58 THE DOCTRINE OF DESCENT. 
rapidly passes through. Although mammals are 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, zz 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 differen- 
