The Presidential Address. 69
oftener than in England, which is seven times a year, and
that they only bring forth eight young at a time, they would
multiply in the course of four years to a million and a
quarter."
Granting this fact, that all organisms tend to increase at a
geometrical rate, it is clear that every species must have in
itself the potentiality of unlimited extension, and must
constantly be endeavouring to extend itself at the expense of
others ; every species must be waiting to fill any vacancy in
the polity of Nature; there must be a perpetual competition
going on—a continual "struggle for existence," which keeps
in check the undue increase of any particular species. Thus
the animals and plants of any region are in a state of nicely
balanced equilibrium, the result of long ages of adjustment
to their surroundings both organic and inorganic. "In
looking at Nature, it is most necessary to keep the foregoing
considerations always in mind—never to forget that every
single organic being may be said to be striving to the utmost
to increase in numbers ; that each lives by a struggle at some
period of its life; that heavy destruction inevitably falls
either on the young or old, during each generation or at
recurrent intervals. Lighten any check, mitigate the de-
struction ever so little, and the number of the species will
almost instantaneously increase to any amount." 5
The science of Geology teaches us that the face of Nature
is undergoing slow but constant change, so that the inorganic
environment of species is by no means immutable ; and since
geological changes must entail rearrangement of life, the
organic environment is similarly in a state of fluctuation.
The equilibrium between a species and its environment is
thus subject in the course of time to be disturbed; new
conditions of life gradually come on, the region inhabited
becomes more or less extended, climatic changes may super-
vene, the amount of atmospheric moisture and the annual
rainfall may increase or diminish, the mean annual tempera-
ture may become higher or lower, new competing forms or
other foes may extend their range into the area—in short,
5 'Origin of Species,' 6th ed., pp. 52—53.