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, theretore, 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 
