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. 
The 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 
