The Theory of Evolution 61 
become current when he wrote in 1828. According to Von 
Baer, the more nearly related two animals are, or rather the 
more nearly similar two forms are (since Von Baer did not 
accept the idea of evolution), the more nearly alike is their 
development, and so much longer in their development do 
they follow in the same path. For example two similar 
species of pigeons will follow the same method of develop- 
ment up to almost the last stage of their formation. The 
embryos of these two forms will be practically identical 
until each produces the special characters of its own 
species. On the other hand two animals belonging to 
different families of the same phylum will have only the 
earlier stages in common. Thus, a bird and a mammal 
will have the first stages similar, or identical, and then 
diverge, the mammal adding the higher characters of its 
group. The resemblance is between corresponding em- 
bryonic stages and not between the embryo of the mammal 
and the adult form of a lower group. 
Von Baer was also careful to compare embryos of the 
same phylum with each other, and states explicitly that 
there are no grounds for comparison between embryos of 
different groups.! 
We shall return again to Von Baer’s interpretation and 
then discuss its value from our present point of view. 
Despite the different interpretation that Von Baer gave 
to this doctrine of resemblance the older view of recapitula- 
tion continued to dominate the thoughts of embryologists 
throughout the whole of the nineteenth century. 
Louis Agassiz, in the Lowell Lectures of 1848, proposed 
for the first time the theory that the embryo of higher 
forms resembled not so much lower adult animals living 
at the present time, as those that lived in past times. 
Since Agassiz himself did not accept the theory of evolu- 
1In one place Von Baer raises the question whether the egg may not bea 
form common to all the phyla. 
