1 6 A STUDY IN HEREDITY 



and to these only when they operate under certain 

 conditions. They must be selective ; that is, 

 they must discriminate between the fit and the 

 unfit, between the superior and the inferior. 

 Under their action individuals of a certain type, 

 the fittest, who excel in a particular quality or 

 set of qualities, must generally survive to con- 

 tinue the race, while the rest of the species in large 

 measure perish. It follows, if an injurious agency 

 is so little injurious as not to influence the death- 

 (or birth-) rate, or so very injurious as not to dis- 

 criminate between the fit and the unfit, that it 

 cannot be a cause of evolution. In the one case 

 the unfit are not eliminated ; in the other the fit 

 do not survive. Haphazard deaths again are not 

 causes of evolution. Thus fire and water may de- 

 stroy many lives in this country, but they do not 

 select for survival any particular type of individual. 



No breeder of plants and animals is able to 

 improve his stock unless he breeds with care, unless 

 he exercises stringent selection. Race-horses, for 

 example, could not have been evolved by the 

 occasional elimination of an inferior animal. All 

 or most inferior animals had to be eliminated. It 

 follows that a breeder cannot at one and the same 

 time improve a species in every or even in many 

 directions ; he must be content with improvement 

 in a very few particulars only. If he sought im- 



