INTERNAL CAUSES OF VARIATION 197 



If this be true, and practical breeders know that it is true, 

 then an interesting and important question at once arises ; 

 namely, What characters are correlated with the highest fertility ? 

 This is important, because these are the ones that will become 

 the dominant characters of the race, certainly unless opposed by 

 the most rigid selection or by other powerful influences. This 

 is genetic selection, an ever-present influence over the line of 

 descent, tending to establish what might be called a natural type. 



Unfortunately we possess no accurate data on this point 

 among domestic animals, but Pearson's work x among men and 

 plants is sufficient to settle the principle that such a definite 

 influence exists. 



He finds, for example, that daughters are not taller than 

 their mothers, but that they are taller than wives in general. 

 Now not all wives are mothers, and these data mean simply 

 that taller women are on the average more fertile. There is 

 thus some correlation between fertility and stature. This is 

 genetic selection, and under it the stature of women (English) 

 may be expected to gradually increase until such correlation is 

 satisfied, unless held back by other influences. 



Mothers are less variable, but daughters more so, than 

 wives in general ; that is, progressive selection exists, for not 

 all daughters marry, and not all who marry produce young. 

 If the standard deviation from the race were the same for each, 

 then no selection would be involved, but it is progressively less 

 from daughter to wife and on to mother. The difference between 

 daughter and ivife is due to preferential mating, the especially 

 ugly individuals being less likely to find a mate ; but the differ- 

 ence between wife and mother is due to relative fertility. 



The fact that in general the mother is nearer the average 

 than is the wife shows that the race is fairly stable ; but the 

 fact that wives are shorter than mothers has but one meaning, 

 that in respect to stature the race is yet unstable. 



Extensive studies in eye color indicate that dark-eyed indi- 

 viduals, both men and women, are slightly more fertile than are 

 the lighter-eyed. This means that the dark-eyed will progress 

 (increase) upon the light-eyed and the race will grow darker-eyed, 



1 Pearson, Grammar of Science, pp. 441-445. 



