342 Darwin, and after Darwin. 



brated animals have risen into vertebrated ? Or why 

 should not all monkeys have become men ? 



The answers are manifold. In the first place, it 

 by no means follows that because an advance in 

 organization has proved itself of benefit in the case 

 of one form of life, therefore any or every other 

 form would have been similarly benefited by a 

 similar advance. The business of natural selection 

 is to bring this and that form of life into the closest 

 harmony with its environment that all the conditions 

 of the case permit. Sometimes it will happen that 

 the harmony will admit of being improved by an 

 improvement of organization. But just as often it 

 will happen that it will be best secured by leaving 

 matters as they are. If, therefore, an organism has 

 already been brought into a tolerably full degree of 

 harmony with its environment, natural selection will 

 not try to change it so long as the environment 

 remains unchanged ; and this, no doubt, is the reason 

 why some species have survived through enormous 

 periods of geological time without having undergone 

 any change. Again, as we saw in a previous chapter, 

 there are yet other cases where, on account of some 

 change in the environment or even in the habits of the 

 organisms themselves, adaption will be best secured 

 by an active reversal of natural selection, with the 

 result of causing degeneration. 



But, it is sometimes further urged, there are cases 

 where we cannot doubt that improvement of organi- 

 zation would have been of benefit to species ; and 

 yet such improvement has not taken place as, for in- 

 stance, in the case all monkeys not turning into men. 

 Here, however, we must remember that the operation 



