NATURAL SELECTION 223 
may more truly be said to struggle with each other. As the mistletoe 
is disseminated by birds, its existence depends on them; and it may 
metaphorically be said to struggle with other fruit-bearing plants, 
in tempting the birds to devour and thus disseminate its seeds. In 
these several senses, which pass into each other, I use for conven- 
ience’ sake the general term of Struggle for Existence. 
GEOMETRICAL RATIO OF INCREASE 
A struggle for existence inevitably follows from the high rate at 
_ which all organic beings tend to increase. Every being, which during 
its natural lifetime produces several eggs or seeds, must suffer destruc- 
tion during some period of its life, and during some season or occasional 
year, otherwise, on the principle of geometrical increase, its numbers 
would quickly become so inordinately great that no country could 
support the product. Hence, as more individuals are produced than 
can possibly survive, there must in every case be a struggle for exist- 
ence, either one individual with another of the same species, or with 
the individuals of distinct species, or with the physical conditions of 
life. It is the doctrine of Malthus applied with manifold force to 
the whole animal and vegetable kingdoms; for in this case there can 
be no artificial increase of food, and no prudential restraint from 
marriage. Although some species may be now increasing, more or 
less rapidly, in numbers, all cannot do so, for the world would not 
hold them. 
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST 
How will the struggle for existence, briefly discussed in’ the last 
chapter, act in regard to variation? Can the principle of selection, 
which we have seen is so potent in the hands of man, apply under 
nature? J think we shall see that it can act most efficiently. Let the 
endless number of slight variations and individual differences occurring 
in our domestic productions, and, in a lesser degree, in those under 
nature, be borne in mind; as well as the strength of the hereditary 
tendency. Under domestication, it may be truly said that the whole 
organization becomes in some degree plastic. But the variability, 
which we almost universally meet with in our domestic productions, 
is not directly produced, as Hooker and Asa Gray have well remarked, 
by man; he can neither originate varieties, nor prevent their occur- 
rence; he can only preserve and accumulate such as do occur. Unin- 
tentionally he exposes organic beings to new and changing conditions 
