XIII VARIATION AND EVOLUTION 143 



footing. If, however, either dominant or recessive 

 be 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 

 sometimes led him to expostulate with the latter for 

 cutting off the supply. On the newer views, however, 

 this difficulty 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 with 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 

 unlikely that most of the various characters which 

 the systematist uses to mark off one species from 

 another, the so-called specific characters, are of this 



