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.
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