CONTRIBUTIONS TO THE THEORY OF EVOLUTION. 
279 
II. On the Inheritance of Fertility in Mankind. By Karl Pearson, F.R.S., and 
Alice Lee, B.A., B.Sc. 
(8.) In commencing an investigation of this kind where the results to be expected 
were quite unknown to us, but wLere we had reason to believe that the cqopon'ent 
strength of inheritance must be very small, we considered that the first thing to be 
done was to investigate the largest possible amount of material. Thus the probable 
errors of our results would be very small and any, however small, correlation between 
fertility in parent and offspring would be brought to light. Attempts might then be 
made to strengthen any correlation discovered by removing so far as possible one 
after another the various factors tending to screen the full effect of the inheritance 
of fertility. 
Such factors are for example ; 
(a.) The age of both husband and wife at the time of marriage. The real fertility 
may be screened by late marriages of one or both parents. The relation of fertility 
to age at marriage has been dealt with by several writers, notably by Duncan and 
Ansell.'“' 
(h.) The duration of marriage. The data may be taken from a marriage not yet 
complete, both parents being still alive. Or from a marriage which is complete one 
or both parents being dead. In the former or the latter case the marriage may be 
complete so far as fertility is concerned, i.e., details of offspring may be available till 
the wife has reached the age of 50 years, which for statistical purposes may be taken 
as an upper limit to fecundity. 
(c.) Restriction of fertility during marriage. It has been shown in a paper on 
Reproductive Selection! that there is evidence of the sensible influence of this factor 
in man. It tends to give fictitious values to the fertility of the younger, rather than 
the elder generation, and so obscures the correlation. 
We have accordingly two problems before us : 
(i.) Supposing these and other factors tending to screen the effects of reproductive 
selection to exist, can we show that it still produces sensible effects in the case of 
man, and thus demonstrate that fertility is really inherited ? 
(ii.) Can we by eliminating these factors so far as possible obtain a lower limit to 
the coefficient of heredity in the case of fertility, and ascertain whether it 
approximates in value to what we might expect from the Law of Ancestral 
Heredity ? 
The first impression of the reader may be that it is only needful to select the 
* J. Mathews Duncan, ‘Fecundity, Fertility, Sterility and Allied Topics,’ second edition, 
Edinburgh, 1871. Charles Ansell, Junr., ‘ Statistics of Families in the Upper and Middle Classes,’ 
London, 1874. 
t ‘ The Chances of Death and other Studies in Evolution,’ vol. I, pp. 77, 89. 
