430 THEORIES OF EVOLUTION 



dowed with a peculiar force, which not only impels the or- 

 ganism to evolve in a definite direction, but also adapts it 

 to its surroundings. A less radical position is taken by 

 certain other scientists who maintain that at certain 

 definite periods the structure of an animal or plant de- 

 termines the general trend of evolution, after which the 

 surrounding conditions mold the species to fit the environ- 

 ment. Several other theories, more or less Lamarckian 

 in principle, have also been proposed as all-sufficient ex- 

 planations of evolution, or as subsidiary theories to ac- 

 count for the orderly and seemingly predetermined course 

 of species development. 



At the present time the majority of biologists are agreed 

 that heritable variations are to a certain extent limited, 

 the range being determined by the structure of the organ- 

 ism concerned. As Huxley wrote, " whales never produce 

 feathers, nor birds whalebone." Nevertheless, each 

 species of whale, bird, or other species of animal or plant 

 may vary in any one of thousands of different directions. 

 Provided the mutation harmonizes with the environment, 

 evolution may accordingly take place along any one of 

 a multitude of different lines, and not along the limited 

 number demanded by the usual theory of Orthogenesis. 



