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 family is the foundation of society.  And family and social life have been the school in which man has been compelled to learn the moral lessons, the application of which has made him what he is.

You must all, I think, have noticed that the different systems of organs succeed one another in a certain definite order; and that each stage from the lowest to the highest is characterized by the predominance of a certain function or group of functions.  This sequence of functions is not a deduction but a fact.  Place side by side all possible genealogical trees of the animal kingdom, whether founded on comparative anatomy, embryology, palaeontology, or all combined.  They will all disclose this sequence of functions arranged in the same order.  Let me call your attention to the fact that this order is not due to chance, but rests upon a physiological basis.  We might almost claim that if the evolution of man from the single cell be granted, no other order of their occurrence is possible.