202 DEVELOPMENT AND HEREDITY. 



of convertible energy cannot maintain the complete 

 development which characterised its ancestors, 

 proves that the formative forces causing development 

 do not all reside in the organism. Some of them 

 must come from the outside. A cursory view of 

 the animal kingdom shows that those races which 

 have been subjected to the greatest variety of action 

 of external influences have attained the greatest 

 complexity of development. 



Wherever any force is withdrawn that has always 

 played a part in the development of any organ, that 

 organ will be changed in its development and will be 

 less perfect. Wherever many or all of the forces of 

 the environment are weakened, we may expect an 

 arrest of development or a case of atavism. When- 

 ever there is a general change in the forces of the 

 environment, we may expect a general change to 

 gradually ensue in the race of organisms. This 

 must, however, be limited in two ways. First, the 

 change cannot exceed certain limits without destroy- 

 ing the species. These limits of change are nar- 

 rower according as the species is more accurately 

 adapted to its environment, i.e. the more its life 

 depends upon minute details. This fact precludes 

 the survival of species under extreme changes, and 

 the changes under which a species can survive are 

 generally so slight that the effect of the change can- 



