EVOLUTION 465 



cent noted in the case of women, and this is accounted 

 for by the reasons given above. Twenty-seven per cent 

 reduction on .3 would give, as we have seen, .219 for the 

 paternal inheritance; we find only .116. The difference 

 between these two is most probably due to the great per- 

 centage of marriages in which the fertility of either father 

 or son or both is not exhausted by the monogamic union. 

 Assuming that the probability of this exhaustion is the same 

 for both generations, a little algebra shows us that in about 

 27 per cent of cases partial or total sterility in marriages 

 is due to the husband. It is the partial sterility which is 

 inherited and leads to the above correlation between the 

 sizes of a man's and of his father's family. In order to 

 test for another race the reality of genetic selection, I have 

 dealt with the fecundity of thoroughbred race-horses.^ 

 The total fertility could not be ascertained from the stud- 

 books, but only the fecundity, i.e. the ratio of foals surviv- 

 ing to be yearlings to the total number of foals possible 

 under the given conditions. The investigation was more 

 difficult owing to a variety of circumstances peculiar to 

 horse-breeding, but the general conclusions reached are 

 the following : — 



{a) Fecundity is inherited between dam and daughter. 



ib) Fecundity is also inherited through the male line, 

 or the sire hands down to his daughter a portion of the 

 fertility of his dam. 



Thus the latent character fecundity in the male was 

 measured for a horse and for his sire, and found to be 

 strongly inherited. 



That fertility and fecundity are inheritable characters 

 thus seems established; but the existence of this differential 

 fertility is the basis of genetic selection. 



Genetic selection is not only vitally important for the 

 theory of evolution, but it is crucial for the stability of 

 civilised societies. If the type of maximum fertility is 

 not identical with the type fittest to survive in a given 



1 Philosophical Transactions, vol. cxcii. pp. 290-315, " Genetic Selection."" 

 Pearson and Bramley-Moore, On the Inheritance of Fecundity in Thoroughbred 



Race-horses. 



