7 
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 
