EFFECT OF BI-PARENTAL REPRODUCTION 131 



duction tending to eliminate individual variations, natural 

 selection tending to preserve those that are favourable. 

 What we should expect according to this theory is, that 

 only such characters would be perpetuated as are the 

 subject of natural selection, and that as soon as they cease 

 to be subjects of selection they would tend to disappear. 

 This we find is actually the case. 1 To take a very crude 

 example : a child is born with six digits on the hand instead 

 of five. There is very little chance that this child will mate 

 with another individual also possessing six digits. If it 

 does not, this character will be diluted in the offspring. 

 Some of the offspring may again possess six digits, but there 

 is just as little chance that they will marry other individuals 

 with six digits, so that this variation is certain to be elimi- 

 nated sooner or later, as it is not an advantage to the 

 individual or a subject of natural selection. 



According to this theory, the mean characters of the race 

 should be more uniform where the individuals travel long 

 distances and spread over large areas of country ; where, hi 

 fact, there is less chance of mating between near relations, 

 and a better chance of a general mixing of the individuals 

 of the race. On the other hand, where individuals remain 

 in the place where they are born and do not travel, marked 

 differences should appear between races that obviously have 

 had a common and comparatively recent origin. This we 

 find to be the case. The snails in Madeira, for instance, vary 

 in almost every different valley. They are hermaphrodites, 

 and moving very slowly and for short distances only, pro- 

 bably do not travel out of their own valley. Every individual 

 is capable of fertilising every other individual, and so in- 

 breeding must occur to a very considerable degree. The 

 result is, that in comparatively few generations, the snails 

 in one valley have become differentiated from those in the 

 next valley. On the other hand, among mammals ranging 

 over considerable areas the characters are more uniform, 

 and in order to find varieties in the same species, we 



1 See p. 66. 



