MEANS AND METHODS OF EVOLUTIONARY CHANGE 355 



not result in changes in the resultant progeny. To be effective, selection, 

 either by man or by nature, must utilize inheritable variations, i.e., muta- 

 tions. 



What Constitutes Success in the "Struggle for Existence"? 



Darwin laid most stress upon survival; individuals having favorable in- 

 heritable variations survive, while their less highly endowed contempo- 

 raries perish. This phenomenon has been termed "the survival of the 

 fittest." The "fittest" were thought of as those individuals which possess in- 

 heritable characteristics enabling them to succeed in the "struggle for ex- 

 istence" in the particular circumstances and environment in which they 

 find themselves. Since they are the survivors, the "fittest" then become the 

 parents of the next generation, members of which inherit the favorable 

 characteristics from their parents. 



A moment's reflection, however, will convince us that survival in itself 

 is not the only, or even the chief, concern. The real point is not survival 

 but contribution to the next generation. Obviously a dead animal cannot 

 become a parent, but some living animals cannot, or do not, become par- 

 ents either. So far as contribution to evolution is concerned, a living ani- 

 mal which does not reproduce might just as well be dead. Indeed, from 

 the standpoint of his species it would probably be better if he were dead, 

 since he consumes food without making any contribution to the species in 

 return. This statement must be qualified for species in which individuals 

 live together in societies. In such species individuals which do not repro- 

 duce may nevertheless contribute to the success of the species by render- 

 ing essential services to the society of which they are a part. Notable ex- 

 amples are such social insects as ants and bees, and man himself (see 

 discussion of the role of cooperation, pp. 520-522). It is the social unit as 

 a whole whose success is measured in terms of contribution to the next 

 generation. Thus, be it on the individual or on the social level, success in 

 the "struggle for existence" means success in contributing to the next gen- 

 eration. Included among the determinants of this success are all factors 

 favoring efi'ective reproduction. The reproductive process is a complicated 

 one, subject to many influences. Fertility is afl"ected by the health and 

 well-being of the individual, and these in turn depend upon a variety of 

 physiological, and even psychological, factors. In the "struggle for ex- 

 istence" premium is placed, then, both on characteristics which make for 

 survival and on characteristics which make for high fertility. 



A somewhat extreme example may help to emphasize the point. Imagine 

 two competing groups of animals, each group consisting at the outset of 



