252 EVOLUTION OF TO-DAY, 



taneously, a step is taken toward the solution of the 

 problem. If any reason can be found why variations 

 should thus be definite, nearly all of the objections 

 above considered will disappear. To do this, scien- 

 tists have thought it necessary to find some internal 

 forces regulating the organism. That some internal 

 force exists seems to be quite conclusive. It is 

 found that the same circumstances do not always 

 produce the same variations. Some animals vary 

 much under circumstances where others vary very 

 little. Variations of any species are not abso- 

 lutely indefinite, but, while they may be quite 

 diverse, are still found in definite directions ; the 

 horse, for example, has no tendency to produce horns. 

 It seems hardly questionable that there have been 

 periods of rapid modification alternating with those 

 of comparative rest. We have seen that it is neces- 

 sary to assume the existence of some sort of internal 

 factor to explain the homology of serial organs, such 

 as that of the arm and the leg. Something of the 

 kind is needed to explain the independent develop- 

 ment of the same complicated organ in different 

 animals, such as the vertebrate and mollusk eye. 

 Many instances are known of a large number of ani- 

 mals in very diverse circumstances varying in the 

 same direction. All Australian marsupials, for ex- 

 ample, have shown a tendency toward the reduction 

 of the second and third digits, and since they live 

 under very different circumstances this cannot be 

 due wholly to the environment. All such facts as 

 these, and numerous others might be mentioned, 

 which certainly indicate that there is something be- 



