EXTINCTION OF GIANT EACES 



407 



An enormous increase of mere bodily size, however, seems in the 

 long run to be always fatal to the race, whose place will be taken 

 by smaller and presumably more active forms. The gigantic 

 amphibians are all extinct, so are the really gigantic reptiles, 

 and of the gigantic mammals only a couple of species of elephants 



FIG. 186. A Hornl>ill, Buceros rJiinoceros, from North- West Borneo. (Drawo 

 from a specimen in the British Museum, Natural History.) 



and a few whales survive, all of which are being rapidly exter- 

 minated in competition with man. 



There is perhaps some justification in recent developments of 

 physiological science for the belief that a race of animals may 

 acquire a momentum of the kind referred to ; that some brake 

 is normally applied to the growth of organisms and organs and 

 that sometimes this brake is removed, leaving the organism to 

 rush onwards to destruction like a car running away down hill. 



Many modern physiologists hold the view that the growth of 



