CHAPTER II 



THE INFLUENCE OF EXTERNAL CONDITIONS IN CAUSING 

 CHANGES IN THE STRUCTURE OF ANIMALS 



Animals and plants are so constituted that one of their chief 

 characteristics is that they respond to their natural environment 

 in such a way as to insure their continued existence. These 

 responses are in the main physiological, and therefore in large 

 part transitory; but in some cases the response is structural, 

 involving a temporary or even a permanent change in form or 

 structure that persists at least so long as the external condi- 

 tions that called it forth remain. The question arises whether 

 these changes, directly induced by the environment, may not 

 give origin to the more fixed characters that have become the 

 permanent inheritance of each species. May not these have 

 been in the first instance adaptive responses to the environ- 

 ment? This leads to the further question of the origin of all 

 the characters of the species, whether adaptive or non-adaptive. 

 In this and in the following chapters the different sides of this 

 question will be considered. 



ADAPTIVE RESPONSES 



External conditions sometimes cause adaptive structural 

 changes in organisms. We are familiar with some effects of 

 this sort in our own bodies. Pressure on the skin, if long con- 

 tinued, causes it to become thicker and more capable of resist- 

 ing the injurious effects of pressure. Sunlight tans the skin 

 and protects it from "burning." It is said that cold causes the 

 fur of some mammals to become thicker, and this change better 

 protects them against the cold. Conversely, it is said that horses 

 and dogs lose their hair to some extent in very warm climates. 



