Transformations Due to Environmental Changes 197 



difficulty in this. If the true provocation to alteration 

 comes from the environment and not from natural 

 selection, the evolution of -certain species and the con- 

 stancy of certain others may be explained simply by the 

 respective alteration or stability of their environment. 



The alteration of the environment could be brought 

 about in the case of a given species not only through 

 natural telluric changes but also for instance by the 

 migration of this species toward other regions, or by the 

 immigration of other species into its territory, often 

 also by the overcrowding of its territory by the species 

 itself. 



Emigration as a cause of variation of environment 

 does not need to be illustrated by examples. 



The immigration of other species can immediately 

 induce a very considerable modification of the environ- 

 ment. The immigration of a bird of prey with rapid 

 flight will have as a result that the birds for instance of a 

 certain native species are compelled to fly more rapidly 

 in order to escape it. This repeated greater effort will 

 develop an increase of their swiftness, an increase which 

 would not have been attained, we must believe, by normal 

 daily exercise. For the normal exercise of a given 

 function after the respective organ has once been formed, 

 does not develop it any further but merely causes it to 

 preserve the degree of development already attained. In 

 this way one can readily see also that if the region 

 ravaged by the bird of prey is only one part of the 

 whole territory inhabited by this aboriginal species, one 

 portion only of that species will be forced to become 

 transformed into a swifter variety while the remaining 

 portion can and must remain unaltered. 



The overcrowding of a given territory by a given 



