CONSEQUENCES IMPOSED BY GANOID FISHES 433 



before the acquisition of the four-chambered heart and the separate pul- 

 monary circulation. During this time it was impossible for the air- 

 breathing vertebrates, especially those of smaller size, to remain active 

 except in warm environments. Such sustained activity, furthermore, as 

 is needed for the present keenness of competition between herbivores and 

 carnivores, or as is necessary for the flight of birds, was impossible. 



But warm-bloodedness and the possibility of continued bodily activity 

 carry with them a far-reaching train of consequences. Hair and feathers 

 come to form a body covering to hold in the body warmth ; the range and 

 quantity of animal life has been thereby enormously extended over the 

 globe. Cold, which in the Permian must have greatly restricted the life 

 of reptiles, in the Pleistocene served as a stimulus to the growth and 

 activity of mammals. 



But the nervous system, and especially the brain, are the parts of the 

 body which stand in most need of pure blood. This is shown not only by 

 those elaborate makeshifts in amphibians and reptiles by means of which 

 purer blood is sent to the head than is sent to the body, but experiment 

 shows that a temporary deficiency of oxygen, as in poisoning by illumi- 

 nating gas, destroys consciousness and even does more or less permanent 

 damage to the brain, when other organs are but little affected. Further- 

 more, there is a correlation between mind and body; so that mental and 

 bodily activity are associated. Mere mental activity should not, however, 

 be confused with intellectuality. Birds, for example, show, as a class, 

 great mental activity, but it is dominantly reflex and emotional rather 

 than judicial and intellectual. 



The Age of Eeptiles was a long era dominated by small and stupid 

 brains in great and armored bodies. Since mind was lacking, the first 

 wave of terrestrial evolution had to put all its stress on mere size and 

 bodily power. With the rise of a perfected circulation the emphasis of 

 evolution became transferred to advances in mental and bodily activity. 

 When, at the end of the Mesozoic, the world-changes modified the older 

 environments, the overspecialized reptiles disappeared and the animals 

 possessing these more efficient processes could take advantage of the new 

 conditions. Then it was that the Age of Mammals dawned. This is a 

 turning point in evolution, for with the radiative expansion of warm- 

 blooded land vertebrates there began a remarkable development of the 

 brain, as shown by the increasing ratio of brain weight to body weight. 

 Through the Tertiary this ratio doubled and redoubled, in geometrical 

 progression, in those leading mammalian orders whose evolution has been 

 traced. The mental factor in connection with a perfected circulation had 

 at last reached a primary importance prophetic of the Age of Mind. 



