ADAPTATION TO ENVIRONMENT 253 



Responses of Mammal Characters to Changing 

 Environment 



Buffon was the first to observe the direct responses of mam- 

 mals to their environment and naturally supposed that en- 

 vironment was the cause of animal modification, chiefly in 

 adaptation to changes of climate. It did not occur to him 

 to inquire whether these modifications were heritable or not, 

 any more than it did to Lamarck. 



It is now generally believed that these reactions are for 

 the most part modifications of the body cells and body chro- 

 matin only, which give rise to what may be known as environ- 

 mental species, as distinguished from true chromatin species 

 which are founded upon new or altered hereditary characters. 

 Of the former order are many geographic varieties and doubtless 

 many geographic species. These visible species of body cell 

 characters are quite distinct from the invisible species of 

 heredity-chromatin characters. Both occur in nature. 



Geologic and secular changes of environment have preceded 

 many of the most profound changes in the evolution of the 

 mammals, which interlock and counteract with their physical 

 and life environments quite as closely as do the reptiles, am- 

 phibians, and fishes; yet a very large part of mammalian evo- 

 lution has proceeded and is proceeding quite independently of 

 change of environment. Thus environment holds its rank as 

 one of the four complexes of the causes of evolution instead of 

 being the cause par excellence as it was regarded in the brilliant 

 speculations of Buffon. 



The interlocking of mammals with their life environment is 

 extremely close, namely, with Bacteria, Protozoa, Insecta, and 

 many other kinds of Invertebrata, with other Vertebrata, as 

 well as with the constantly evolving food supply of the plant 



