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."' 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 existenee,'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. 



