SELECTION 109 



Different Kinds of Selection. — There 

 are two main modes of natural selection. 

 There is the ordinary "lethal selection," 

 which works by the discriminate elimination 

 of the relatively less fit; and there is "repro- 

 ductive selection," which works through the 

 increased and more effective reproductivity 

 incident on the success of the more fit. When 

 Darwin says "natural selection acts by life 

 and death ... by the survival of the fittest 

 and by the destruction of the less well-fitted 

 individuals," he describes lethal selection, 

 and many use the term natural selection in 

 this sense only. But when Weismann says: 

 "Those that are best adapted in colour 

 will secure the most abundant food and will 

 reproduce most prolifically, and they will 

 thus have a better prospect of transmitting 

 their usual colouring to their offspring," he 

 is obviously describing reproductive selec- 

 tion. 



Karl Pearson draws a distinction between 

 "secular selection," which is Darwin's natu- 

 ral selection, and "periodic selection," which 

 is less easily detected. The difference is 

 this: in the ordinary process of natural 

 selection a change in the mean value of the 

 selected character must be effected from one 

 generation to another. But it might also 

 happen that the extreme deviations from the 



