II ON THE TENDENCY OF VARIETIES, ETC. 23 
but the difficulty is overcome by assuming that such varieties 
have strict limits, and can never again vary further from the 
original type, although they may return to it, which, from the 
analogy of the domesticated animals, is considered to be highly 
probable, if not certainly proved. 
It will be observed that this argument rests entirely on the 
assumption that varieties occurring in a state of nature are 
in all respects analogous to or even identical with those of 
domestic animals, and are governed by the same laws as 
regards their permanence or further variation. But it is the 
object of the present paper to show that this assumption is 
altogether false, that there is a general principle in nature 
which will cause many varieties to survive the parent species, 
and to give rise to successive variations departing further and 
further from the original type, and which also produces, in 
domesticated animals, the tendency of varieties to return to 
the parent form. 
The Struggle for Ewistence 
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. 
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 
