714 LIFE : OUTLINES OF GENERAL BIOLOGY 



different niches of opportunity, and even by more or less psycho- 

 logical antipathies, such as hares have for rabbits, and some amorous 

 pigeons for other races. 



What is most difficult to understand in the origin of a new species 

 is the fact that along with the new features of structure and of habit 

 there is usually associated some degree of sterility with the original 

 stock and with other species. This may be in part correlated with a 

 biochemical incompatibility, and also with a change in the number 

 of the chromosomes which makes cross-breeding impossible or 

 difficult. 



Finally, there is the factor on which Darwin laid most empha.sis, 

 that the new departure will succeed in establishing itself in direct 

 proportion to the survival value of its peculiar features. The in- 

 cipient variety may find itself in a slightly different environment, 

 to which it is better suited than was the original stock. Thus it may 

 evolve alongside of the old species, though in many cases, no doubt, 

 it becomes a successful supplanter. 



