xiii VARIATION AND EVOLUTION 149 



benefit of one class more than of another. Moreover, it 

 makes no difference whether p represents the homo- 

 zygous dominants or whether it stands for the recessives. 

 A population containing a very small proportion of domi- 

 nants and one containing a similar proportion of recessives 

 are equally stable. The term dominant is in some re- 

 spects apt to be misleading, for a dominant character 

 cannot in virtue of its dominance establish itself at the 

 expense of a recessive one. Brown eyes in man are 

 dominant to blue, but there is no reason to suppose that 

 as years go on the population of these islands will become 

 increasingly brown eyed. Given equality of conditions 

 both are on an equal footing. If, however, either domi- 

 nant or recessive be favoured by selection the conditions 

 are altered, and it can be shown that even a small advan- 

 tage possessed by the one will rapidly lead to the elimina- 

 tion of the other. Even with but a 5 per cent selection 

 advantage in its favour it can be shown that a rare sport 

 will oust the normal form in a few hundred generations. 

 In this way we are freed from a difficulty inherent in the 

 older view that varieties arose through a long-continued 

 process involving the accumulation of very slight varia- 

 tions. On that view the establishing of a new type was 

 of necessity a very long and tedious business, involving 

 many thousands of generations. For this reason the biol- 

 ogist has been accustomed to demand a very large sup- 

 ply of time, often a great deal more than the physicist is 



