38 ANIMAL LIFE PAST AND PRESENT. 



namely, the replacement, with the advance of time, of 

 a lower by a higher type of organisation. The Bats, 

 indeed, which belong to the highest class of animals, 

 appear to have been the latest in which the power of 

 flight has been developed ; but since most of them are 

 of comparatively small size, and of more or less com- 

 pletely crepuscular and nocturnal habits, they have 

 never entered seriously into competition with the Birds, 

 so that both groups are found existing side by side in 

 full development. 



