WHY DO ANIMALS BECOME EXTINCT? 237 



been the effect of some sudden increase in the 

 numbers of such a pest on the forests of the 

 past. Trees might resist the attacks of ene- 

 mies and the destruction of their leaves for 

 two or three years, but would be destroyed by 

 a few additional seasons of defoliation. 



Ordinarily the abnormal increase of any in- 

 sect is promptly followed by an increase in the 

 number of its enemies ; the pest is killed off, 

 the destroyers die of starvation and nature's 

 balance is struck. But if by some accident, 

 such as two or three consecutive seasons of 

 wet, drought, or cold, the natural increase of 

 the enemies was checked, the balance of nature 

 would be temporarily destroyed and serious 

 harm done. That such accidents may occur 

 is familiar to us by the damage wrought in 

 Florida and other Southern States by the un- 

 wonted severity of the winters of 1893, 1895, 

 and 1899. 



If any group of forest trees was destroyed in 

 the manner suggested by Professor Shaler, the 

 effects would be felt by various plants and ani- 

 mals. In the first place, the insects that fed 

 on these trees would be forced to seek another 



