammals are never actual 

 fish, there is much that is fish-like in the embryonic 

 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 grad- 

 ually perfected by progressive histological and morpho- 

 logical 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 unfounded hypothesis), the mammal must be con- 

 sidered as a more highly developed fish; and, in that 

 case, it is quite logical to say that the embryo of the verte- 

 brate 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 capa- 

 ble of satisfying us. I think not. It is by no means a 

 merely histological and morphological differentiation 



