464 THE GRAMMAR OF SCIENCE 



Father and son, looo cases, marriages of both having 

 lasted at least fifteen years. Inheritance of size of 

 family — 



Coefficient of heredity = . I 1 6 ± -02 I. 



Now, of course, many marriages are fertile after 

 fifteen years. We really ought to confine ourselves to 

 marriages which last from about twenty to fifty years of 

 age in both husband and wife to get the full strength of 

 inheritance. Unfortunately sufficiently ample data were 

 not forthcoming from our records for such an investiga- 

 tion. Nor even then could we allow for a possible limita- 

 tion of the family. Hence we must conclude that the 

 above values give a minimum limit to the intensity with 

 which fertility is inherited among mankind. Now we 

 may draw some important conclusions from them. 



{a) The inheritance from the mother is just about 

 twice that from the grandmother. This is exactly what 

 we have seen holds for the theoretical intensities .3 and 



.15. 



{b) These minimum values are both about 27 per 



27 



cent less than the values to be expected, i.e. -3 — 7^ x -3 



= .219 instead of .21 3, and .15 — 7^0 -^ -^ 5 = .109 instead 

 of. I 12. 



{c) The woman inherits fertility equally through the 

 male and the female lines ; for the reader must observe 

 that we have correlated the paternal, and not the maternal 

 grandmother with the grand-daughter. 



This is a most important result, we see a sexual 

 character of the female transmitted through the male 

 line and with equal intensity for .112, the approximate 

 half of .213 is what we should expect in the maternal 

 grandmother. This is a case of what Darwin has termed ^ 

 " transmission without development," and we see that such 

 transmission can equal in intensity direct inheritance. 



{d) Lastly, we find the inheritance of fertility from 

 father to son even still further reduced than the 27 per 



1 Descent of Man, second edition, p. 227, "Laws of Inheritance. " 



