90 THE THEORY OF EVOLUTION 



be followed out in surprising detail. Indeed, the 

 changes within the eras and periods are as marked 

 as those which distinguish these divisions from one 

 another. 



The vegetation of the latest part of the Mesozoic 

 era is substantially the same as that of the Cenozoic, 

 the great revolution in plant life taking place within 

 the Cretaceous period, but the characteristic Meso- 

 zoic plants were conifers and cycads, the latter in 

 great variety and spread all over the earth, even 

 to the Antarctic continent. Birds are rare and far 

 more primitive than those of the Cenozoic, and the 

 same is true of the mammals, all of which were tiny 

 creatures of very low grade. The conspicuous and 

 all-dominating group was that of the reptiles, so 

 that the Mesozoic is appropriately called the "Age 

 of Reptiles." These creatures swarmed in the sea, on 

 the land and in the air and many attained gigantic 

 size, incomparably the largest land animals that 

 ever lived. The contrast between the Cenozoic and 

 Mesozoic is well summarized by the fact that while 

 in the Cenozoic only five orders of reptiles have 

 been found, in the Mesozoic there were no less 

 than twenty. Like the modern type of vegetation, 

 the bony fishes came in in the Cretaceous, while 

 before that period the fish-fauna was almost en- 

 tirely composed of ganoids and sharks. Many of 

 the Mesozoic invertebrates were of modern type, 

 but there were several highly characteristic groups. 

 Among the Mollusca the bivalves and gastro- 



