IN ORGANIC EVOLUTION 79 



very different matter in different parts of the 

 world of life. The competition with like forms, 

 the struggle with unlike forms, the compromise 

 with hard conditions of life, all these change at 

 every angle, and the character of Natural Selec- 

 tion changes with them. The individual animals 

 are mobile, as plants are not. They shift about 

 and occupy their range more perfectly, while in 

 plants their pollen and their seed have great ad- 

 vantages over animals. With a plant everything 

 depends on where its seed is dropped. Where 

 an animal is born or hatched is a matter of rela- 

 tive indifference. With plants, some seed is sure 

 to reach almost every available point within the 

 range of the species, while the vast majority of 

 seeds never have a chance to germinate. All 

 these, and every other point of difference, be- 

 tween one group of organisms and another, affect 

 the nature and relative value of the different fac- 

 tors in divergence. They tend also to obscure 

 the laws of distribution. But no law is invah- 

 dated by the occurrence of exceptions which come 

 under some other rule or law. 



The obvious immediate factor in the spUtting 

 apart of races or species is, therefore, in all 

 groups, that of isolation. Behind this hes the 

 primal factor of variation, continuous or discon- 

 tinuous. Fluctuation, saltation or mutation, all 

 these are one for the purposes of our present dis- 

 cussion. With these come the factor of heredity 

 and the factor of selection, to which we must 



