A VIEW IN PERSPECTIVE. 133 



have been constantly occurring in the past we have 

 abundant proof, and it is certain they always induce 

 changes in living nature. Indeed, the periods of 

 greatest geological disturbances have always been 

 those of most rapid evolution of animals (e. g., the 

 end of the Carboniferous (4) and Cretaceous (8). It 

 would seem, however, that more restricted causes 

 may frequently have been the origin of organic 

 change. A group of animals may migrate into a 

 new territory, and finding there a new field with a 

 less powerful set of competitors, it will be able to 

 expand itself to a much greater extent than in the 

 old home. That this is a result of such migration is 

 well known from the study of animals to-day. Un- 

 doubtedly such local changes have occurred in the 

 past and have constituted one of the factors which 

 have caused the periodic expansion of animals. 



There is, however, another factor probably of even 

 greater importance, but one which cannot be so 

 easily understood or explained. Internal changes in 

 the organism itself will produce new fields of expan- 

 sion. It is well known that animals are constantly 

 varying, now in one direction and now in another. 

 These variations are continuous, and seem to have 

 no direct relation to any change in environment. 

 Of the many variations that appear, some are use- 

 less, and soon disappear. Others are of some value, 

 and will be preserved by natural selection. Now it 

 will occasionally happen that the variations may be 

 of such a character as to make a radical change in 

 the organism and fit it for entirely new conditions. 

 For instance, we may suppose that all during the 



