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. 1 



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- 



1 In one place Von Baer raises the question whether the egg may not be a 

 form common to all the phyla. 



