XV VARIATION AND EVOLUTION i8i 



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 dominant or recessive 

 b.e favoured by selection the conditions are altered, 

 and it can be shown that even a small, advantage 

 possessed by the one will rapidly lead to the 

 elimination 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 variations. 

 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 biologist has been accustomed to demand a very 

 large supply of time, often a great deal more than 

 the physicist is disposed to grant, and this has some- 

 times led him to expostulate with the latter for cutting 

 off the supply. On the newer views, however, this diffi- 

 culty need not arise, for we realise that the origin and 

 establishing of a new form may be a very much more 

 rapid process than has hitherto been deemed possible. 

 One last question with regard to evolution. How 

 far does Mendelism help us in connection v\ith the 

 problem of the origin of species ? Among the plants 

 and animals with which we have dealt we have been 

 able to show that distinct differences, often con- 

 siderable, in colour, size, and structure, may be 

 interpreted in terms of Mendelian factors. It is not 



