DARWIN AND PALEONTOLOGY 241 



fore the actual occurrence of either. It is only 

 through some such restraining or limiting law of 

 hereditary kinship and potential that we can 

 explain the marvelous uniformity which exists, 

 for example, in the fundamental structure of the 

 grinding teeth of the manmialia. 



Hypothetical interpretation. This is not to be 

 understood as an internal perfecting tendency. 

 Such an interpretation may be abandoned at once 

 so far as it applies to the independence of these 

 hereditary origins from other causes. While a 

 phenomenon of heredity, the definite origin of 

 adaptive structures is, in a manner which is en- 

 tirely incomprehensible to us, related to ontogeny 

 and environment, to new habits and new condi- 

 tions of life, there is a marvelous nexus be- 

 tween the internal and the external. Thus, for 

 example, if a monkey or lemur begins to imitate 

 the habits of the hoofed animals by becoming 

 herbivorous, it also begins to acquire the dental 

 cusps of the herbivorous hoofed animals in ap- 

 proximately the same order. This principle of 

 the relation of the internal and the external hes 

 at the basis bf many of the phenomena of par- 

 allelism. For example, some of the Eocene 

 monkeys so closely parallel the Eocene hoofed 

 mammals in dental structure that they were first 

 placed in the same taxonomic order. Some un- 

 known relation between external and internal 

 causes appears to evoke the potential of similar 

 development. Thus there appear to be accelera- 



