136 THE LIVING WORLD. 



As already stated, it is very seldom that we can 

 determine what were the circumstances which have 

 produced the rapid development of certain types. 

 We may suppose that the marvellous expansion of 

 the reptiles in the Mesozoic was due to the fact that 

 these were the first true air-breathing vertebrates, 

 and had therefore the whole land to themselves. 

 Such an unoccupied field would undoubtedly have 

 had a tendency to stimulate them into rapid expan- 

 sion. We may suppose also that the rapid appear- 

 ance of the numerous order of insects in the Jurassic 

 (7) and Cretaceous (8) was correlated with, if not 

 caused by, the appearance of flowers and the acquir- 

 ing of the habit, on the part of some insects, of 

 feeding upon them. So too the rapid expansion of 

 birds in the Cretaceous and subsequent age was due 

 to the acquiring of aerial powers, which powers were 

 of course due to internal variations in the direction 

 of the production of wingsl Perhaps the expansion 

 of mammals was due to the change in reproductive 

 habits as above mentioned. In a few other cases it 

 is possible to make a guess as to the causes which 

 produced expansion of certain groups of animals, 

 but in general we must rest satisfied with ignorance 

 of the details, and with only the general explanation 

 that expansion is due to the occupancy of a new 

 field in nature. 



A General Advance from the Earliest Ages. 



The next point for us to notice is that there has 

 been a general advance in the animal kingdom from 

 the earliest ages until now. Taken as a whole, there 



