ON THE EFFECT OF A DIFFEKENTIAL FEKTILITY 
ON DEGENERACY. 
By KARL PEARSON, F.R.S. 
A NEW YEAR'S GREETING TO FRANCIS GALTON, 1910. 
(1) I HAVE indicated in several earlier papers* the very marked changes 
which are produced when there is correlation between fertility and any character 
in a species. I have termed the changes which result in the mean and variability 
of the character genetic or reproductive selection, and have shown that if such 
correlation exists reproductive selection may wholly defeat or largely neutralise 
the results of natural selection. In actual wild life, it is very difficult to find any 
character sensibly correlated with fertility, and it would appear that the low value 
of correlations of this character is an essential condition for rapid progress by 
natural selection. Nature has in some way — at present not clear to us — prevented 
this reversal of the survival of the fitter by suppressing all marked correlation 
between the physical characters and the fertility of a species. Half a century ago, 
I think, this absence of correlation between fertility and physique would have 
been found to hold for man in this country. It is practically certain that it does 
not hold to-day. Artificially a differential fertility has been created ; the better 
mental as well as physical characters can be shown to be associated with a lessened 
fertility and a reproductive selection has been called into play, which not only 
impedes, but possibly reverses natural selection. The object of the present paper 
is to obtain — at any rate to a first approximation — some measure of the secular 
changes in a race which must flow from a correlation of fertility with any character 
of an organism. The problem in the case of bisexual reproduction is not a wholly 
straightforward one. It is influenced in the first place by the intensity of assorta- 
tive mating in the species. In the next place we have to ask : Does the fertility 
depend on the intensity of the character in one or in both parents ? And lastly 
we must find a reasonable form for the relation between fertility and the intensity 
of the character. These points will be considered in the following sections. 
* See Phil. Trans. Vol. 187, p. 258 ; Vol. 192, pp. 259, 314 ; R. S. Proc. Vol. 59, p. 303. The 
Chances of Death, Vol. I. p. 63 et seq. 
