Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

'28 ON THE TENDENCY OF VARIETIES TO DEPART
original type, and which also produces, in domesticated animals, the tendency of varieties to return to
the parent form.
Tlze Struggle for Existence.
The life of wild animals is a struggle for existence. 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 offspring. 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 inexplicable-the excessive abundance of some species,
while others closely allied to them are very rare.
TIze 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