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 

 hfe. 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 ? I think we shaU 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 



