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 suc- 

 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 is in 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, i. 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 8 1 of The Chances 

 of Death, the la.st 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 vcra 

 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 



