SPECIES AND THEIR ORIGIN 315 



animal may determine which of two will be killed 

 and eaten and which escape ; a slight difference in 

 resistance to low temperature will determine which 

 of two plants will be killed by frost and which sur- 

 vive. This universal phenomenon was termed by 

 Darwin, " The Survival r>f fli^ Fitt^g^" It may be 

 better called, perhaps, the survival of the best adapted, 

 since the criterion of survival, or of " Natural Selec- 

 tion," as Darwin called it, is the degree to which the 

 organism is adapted to its environment. Since " like 

 tends to produce like," Darwin held that the individ- 

 uals that have survived on account of their favor- 

 able variations will tend to reproduce individuals 

 of the same type. But the inorganic environment is 

 no more stable than organic nature. Excluding 

 the titanic changes which, Geology teaches us, 

 have been going on for long periods of time, the 

 minor changes of climate and physical conditions 

 are constantly nullifying the delicate adjustments 

 that have come into existence through the natural 

 selection just described. New criteria for the selec- 

 tion of the survivors in the struggle will become 

 operative, and in consequence a different type will be 

 preserved. It is not even necessary for the environ- 

 mental changes to become profound. As we have 

 seen, in connection with fortuitous variation, free 

 interbreeding tends to maintain a single mode, but 

 if a group of variants should be segregated and 

 prevented from mutual intercrossing, no other 

 factor than chance need be called upon to bring 

 about a divergence of two modes. Darwin's atten- 



