The cerebellum increases with the greater locomotive powers of the
animal. But its development is evidently limited. The large brain,
or cerebrum, is in fish hardly as heavy as the mid-brain; in
amphibia the reverse is true. In higher recent reptiles the cerebrum
would somewhat outweigh all the other portions of the brain put
together. In mammals it extends upward and backward, has already in
lower forms overspread the mid-brain, and is beginning to cover the
small brain. But this was not so in the earliest mammals. Here the
cerebrum was small, more like that of reptiles. But during the
tertiary period the large brain began to increase with marvellous
rapidity. It was very late in arriving at the period of rapid
development, but it kept on after all the other organs of the body
had settled down into comparative rest, perhaps retrogression.

We have given thus a rapid sketch in outline of the changes in the
most characteristic systems between fish and mammals. Some of the
changes which took place in mammals were along the same lines, but
one at least is so new and unexpected that this highest class
demands more careful and detailed examination.

The mammal is a vertebrate. Hence all its organs are at their best.
But mammals stand, all things considered, at the head of
vertebrates. The skeleton is firm and compact. The muscles are
beautifully moulded and fitted to the skeleton so as to produce the
greatest effect with the least mass and weight of tissue. The
sense-organs are keen, and the eye and ear especially delicate, and
fitted for perception at long range. Yet in all these respects they
are surpassed by birds. As a mere anatomical machine the bird always
seems to me superior to the mammal. It is not easy to see why it
failed, as it has, to reach the goal of possibility of indefinite