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 l Or Riddle's paper on Pigment and Mendelism 1
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.
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. Ii, 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 !