2 Colouration in Animals and Plants. 



individual a better chance, in the struggle for existence, of obtaining 

 food or avoiding danger. It is based on a few well-known and 

 universally admitted facts or laws of nature : namely, the law of 

 multiplication in geometrical progression causing the birth of many 

 more individuals than can survive, leading necessarily to the struggle 

 for existence ; the law of heredity, in virtue of which the offspring 

 resembles its parents ; the law of variation, in virtue of which the 

 offspring has an individual character slightly differing from its 

 parents. 



To illustrate these laws roughly we will take the case of a bird, 

 say, the thrush. The female lays on the average five eggs, and if all 

 these are hatched, and the young survive, thrushes would be as seven 

 to two times as numerous in the next year. Let two of these be 

 females, and bring up each five young ; in the second year we shall 

 have seventeen thrushes, in the third thirty-seven, in the fourth 

 seventy-seven, and so on. Now common experience tells us not 

 merely that such a vast increase of individuals does not take place, 

 but can never do so, as in a very few years the numbers would 

 be so enormously increased that food would be exhausted. 



On the other hand, we know that the numbers of individuals 

 remain practically the same. It follows, then, that of every five 

 eggs four fail to arrive at maturity ; and this rigorous destruction of 

 individuals is what is known as the struggle for existence. If, instead 

 of a bird, we took an insect, laying hundreds of eggs, a fish, laying 

 thousands, or a plant, producing still greater quantities of seed, we 

 should find the extermination just as rigorous, and the numbers of 

 individuals destroyed incomparably greater. Darwin has calculated 

 that from a single pair of elephants nearly nineteen millions would 

 be alive in 750 years if each elephant born arrived at maturity, lived 

 a hundred years, and produced six young — and the elephant is the 

 slowest breeder of all animals. 



The struggle for existence, then, is a real and potent fact, and 

 it follows that if, from any cause whatever, a being possesses any 

 power or peculiarity that will give it a better chance of survival 

 over its fellows — be that power ever so slight— it will have a very 

 decided advantage. 



Now it can be shown that no two individuals are exactly alike, 

 in other words, that variation is constantly taking place, and that 

 no animal or plant preserves its characters unmodified. This we 

 might have expected if we attentively consider how impossible it 



