A VIEW IN PERSPECTIVE. 115 



a general development of new forms at this period 

 would indicate that some highly important change 

 in the condition of the surrounding living world 

 occurred to act as a stimulus. Although it is im- 

 possible to say positively what these new conditions 

 were, we may perhaps find a partial explanation in 

 the reduction of the amount of carbonic acid in the at- 

 mosphere. During the Carboniferous (4) era, vegeta- 

 tion had been rapidly at work drawing the CO 2 from 

 the air and storing it up in the form of coal. Roughly 

 speaking, we may say that all of the carbon now 

 stored in the coal beds was in the atmosphere in the 

 form of CO 2 , previous to the Carboniferous age. 

 Plainly, the withdrawal of this CO 3 by the Carbon- 

 iferous vegetation rendered the air much better 

 fitted for the terrestrial life of animals, and it is 

 interesting to find that during this period or just 

 after it the first air-breathing vertebrates came into 

 existence. Though this may not be a sufficient 

 explanation for the whole of the marvellous ex- 

 pansion of type in the early Mesozoic, and especially 

 of marine types, we are nevertheless probably correct 

 in regarding it as one of the most important factors 

 in this expansion. At all events, it is true that some 

 impulse acted upon the animal kingdom at about 

 the beginning of the Mesozoic which produced an 

 exceptionally rapid advance and a divergence of 

 form. 



It is especially noticeable that this era, which 

 was so marked in the life history of animals, was 

 not an era of especial note in the vegetable king- 

 dom. It was not until toward the Cretaceous (8) 



