respiration takes place by gills alone, the circulation is simple;
the blood flows from the heart to the gills, and thence directly all
over the body; the oxygenated blood from the gills does not return
directly to the heart. But the blood from the lungs does return to
the heart; and there at first mixes in the ventricle with the impure
blood which has returned from the rest of the body. Gradually a
partition arises in the ventricle, dividing it into a right and left
half. Thus the two circulations of the venous blood to the lungs,
and of the oxygenated blood over the body, are more and more
separated until, in higher reptiles, they become entirely distinct.

As the animal came on land and breathed the air, more completely
oxygenated blood was carried to the organs, and their activity was
greatly heightened. As more and more heat was produced by the
combustion in muscular and nervous tissues, and less was lost by
conduction, the temperature of the body rose, and in birds and
mammals becomes constant several degrees above the highest summer
temperature of the surrounding air.

The changes in the brain affect mainly the large and small brain.
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
vertebrat