gments and is highly
differentiated. It has a special centre of perception, thought, and
will; it is an organ of mind. The vertebrate has the physical and
mental advantages of large size.

First the definite form and mode of developing a vertebra is
attained. Then the vertebral column is perfected. The fins are
modified into legs. The lungs increase in size and the heart becomes
double. The animal emerges on land; and, with a better supply of
oxygen and less loss of heat, all the functions are performed with
the highest possible efficiency. First, apparently, amphibia, then
reptiles, and finally mammals of enormous size and strength
appeared. It looked as if the earth were to be an arena where
gigantic beasts fought a never-ending battle of brute force. But
these great brutes reproduced slowly, had therefore little power of
adaptation, were fitted to special conditions, and when the
conditions changed they disappeared. The bird tried once more the
experiment of developing the locomotive powers to the highest
possible extent. It became a flying machine, and every organ was
moulded to suit this life. Every ounce of spare weight was thrown
aside, the muscles were wonderfully arranged and of the highest
possible efficiency. The body temperature is higher than that of
mammals. The whole organization is a physiological high-pressure
engine. The sense-organs are perhaps the finest and keenest in the
whole animal kingdom. The brain is inferior only to that of mammals.
The experiment could not have been tried under more favorable
conditions; it was not a failure, it certainly was not a success
when compared with that of mammals.

The possibilities of every system except one had been practically
exhausted. Only brain development remained as the last hope of
success. Here was an untried line, and the mammals followed it.
During the short tertiary period the brain in many of their genera
seems to have increased tenfold. By the arboreal life of the highest
forms the hand is developed as the instrument of the thinking brain.
The battle is beginning to become one of wits, and the crown will
soon pass from the strongest to the shrewdest. Mind, not muscle,
much less digestion or reproduction, is the goal of the animal
kingdom. And we shall see later that the mammalian mode of
reproduction and of care of the young led to an almost purely mental
and moral advance. For these could have but one logical outcome,
family life. And the fami