SURVIVAL OF THE FITTEST 403 



of reproductive age. The others must in some way be destroyed. 

 Many seeds and eggs are devoured by animals and many are 

 not brought into an environment favorable for development. 

 But one need not observe very closely to discover that the 

 number of young is always much greater than the number of 

 adults. The Nemesis of destruction follows the young through- 

 out the period of development, and indeed throughout life, 

 but the ratio of mortality is greatest during the earlier 

 stages. 



817. Now what is it that determines which one of a thousand 

 young should survive? Is it merely a matter of chance or is 

 there a difference between individuals which gives certain ones 

 an advantage? Let us consider an imaginary concrete example. 

 Suppose a litter of young rabbits in a nest at the edge of a forest. 

 These young are more or less like their parents (heredity), 

 but are not all exactly alike (individual variation). One soon 

 shows itself to be puny, is ill-nourished, and perhaps falls a 

 prey to disease or, being less active, is the first to fall a prey to 

 the weasel or other predacious animal. Its more active mates 

 escape the first assault, but one is particularly light in color 

 and is readily seen at night in the open field, where both he 

 and the owl are seeking their food. Perhaps one is too dark and 

 his color fails to blend with the dead grass where he attempts 

 to hide. One may be rather stupid. He fails to sense the 

 enemy until it is too late. But among the rest there is one 

 just the right color, that of his successful parents. He is alert, 

 strong of limb and a nimble dodger. He runs fast, dodges 

 quickly and has the instinct to hide at the right time and place. 

 He is the one most likely to survive to the season when he estab- 

 lishes a family of his own. 



8 1 8. This is a purely imaginary case, but no unreasonable 

 supposition is made. That rabbits vary in regard to such 

 characters cannot be questioned, nor that deficiency in such 

 matters may be fatal to the individual. The matter may be 



