THEORY OF EVOLUTION 15 



later disappear before the adult stage is 

 reached. We find, moreover, that the young 

 sometimes resemble in a most striking way the 

 adult stage of groups that we place lower in the 

 scale of evolution. 



Many years before Darwin advanced his 

 theory of evolution through natural selection, 

 the resemblance of the young of higher ani- 

 mals to the adults of lower animals had at- 

 tracted the attention of zoologists and various 

 views, often very naive, had been advanced 

 to account for the resemblance. Among 

 these speculations there was one practically 

 identical with that adopted by Darwin and the 

 post-Darwinians, namely that the higher ani- 

 mals repeat in their development the adult 

 stages of lower animals. Later this view be- 

 came one of the cornerstones of the theory of 

 organic evolution. It reached its climax in the 

 writings of Haeckel, and I think I may add 

 without exaggeration that for twenty-five years 

 it furnished the chief inspiration of the school 

 of descriptive embryology. Today it is taught 

 in practically all textbooks of biology. Haeckel 

 called this interpretation the Biogenetic Law. 



