XV VARIATION AND EVOLUTION 179 



It is interesting to recall that in earlier years 

 Darwin was inclined to ascribe more importance to 

 " sports " as oppose<l to continuous minute variation, 

 and to consider that they might play a not incon- 

 siderable part in the formation of new varieties in 

 nature. This view, however, he gave up later, be- 

 cause he thought that the relatively rare sport, or 

 mutation, would rapidly disappear through the 

 swamping effects of crossing with the more abun- 

 dant normal form, and so, even though favoured by 

 natural selection, would never succeed in establishing 

 itself. Mendel's discovery has eliminated this diffi- 

 culty. For suppose that the sport differed from the 

 normal in the loss of a factor and were recessive. 

 When mated with tlie normal this character would 

 seem to disappear, though, of course, half of the 

 gametes of its progeny would bear it. By continual 

 crossing with normals a small proportion of hetero- 

 zygotes would eventually be scattered among the 

 population, and as soon as any two of these mated 

 together the recessive sport would appear in one 

 quarter of their offspring. 



A suggestive contribution to this subject was 

 recently made by G. H. Hardy. Considering the 

 distribution of a single factor in a mixed popula- 

 tion consisting of the heterozygous and the two 

 homozygous forms, he showed that such a population, 

 breeding at random, rapidly fell into a stable con- 

 dition with regard to the proportion of these three 

 forms, whatever may have been the proportion of 

 the three forms to start with. Let us suppose, for 

 instance, that the population consists of/ homozygotes 

 of one kind, r homozygotes of the other kind, and 



