THE BRAIN. 59 



The author has since continued this line of investigation, and has 

 ascertained that the same general law of brain growth is true for Birds 

 and Reptiles from the Jurassic to the present time. 



To this general law of brain growth two additions may now be 

 made, which briefly stated are as follows : 



(1.) The brain of a mammal belonging to a vigorous race, fitted for a 

 long survival, is larger than the average brain, of that period, in the same 

 group. 



(2.) TJie brain of a mammal of a declining race is smaller than the 

 average of its cotemporaries of the same group. 



An example of the first of these statements is seen in figure 70, 

 page 64, representing the genus Colonoceras, one of the Eocene ancestors 

 of the Rhinoceros. The second case is illustrated by figure 82, page 67, 

 of the Hippopotamus, evidently one of the last members of a long line. 

 A study of a larger number of extinct and recent specimens will make 

 these conclusions more apparent. 



The results of this study of the whole subject of brain growth, tlie 

 author intends to bring together in a separate memoir. Some of the 

 principal facts, however, may be appropriately presented in the present 

 volume in connection with the brain characters of the Dinocerata, which 

 naturally form the beginning of one series in the investigation. 



In any comparison of the size of the brain in different animals, 

 whether in the same group or in others widely diflterent, it is important 

 to bear in mind that : 



1. The brain of small animals is proportionall}^ larger in bullc than 

 that of large animals. 



2. The brain of young animals is proportionally larger than in those 

 fully adult. 



In a general comparison of brain growth of manmials, the first of 

 these facts can have only a limited eifect, which would not change, 

 essentially, ther general results. The effects of the second fact may be 

 readily eliminated by confining the com2)arison to adult animals. 



