Transformations Due to Environmental Changes 197 
difficulty in this. If tie 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 
