346 THE INDUCTIONS OF BIOLOGY. 



tions; and if their constitutions were dissimilar, the proba- 

 bility that their married grandchildren will have offspring will 

 be greater than if their constitutions were similar. Or the 

 brothers and sisters from whom these cousins descended, in- 

 stead of severally inheriting the constitutions of their parents 

 in tolerably equal degrees, may have severally inherited them 

 in very different degrees : in which last case, intermarriages 

 among the cousins will be less likely to prove infertile. Or 

 the brothers and sisters from whom these cousins descended, 

 may severally have married persons very like, or very 

 unlike, themselves; and from this cause there may have 

 resulted, either an undue likeness, or a due unlikeness, 

 between the married cousins.* These several causes, con- 

 spiring and conflicting in endless ways and degrees, will 

 work multiform effects. Moreover, differences of segrega- 

 tion will make the reproductive centres produced by the 

 same nearly-related organisms, vary considerably in their 

 amounts of unlikeness; and therefore, supposing their 

 amounts of unlikeness great enough to cause fertilization, this 



* I omitted to name here a cause which may be still more potent in pro- 

 ducing irregularity in the results of cousin-marriages. So far as I can learn, 

 no attempt has been made to distinguish between such results as arise when 

 the related parents from whom the cousins descend are of the same sex and 

 those which arise when they are of different sexes. In the one case two 

 sisters have children who intermarry ; and in the other case a brother and 

 a sister have children who intermarry. The marriages of cousins in these 

 two cases may be quite dissimilar in their results. If there is a tendency 

 to limitation of heredity by sex — if daughters usually inherit more from the 

 mother than sons do, while sons inherit more from the father than from the 

 mother, then two sisters will on the average of cases be more alike in con- 

 stitution than a sister and a brother. Consequently the descendants of two 

 sisters will differ less in their constitutions than the descendants of a brother 

 and a sister ; and marriage in the first case will be more likely to prove in- 

 jurious from absence of dissimilarity in the physiological units than marriage 

 in the second. My own small circle of friends furnishes evidence tending 

 to verify this conclusion. In one instance two cousins who intermarried are 

 children of two sisters, and they have no offspring. In another the cousins 

 who intermarried are children of two brothers, and they have no offspring. 

 In the third case the cousins were descendants of two brothers and only one 

 child resulted. 



