an ultimate mystery. It is so, and that is an
end of it. But why it should be so, we cannot see. Nor can we see why
there should be any of the particular kinds of form that there are. To
explain this, Aristotle ought to have shown that the forms constitute
a systematic unity, that they can be deduced one from another, just as
we saw that Plato ought to have deduced all the Ideas from one
another. Thus Aristotle asserts that the form of plants is nutrition,
of animals sensation, and that the one passes into the other. But even
if this assertion be true, it is a mere fact. He ought not merely to
have asserted this, but to have deduced sensation from nutrition.
Instead of being content to allege that, as a fact, nutrition passes
into sensation, he ought to have shown that it must pass into
sensation, that the passage from one to the other is a logical
necessity. Otherwise, we cannot see the reason why this change occurs.
That is to say, the change is not _explained_.

Consider the effects of this omission upon the theory of evolution. We
are told that the world-process moves towards an end, and that this
end is the self-realization of reason, and that it is proximately
attained in man, because man is a reasoning being. So far this is
quite intelligible. But this implies that each step in evolution is
higher than the last because it approaches nearer to {337} the end of
the world-process. And as that end is the realization of reason, this
is equivalent to saying that each step is higher than the last because
it is more rational. But how is sensation more rational than
nutrition? Why should it not be the other way about? Nutrition passes
through sensation into human reason. But why should not sensation pass
through nutrition into human reason? Why should not the order be
reversed? We cannot explain. And such an admission is absolutely fatal
to any philosophy of evolution. The whole object of such a philosophy
is to make it clear to us why the higher form is higher, and why the
lower is lower: why, for example, nutrition must, as lower, come
first, and sensation second, and not _vice versa_. If we can see no
reason why the order should not be reversed, this simply means that
our philosophy of evolution has failed in its main point. It means
that we cannot see any real difference between lower and higher, and
that therefore we have merely change without development, since it is
indifferent whether A passes into B, or B into A