MEANS AND METHODS OF EVOLUTIONARY CHANGE 357 



and competition than are the examples we have given. Resistance to dis- 

 ease is highly important. Any structural or physiological improvement con- 

 tributing to vitality and fecundity will tend to confer a differential ad- 

 vantage on some individuals as compared to others. Ability to produce 

 large numbers of viable offspring confers an advantage, particularly in 

 species in which the parents do not care for the young after hatching or 

 birth. Alternatively, increased perfection of postnatal care confers ad- 

 vantages in species which produce few offspring per parent. In the former 

 instance more than the laying of large numbers of eggs is involved. The 

 eggs must be viable, and they must be efficiently fertilized. Young which 

 develop quickly have an advantage over those which develop slowly. 

 When two competing strains differ in speed of individual development, that 

 strain which produces mature offspring in less time will, other things being 

 equal, contribute more of its genes to future generations than will a 

 strain in which sexual maturity is attained more slowly. Offspring of the 

 first strain may already have mated in their turn before offspring of the 

 second strain have matured sufficiently to do so. 



These examples are given to emphasize the fact that complex and subtle 

 factors are involved in determining which individuals shall contribute 

 most to the next generation. The familiar matters of escape from preda- 

 tors and competition for food are but two among many important factors. 



Essence of Natural Selection 



A brief statement of natural selection may help to bring our discussion 

 into focus. Reduced to its essentials, natural selection results from the cu- 

 mulative action of all forces tending to insure that individuals possessing 

 one genetic constitution shall leave larger numbers of offspring than will 

 individuals possessing some other genetic constitution. Thus if a mutation 

 contributes in any way to the leaving of larger numbers of offspring it will 

 be perpetuated in increased proportion in the next generation, since it will 

 be carried by those "larger numbers of offspring." Contrariwise, if the mu- 

 tation interferes in any way with the leaving of larger numbers of off- 

 spring it will be perpetuated in decreased proportion in the next genera- 

 tion, since it will be carried by but a decreased number of individuals in 

 that seneration. 



We readily appreciate that if natural selection continues for several gen- 

 erations, individuals lacking the favorable mutation may be completely 

 eliminated, with the result that the mutation becomes "standard equip- 

 ment" for the entire population. 



It will be evident, moreover, that these principles apply not only to indi- 



