EVOLUTION 463 



the number of a moth's eggs hatched out under a given 

 environment, would undoubtedly be the best and most 

 direct method of answering the problem by experiment. 

 Great care would, however, in view of my remarks on 

 p. 449, have to be taken to preserve the same environ- 

 ment for moths and eggs in successive generations. Fail- 

 ing such experiments at present, I have turned to the 

 records of men and horses for evidence with regard to 

 the inheritance of fertility and fecundity. 



If we take the fertility of a given pair of human beings 

 we find that it is largely affected in civilised communities 

 by social customs and habits. The fertility of the given 

 pair depends upon the age of both husband and wife at 

 marriage ; it depends upon the duration of the marriage, 

 and also on the amount of restraint during marriage. 

 These and other factors tend to screen the intensity of 

 what we may term the inheritance of constitutional 

 fertility in man. Further, when we come to deal with 

 the case of the human male, the fertility is not exhausted 

 by monogamic union ; our attempt to correlate the size 

 of a father's family with the size of his son's will only 

 give results when the man is partially sterile, or becomes 

 so before the end of the fecund period of marriage. It 

 would be impossible here to enter into the whole detail of 

 the investigations made on data drawn from the peerage, 

 baronetage, landed gentry, and family histories,^ but the 

 following three results may be taken to illustrate the 

 inheritance of fertility in man : — 



Mother and daughter, 1000 cases, marriages of both 

 having lasted at least fifteen years. Inheritance of size 

 of family — 



Coefficient of heredity = .213 ±.020. 



Paternal grandmother and grand-daughter, 1000 cases, 

 marriages of both having lasted at least fifteen years. 

 Inheritance of size of family — 



Coefficient of heredity = .1 12 ±.021. 



1 See Genetic Selection, Inheritance of Fertility in Man, Pearson and 

 Lee. Philosophical Transactions, vol. cxcii. pp. 279-2S9. 



