258 
PROFESSOR KARL PEARSON, MATHEMATICAL 
Introductory. 
I UNDERSTAND by a factor of evolution any source of progressive change in the 
constants—mean values, variabilities, correlations—which suffice to define an orsan 
or character, or the interrelations of a group of organs or characters, at any stage in 
any form of life. To demonstrate the existence of such a factor we require to show 
more than the plausibility of its effectiveness, we need that a numerical measure of 
the changes in the organic constants shall be obtained from actual statistical data. 
These data must be of sufficient extent to render the numerical determinations larcre 
O 
as comj)ared with their probable errors. 
In a “ Note on Reproductive Selection,” published in the ‘ Roy. Soc. Proc.,’ vol. 59, 
p. 301, I have pointed out that if fertility be inherited or if it be correlated with 
any inherited character—those who axe thoroughly conversant with the theory of 
correlation will recognise that these two things are not the same—then we have a 
source of progressive change, a vera causa of evolution. I then termed this factor 
of evolution Rejjrod.uctive Selection. As the term has been objected to, I have 
adopted Genetic SelectioJi as an alternative. I mean by this term the influence of 
different grades of reproductivity in producing change in the predominant type. 
If there be two organs A and B both correlated with fertility, but not necessarily 
correlated with each other,* then genetic or reproductive selection may ultimately 
cause the predominance in the population of two groups, in which the organs 
A and B are widely different from their primitive types—‘ widely different,’ because 
reproductive selection is a source of ‘progressive change. Thus this form of selection 
can be a source, not only of change, but of differential change. As this differentia¬ 
tion is progressive, it may amount in time to that degree of divergence at which 
crossing between the two groups begins to be difficult or distasteful. We then 
reach in genetic or reproductive selection a source of the origin of species. 
When I assert that genetic (reproductive) selection is a factor of evolution, I do 
not intend at present to dogmatise as’ to the amount it is playing or has played in 
evolution. I intend to isolate it so far as possible from all other factors, and then 
measure its intensity numerically. If this be sensible, then the demonstration that 
it is a factor is complete. How far it may be held in check by other factors— 
e.g., natural or sexual selection—is a matter for further inquiry. If three forces, 
Fj, Fo, F 3 hold a system sensibly in equilibrium, then Fi cannot be asserted to be 
non-effective because no progressive change is visible; its absence would soon bring 
to light its effectiveness. 
The manner in which genetic (reproductive) selection is to some extent held in 
check will be clearer when my memoir on the influence of directed selection on 
* If Joi, be the correlation of two organic characters A and B, and C be third character, there is a 
considerable range of vadues of and tor which may be zero (see Yole, ‘ Roy. Soc. Proc.’ 
\ol. 60, p. 486). 
