THE DOUBLE-CRESTED CORMORANT. 
19 
Food supply is largely dependent upon conditions outside 
the species and without its control; inasmuch as food habits 
can be changed, within certain broad lines, they are flexible. 
Should the usual source of supply fail, the species can usually 
adapt itself to others. However, food supply must be obtained 
and is a most important agent in limiting the numbers of any 
and all species. It is also a necessity that occurs throughout 
the life of an individual. It avails nothing that food should be 
abundant for the adult if it is scarce or absent during develop- 
ment. The effect of starvation is immediate. A subnormal 
birth-rate reduces population slowly, acting over generations. 
Reduction of numbers through enemies may be much quicker 
and completed within a few seasons or less, but starvation acts 
almost instantaneously and in most cases is an operation of 
days instead of weeks, years, or generations. 
The effect of enemies upon a species is complicated in 
results; the species preying and preyed upon re-acting on each 
other in various ways. In general, a species is seldom if ever 
absolutely exterminated by these means. As soon as a food 
animal becomes too scarce to be profitably hunted, its pursuit 
is neglected and thereafter only occasional or accidental in- 
dividuals are taken. On the other hand, any marked increase 
of food animals is followed by an increased attention from their 
present enemies and an influx of new ones from adjoining ter- 
ritories. If reduction of food supply stopped when normality 
was reached the result would be comparatively simple, but, 
while the number of enemies in the locality is supernormal their 
food supply has at this stage been reduced to normal. Attention 
is eventually turned towards other food sources, but, as the 
enemy population is temporarily greater than the supporting 
powers of the habitat, it is unusually active and keen, hunting 
more carefully, with greater persistence than usual, and con- 
sequently with greater effect. For a while, at least, the re- 
duction process is thus continued and the food supply, or species 
preyed upon, falls below normal. Finally, however, the enemies 
also are reduced either through starvation or movement to 
other localities until their population is also brought below normal 
numbers in harmony with the reduced resources of the habitat. 
