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 
