74 ' Evolution and Adaptation 



comparable with that of the embryo of amphioxus, and not 

 with the persistent notochord in the adult amphioxus. Here 

 also it is of the first importance to find that the notochord 

 appears both in the embryo bird and in amphioxus at the very 

 beginning of the development. The embryo bird is not fish- 

 like except in so far as there are certain organs in the embryo 

 fish that are retained in the adult form. The embryo bird 

 bears the same relation to the embryo fish that the early 

 segmentation stages of the mollusk bear to the early seg- 

 mentation stages of the annelid. There are certain obvious 

 resemblances between this view and that of Von Baer, but 

 there are also some fundamental differences between the two 

 conceptions. 



Von Baer thought that within each group the embryonic 

 development is the same up to a certain point. He supposed 

 that the characters of the group are the first to appear, then 

 those of the order, class, family, genus, and, finally, of the 

 species. He supposed that two similar species would follow 

 the same method of development until the very last stage was 

 reached, when each would then add the final touches that 

 give the individual its specific character. We may call this 

 the theory of embryonic parallelism. Here there is an impor- 

 tant difference between my view and that of Von Baer, for I 

 should not expect to find the two embryos of any two species 

 identical at any stage of their development, but at most there 

 might exist a close resemblance between them. 



Von Baer's statement appears to be erroneous from a mod- 

 ern point of view in the following respects. We know that in 

 certain large groups some forms develop in a very different way 

 from that followed by other members of the group, as shown 

 by the cephalopods, for instance, in the group of mollusks. 

 Again, it is entirely arbitrary to assume that the group- 

 characters are the first to appear, and then successively 

 those of the order, family, genus, species. Finally, as has 

 been said above, we do not find the early embryos of a 



