e 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 organization would have been of benefit to
species; and yet such improvement has not taken place--as, for instance,
in the case all monkeys not turning into men. Here, however, we must
remember that the operation of natural selection in any case depends
upon a variety of highly complex conditions; and, therefore, that the
fact of all those conditions having been satisfied in one instance is no
reason for concluding that they must also have been satisfied in other
instances. Take, for example, the case of monkeys passing into men. The
wonder to me appears to be that this improvement should have taken place
in even one line of descent; not that, having taken place in one line,
it should not also have taken place in other lines. For how enormously
complex must have been the conditions--physical, anatomical,
physiological, psychological, sociological--which by their happy
conjunction first began to raise the inarticulate cries of an ape into
the rational speech of a man. Therefore, the more that we appreciate the
superiority of a man to an ape, the less ought we to countenance this
supposed objection to Darwin's theory--namely, that natural selection
has not effected the change in more than one line of descent.

Even in the case of two races of mankind where one has