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 he 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 beeps 
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 them 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 fives by a struggle at some 
period of its fife; 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 fife, 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. 
