CHAPTER VIII 

 LIMITATION OF FAMILIES 



ELEPHANTS may live until they are at least a century old, and do 

 not begin to breed until they are well over twenty years of age. 

 They are probably the slowest breeders of all animals, and a pair 

 living to their full range of life under the most favourable conditions 

 would not bring into the world more than six young ones. None 

 the less, as Darwin calculated, if we could suppose that all the 

 descendants of a single pair of elephants were to live to their full 

 time of life, and to produce their six offspring, then at the end of 

 the fifth century the single pair of elephants would be represented 

 by over fifteen millions of living descendants. At the other end 

 of the scale we may place a fish like the turbo t, which can produce 

 as many as fifteen million eggs in a season. I do not know how 

 long it will live, if it escape being caught, but certainly it is capable 

 of living a good many years. If all the descendants of a single 

 pair of turbot were to survive, even the enormous area of the 

 oceans would soon be filled with a solid mass of fish. A pair of 

 London sparrows are able to rear three or four clutches of eggs in 

 the course of a year, and each clutch contains from five to six eggs. 

 The prolificness of animal life is enormous. Whether animals live 

 a short time or a long time, whether they produce many or few 

 young in a season, a sum in arithmetic shows that the air, and the 

 surface of the earth, and the waters could soon be filled with the 

 incredible swarms of progeny. 



And yet we know that on the whole the relative numbers of 

 different species of animals remain stationary. Now and again 

 there is a grasshopper year, or a vole year, or a wasp year, when 

 the destructive forces seem to have been swamped by natural 

 increase. Some species of animals, such as man himself, are steadily 

 gaining ground ; others, like the bison in America or the antelopes 

 and zebras in South Africa, are disappearing ; but on the whole 

 the balance of life is preserved and, with occasional fluctuations, 

 species neither gain nor lose very much in numbers. 



