28 ON THE TENDENCY OF VARIETIES TO DEPART 



original type, and which also produces, in domesti- 

 cated animals, the tendency of varieties to return to 

 the parent form. 



The Struggle for Existence. 



The life of wild animals is a struggle for exist- 

 ence. The full exertion of all their faculties and 

 all their energies is required to preserve their own 

 existence and provide for that of their infant off- 

 spring. The possibility of procuring food during the 

 least favourable seasons, and of escaping the attacks 

 of their most dangerous enemies, are the primary 

 conditions which determine the existence both of 

 individuals and of entire species. These conditions 

 will also determine the population of a species; and by 

 a careful consideration of all the circumstances we 

 may be enabled to comprehend, and in some degree 

 to explain, what at first sight appears so inex- 

 plicable the excessive abundance of some species, 

 while others closely allied to them are very rare. 



Tlie Law of Population of Species. 



The general proportion that must obtain between 

 certain groups of animals is readily seen. Large 

 animals cannot be so abundant as small ones ; the 

 carnivora must be less numerous than the herbivora ; 

 eagles and lions can never be so plentiful as pigeons 

 and antelopes; and the wild asses of the Tartarian 

 deserts cannot equal in numbers the horses of the 

 more luxuriant prairies and pampas of America. The 



