Eugenics as a Creed and the Last Decade of Galton's Life 395 



I don't understand Wee ling's temper; his brother and sister are very frisky, but angelic 

 in temper. I gave you Wee Ling because we had decided he was the most intelligent*. I hope 

 you got the letter about the midparent. Yours always affectionately, Karl Pearson. 



Have you seen the Whethams' book 1 Or Riddle's paper on Pigment and JVTendelism t 



7, Well Road, Hampstead, N.W. October 31, 1909. 

 My dear Francis Galton, I think I have got out a general theory of the problem you 

 suggested on the following lines : Given a differential fertility, what changes will it make 

 ( 1 ) in the mean and variability of the offspring and (2) how will it change the coefficient of heredity 

 in the population. From these results I can at once deal with your special problem of a certain 

 percentage of the population having a desirable character but lessened fertility. The chief 

 difficulty is the form of the law of fertility. Now the distribution of the size of families in 

 any population is not Gaussian, it rises steeply and falls slowly, thus : 



It certainly does not closely approach any mere straight line relation. It would therefore 

 seem reasonable to suppose such a curve to give the fertility distribution with any character. 

 It seemed to me better than taking a straight line to see where we arrive by supposing the 

 fertility is somewhere a maximum and drops in Gaussian fashion on either side of the modal value. 

 To take the cases in which the fertility is greatest with the worst values of the character we 

 have only to place the fertility curve much to one side, e.g. 



Oza.r&cter Axis 



To the right of the line AB no individuals occur and accordingly the fact that there would 

 be fertility, if individuals occurred, is of no importance. We can also take the case when a very 

 small part of the population is fertile, thus : 



* See our Vol. n, p. 76, as to breeding for intelligence in dogs. Unfortunately Wee Ling, 

 while markedly intelligent, and long a dear friend of the biographer's family, turned out to be 

 incapable of reproducing his kind ! 



50—2 



