HEREDITY AND EVOLUTION 183 



the entire idea of evolution. In fact, natural selection is 

 but one of many theories by which evolution is explained. 



In introducing the discussion of natural selection Darwin 

 makes use of the principle of artificial selection among 

 domesticated animals. According to Darwin, something 

 similar to this artificial selection by man goes on in nature, 

 producing the different species of animals as we know them 

 today. Stated briefly, Darwin's theory of natural selection 

 rests on a series of arguments as follows: (1) there is a 

 tendency for each species of animal to produce more in- 

 dividuals than can survive, because of lack of food, shel- 

 ter, or other natural limits to numbers (overpopulation) ; 

 (2) this overproduction leads to a conflict or contest be- 

 tween all individuals of the same species (struggle for exist- 

 ence) ; (3) in this contest all individuals having a handicap, 

 or weakness, of any sort would be at a disadvantage in the 

 struggle and would be the first to be starved or killed 

 (elimination of the unfit) ; (4) thus only the best individ- 

 uals, those having some superior trait of cunning, speed, 

 strength, or the like, would live (survival of the fittest). 

 According to this argument only the superior individuals of 

 each generation would live and have opportunity to produce 

 young. The very same advantages which permitted the 

 parents to triumph over their less well-fitted brothers would 

 be passed on to the next generation by heredity. Thus new 

 traits, no matter how small, if they gave the owner ad- 

 vantages over his neighbors, would become emphasized. 

 According to this belief Nature in a metaphorical sense 

 chooses those individuals best suited to improve the race in 

 a manner highly comparable to the part played by man in 

 selecting his breeding stock from among his flocks of domes- 

 tic animals. 



In order to understand the principle of natural selection 

 we must consider for a moment the struggle for existence. 



