220 COMPARATIVE ZOOLOGY. 
rare. The number of any given species is not determined 
by the number of eggs produced, but by its conditions.” 
Aquatic birds outnumber the land birds, because their 
food never fails, not because they are more prolific. The 
Fulmar-petrel lays but one egg, yet it is believed to be the 
most numerous bird in the world. 
The main checks to the high rate of increase are: ei- 
mate (temperature and moisture), acting directly or indi- 
rectly by reducing food; and other animals, either rivals 
requiring the same food and locality, or enemies, for the 
vast majority of animals are carnivorous. Offspring are 
continually varying from their parents, for better or worse. 
If feebly adapted to the conditions of existence, they will 
finally go to the wall. But those forms having the slight- 
est advantage over others inhabiting the same region, be- 
ing hardier or stronger, more agile or sagacious, will sur- 
vive. Should this advantageous variation become heredi- 
tary and intensified, the new variety will gradually extir- 
pate or replace other kinds. This is what Mr. Darwin 
means by WVatural Selection, and Herbert Spencer by the 
Survival of the Fittest. 
