DEVELOPMENT. 
227 
The total extinction of any one species is exceedingly 
rare. The number of any given species is not determined 
by the number of eggs produced, but by its surrounding 
conditions. 123 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 be¬ 
lieved to be the most numerous bird in the world. 
The main checks to the high rate of increase are: cli¬ 
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, 
being hardier or stronger, more agile or sagacious, will 
survive. Should this advantageous variation become 
hereditary and intensified, the new variety will gradually 
extirpate or replace other kinds. This is what Mr. Dar¬ 
win means by Natural Selection , and Herbert Spencer by 
the Survival of the Fittest . 
