CONTRIBUTIONS TO THE THEORY OF EVOLUTION. 
259 
variation and correlation is published. Meanwhile Mr. Filon and I have shown 
that even a random selection of one organ alters the whole system of correlated 
organs.* Hence genetic (reproductive) selection indirectl}’ modifies not only organs 
A and B, but all correlated organs. These modifications must be consistent wdth 
the maintenance of stamina, physique and fitness to the environment, if the change 
is not to be counteracted by natural selection. 
So far as man is concerned, I have shownt that in the case of civilised man, the 
selective death-rate—he., natural selection—does not appear to counteract repro¬ 
ductive selection. A small element of the population produces the larger part of the 
following generation. I thus concluded that fertility were inherited, reproductive 
selection was not only a factor of evolution, but in civilised man a very sensible 
factor, he., an apparently incompletely balanced factor. 
In the three years which have intervened since waiting the essay just referred to, 
members of the Department of Applied Mathematics in University College, as w-ell 
as other friends, have occupied their spare time in the collection of data as to fertility 
and fecundity in the cases of man and of the thoroughbred racehorse. About 
16,000 extracts were made in the case of man, and more than 7000 in the case of 
thoroughbred racehorses. In the course of the work, which proved far more laborious 
than we had anticipated, many difficulties and pitfalls appeared. But as a general 
conclusion it seems certain that: Both fertility and fecundity are inherited, and 
'probably in the manner prescribed by the Laiv of Ancestral Heredity. 
The object of this memoir is.to set forth the theory and data by aid of wffiich 
this conclusion was reached. It will be seen that it completes the establishment 
of genetic or reproductive selection as a factor of evolution by determining the much 
disputed point as to whether fertility is or is not inherited. 
I. Theory of Genetic or Reproductive Selection. By Karl Pearson, F.B.S. 
(1.) While the physical result of fertility in an individual is measurable, the 
quality of fertility or fecundity in an individual differs from other physical characters 
in that it does not allow of direct measurements except when the potentiality is 
exerted and the effects recorded. At present we are not able to measure any series 
of organs or characters in individuals and so ascertain their fertility or fecundity. 
At the same time there is little doubt that these characters are functions of the 
physical and measurable organs and characters of the body. Such organs and 
characters we have good ground for supposing to be inherited according to the Law 
* “ Contributions to the Theory o£ Evolution.—IV. On the Influence of Random Selection on Variation 
and Correlation,” ‘ Phil. Trans.,’ A, vol. 191, p. 234 et seq. 
t *■ The Chances of Death and other Studies in Evolution. Reproductive Selection,” vol. 1, p. 63. 
X See ‘ Roy. Soc. Proc.,’ vol. 62, p. 386. 
2 L 2 
