234 THE WOKLD OF LIFE 



of carbon from the air by plants and from the water by marine 

 animals and by chemical action. The superabundance of this 

 gas during the early stages of the life-world facilitated the 

 process of clothing the land with vegetation soon after it ap- 

 peared above the waters; while its absorption by water was 

 equally useful in rendering possible the growth of the calcare- 

 ous framework or solid covering of so many marine animals. 

 With the progressive cooling of the earth and the increased 

 area of land-surface, more and more of the atmospheric carbon 

 became solidified and inactive, thus rendering both the air and 

 the water better fitted for the purposes of the higher, warm- 

 blooded, and more active forms of life. This process will, I 

 think, enable us partially to understand the fundamental 

 changes in life-development which characterised the three great 

 geological areas; but it does not seem sufficient to explain tbe 

 very sudden and complete changes that occurred, and, more 

 especially, the almost total extinction of the lower or earlier 

 types just when they appear to have reached their highest and 

 most varied structure, their greatest size of body, and their 

 almost world-wide distribution. Before attempting a solution 

 of this difficult problem an outline must be given of the latest, 

 and in some respects the most interesting, of the geological 

 eras — the Tertiary, or, as more frequently termed by geolo- 

 gists, the Cainozoic. 



