94 ANALYSIS OF THE FOUR PRINCIPLES. 
’ 
sively changed, * * * owing to reproductive selection,’’ we are 
led to suppose that reproductive selection depends in part on that 
fertility which depends on the fitness of the pelvis of the mothers for 
fulfilling the functions of pregnancy and parturition. We also reflect 
that in all mammalian mothers there are organs on the fitness of 
which for furnishing nourishment to the young must depend the stc- 
cess of the process of reproduction. Does this form of survival of the 
fittest (or failure of the least fitted to leave mature offspring) come 
under the principle of reproductive selection as defined by our author? 
Again, we know that, in the case of many species, the males who are 
best fitted for driving off rivals, or who are best fitted for attracting 
the females, leave the most offspring. Is their success in reproducing 
an example of reproductive selection? According to Darwin’s nomen- - 
clature this process is called sexual selection; but Karl Pearson’s 
definition of sexual selection is: ‘‘ All differential mating due to taste, 
habit, or circumstance, which prevents a form of life from freely inter- 
crossing. If this goes on for a sufficient period during which differen- 
tiation of type isin progress, the principle of correlation may account 
for a sufficient differentiation in reproductive organs or functions 
to render intercrossing physiologically or mechanically difficult, dis- 
tasteful, or even impossible, and accordingly give rise to the relative 
or absolute sterility of the differentiated types, 7. e., to the origin of 
species.” (Grammar of Science, p. 418.) It is very pleasant to find 
such full recognition of the principle of isolation, even though it be 
under another name; but the question at present is, where does he place 
the survival through successful propagation of those who are best able 
to win partners? Does he classify it as a form of natural selection, or 
as a form of sexual selection, which has been so defined as to be equiv- 
alent to isolation; or is it included under reproductive selection? 
After reading the passage already quoted from page 81 of The Chances 
of Death, the last of these three suppositions seems the most probable ; 
and still more so after reading the following definition of reproductive 
selection given on pages 65 and 66: 
If there be any sensible correlation between fertility and the size of any organ or 
intensity of any characteristic in male or female—that is, if deviations in excess 
(or defect) from the mean of this organ correspond to a greater fertility than devi- 
ations in defect (or excess)—then under the action of heredity we have a vera 
causa of progressive evolution in this organ; for an increasing number of individ- 
uals will be born with the organ in excess (or defect), and consequently the mean, 
and most probably the variation about the mean, of the general population will be 
progressively modified. The result is somewhat similar to that due to artificial 
selection in the case of domestic animals, where without extermination greater 
fertility is given to selected parents by pairing them only, or by pairing them more 
