6 



The number of species of living animals and plants on the 

 earth is vast, and the number of individuals entirely beyond 

 human comprehension. The chief efforts of every individual 

 of each species go to preserve its life and to produce seed or 

 offspring and so multiply its kind, but always and everywhere 

 similar efforts of the many other organisms by which each 

 species is surrounded tend to hold its multiplication in check. 

 Huxley says that if there were but a single plant in the world, 

 and that plant should produce but fifty seeds each year and 

 multiply unchecked, its progeny would cover the globe in nine 

 years. The oak produces quantities of acorns. Were each seed 

 to develop into a tree the earth in time would be covered with 

 oaks, and all other trees would be crowded out. But many 

 mammals, birds and insects feed on acorns and so prevent their 

 germination; others feed on the seedling trees and destroy 

 bark, leaf and wood, so that on the average each mature oak 

 during all the years of its long life succeeds in producing but one 

 other to live and take its place. The fulmar petrel, so says 

 Darwin, lays but one egg, yet it is believed to be the most 

 numerous species of bird in the world. Wallace estimates that 

 the unchecked increase of any pair of birds having four young 

 each year would amount to 2,000,000,000 birds in fifteen years. 

 However, such an enormous multiplication never happens be- 

 cause snakes, turtles, crows, hawks, jays, squirrels, raccoons, 

 cats, foxes and many other creatures eat birds or their eggs or 

 young. Many birds are destroyed by the elements; they are 

 starved, frozen and drowned, and their increase is checked so 

 that commonly in nature but one pair of birds succeeds another. 



In the insect world the possibilities of unchecked increase are 

 still more formidable than among mammals or birds. Huxley 

 reckons that the young of a certain plant louse, increasing un- 

 checked, in one year would equal in bulk the entire human 

 population of the Chinese Empire. Such increases in number, 

 however, are impossible because of the many forces always 

 working to check them. Insects in all their forms are killed 

 and eaten continually by a host of other creatures. 



Each animal species while striving mightily to increase its 

 numbers also works to hold others in check. Forbes likens the 

 whole system of life with all its interrelations to a series of ex- 



