144 THE LIVING WORLD. 



of the utmost importance in these contests, and 

 through all the history of the vertebrates we find 

 that the development of each class was marked 

 by an increase in size. This tendency toward in- 

 crease in size continued until the Jurassic (7), when 

 it reached its culmination in the huge reptiles of 

 this period, some of which were the largest animals 

 that ever lived. Of course, we do not mean that 

 every order of vertebrates became of large size, for 

 many of the smaller ones did establish their right to 

 live. Many small fishes and small reptiles did suc- 

 ceed in perpetuating themselves. Nevertheless we 

 can recognize the truth of the general fact that from 

 the Devonian (3) to the Cretaceous (8) or Tertiary 

 (9) a tendency towards increase in size marked the 

 history of the animal kingdom. 



This increase in size, however, was sure to reach 

 a limit. Great size is only to be possessed at the 

 expense of activity and agility, and the great reptiles 

 of the Jurassic probably became so large that it was 

 a matter of greater and greater difficulty for them to 

 procure sufficient food. As we have already seen, 

 from the Tertiary period the mammals began to 

 supersede the reptiles as the monarchs of creation. 

 Even among the h'ighest class of animals we find for 

 a long time a tendency to produce animals of great 

 size. Some of the mammal orders of the Tertiary 

 (9) attained a size which almost made them rivals in 

 bulk with the reptiles. But we soon notice in the 

 history of this class a tendency to develop small 

 size and agility instead of huge. bulk. Of the eden- 

 tates, the huge Megatherium disappeared, and only 



