45^ EVOLUTION, GENETICS, AND EUGENICS 



ancestry might show in other characteristics, such as curly hair, broad 

 nose, thick lips, etc. It is difficult to get any wholly satisfactory 

 evidence either for or against this explanation. That published by 

 Davenport can scarcely be considered conclusive, for the data studied 

 are derived from a population in which illegitimacy, by Davenport's 

 own statement, is as high as 72 per cent. On the whole, it seems prob- 

 able that segregation of skin pigmentation in mulattoes is either in- 

 complete or rarely complete, because multiple or modifying factors 

 are involved." 



Having given examples of pedigrees of physical characters exhibit- 

 ing all of the principal modes of Mendelian heredity, let us now pass to 

 a consideration of the facts about the heredity of mental traits in 

 man as revealed by pedigree studies. 



HEREDITY OF MENTAL TRAITS IN MAN 



The studies of the heredity of mental traits in man consist prin- 

 cipally of those of exceptional genius and of definite mental defects 

 and abnormalities. Very little effort has been expended upon the 

 heredity of normal average intelligence, such as the great majority of 

 us possess. 



Long ago, in 1869, Sir Francis Galton published a fascinating book 

 on The Heredity of Genius, in which he presented a large number of 

 pedigrees of families showing definite and special types of genius, such 

 as musical genius, genius for astronomy and mathematics, genius for 

 science, etc. Among those described in some detail are the Bach 

 family of great musicians in Germany, the Herschell family of great 

 astronomers in England; and he might have added his own pedigree 

 composed of the interrelated families of Darwins, Galtons, and Wedge- 

 woods, the present generation of which consists of many outstanding 

 and influential persons (Fig. 85). No family today shows more plain- 

 ly than this one what can be done in the way of racial improvement 

 through a certain amount of concentration of good germ plasm by 

 judicious inbreeding (cousin marriages), when the inbreeding stocks 

 are strong and sound. Charles Darwin, the most eminent member of 

 this family connection was the son of an eminent physician, a member 

 of the Royal Society. One of Darwin's grandfathers was the well- 

 known Erasmus Darwin, referred to in the History of Evolution (chap, 

 ii). The other grandfather was Josiah Wedgewood, F.R.S., founder 

 of the Wedgewood potteries. Charles Darwin married his own cousin, 

 Emma Wedgewood. Four of Darwin's sons are eminent members oi 



