an influence which is exactly comparable to that of artificial
selection. By Conditions of Existence I mean two things,--there are
conditions which are furnished by the physical, the inorganic world,
and there are conditions of existence which are furnished by the organic
world. There is, in the first place, CLIMATE; under that head I include
only temperature and the varied amount of moisture of particular places.
In the next place there is what is technically called STATION, which
means--given the climate, the particular kind of place in which an
animal or a plant lives or grows; for example, the station of a fish
is in the water, of a fresh-water fish in fresh water; the station of a
marine fish is in the sea, and a marine animal may have a station higher
or deeper. So again with land animals: the differences in their stations
are those of different soils and neighbourhoods; some being best adapted
to a calcareous, and others to an arenaceous soil. The third condition
of existence is FOOD, by which I mean food in the broadest sense, the
supply of the materials necessary to the existence of an organic being;
in the case of a plant the inorganic matters, such as carbonic acid,
water, ammonia, and the earthy salts or salines; in the case of the
animal the inorganic and organic matters, which we have seen they
require; then these are all, at least the two first, what we may
call the inorganic or physical conditions of existence. Food takes a
mid-place, and then come the organic conditions; by which I mean the
conditions which depend upon the state of the rest of the organic
creation, upon the number and kind of living beings, with which an
animal is surrounded. You may class these under two heads: there are
organic beings, which operate as 'opponents', and there are organic
beings which operate as 'helpers' to any given organic creature. The
opponents may be of two kinds: there are the 'indirect opponents', which
are what we may call 'rivals'; and there are the 'direct opponents',
those which strive to destroy the creature; and these we call 'enemies'.
By rivals I mean, of course, in the case of plants, those which require
for their support the same kind of soil and station, and, among animals,
those which require the same kind of station, or food, or climate; those
are the indirect opponents; the direct opponents are, of course, those
which prey upon an animal or vegetable. The 'helpers' may also be
regarded as direct and