Ch. XVIII.] CONTINUITY OF LAXD AND AIR. 337 



tlie much greater variety in some classes of terrestrial 

 animals than in others. The land has often been sub- 

 merged in geological history, and the classes that were 

 best fitted to escape the impending catastrophes would 

 be most likely to preserve the varieties that had been 

 developed. The atmosphere has always been con- 

 tinuous, and the animals that could use it as a high- 

 way had great advantages over those that could not, and 

 so we find the slow-moving terrestrial mollusks few in 

 number compared with the multitudinous hosts of 

 stron^-nvin"; insects ; similarly, the mammals are far 



/ O - J 



out-numbered by the birds of the air, that can pass 

 from island to island, and from country to country, 

 unstopped by mighty rivers or wide arms of the sea. 



