October 28, 1921] 



SCIENCE 



393 



sirable, where possible, to breed experimentally 

 mammals, if any are available, which show the 

 same trait that we are studying in humans. 

 This is often possible, and such study will 

 afford a control of results gained on man. 

 Thus have been studied hare-lip in dogs, fecun- 

 dity in sheep, instincts in dogs, polydactylism 

 in fowls. 



In other studies the method employed will be 

 that of accumulation of statistics, their tabu- 

 lation and analysis. Thus we investigate 

 mate selection, the relative fecundity and 

 relative mortality of the various stocks and 

 the effect on the germ plasm of a country of 

 the different immigrant races. 



Some of the results of analytical study of 

 these eugenical data are fairly well established. 

 A few clearly simple Mendelian traits have 

 been found. Such is eye color in which brown 

 is dominant over its absence. It is possible 

 that in some cases additional factors may be 

 present, but the rule serves as a first approxi- 

 mation. Dominant, also, appears to be curli- 

 ness of the hair as contrasted with recessive 

 straight. And there are various diseases and 

 defects that appear either as simple dominants 

 or recessives, such as abnormalities in number 

 and form of fingers and toes, which are mostly 

 dominant over the normal condition; various 

 defects of the eye such as cataract, certain 

 types of congenital deafness, various abnormal- 

 ities of skin, and hair and nails. 



Other, and probably many other, traits are 

 due to multiple factors — so often this is true 

 as to suggest the hypothesis that in mammals, 

 as contrasted with insects, traits are genetically 

 relatively complex. Thus stature and build 

 and proportions of parts and pigmentation of 

 hair and skin are dependent on multiple fac- 

 tors. Indeed, there seems to be evidence that 

 uegro skin color is dependent upon two pairs 

 of factors which merely reinforce each other. 



Other traits are associated with sex in the 

 remarkable fashion called sex-linked. That is, 

 they are usually found only in the male sex 

 and are inherited through the mother, though 

 she, herself, is not affected. In such cases one 

 usually finds male relatives of the mother who 

 are affected. Such are color blindness, hemo- 



philia and atrophy of the optic nerve. The 

 facts of sex-linked heredity bring home, even 

 to the layman, the lesson that heredity is a 

 matter of the gametes ; and that bodily appear- 

 ance often gives no hint of the nature of the 

 particular germ-cells carried and, in so far, of 

 what the inheritance shall be. The parents of 

 an albino may have pigmented hair and skin, 

 but both carry gametes which lack the capacity 

 of forming pigment. 



Our knowledge of the inheritance of these 

 physical traits is sufficiently precise to be ap- 

 plied practically in cases of doubtful parent- 

 age. If the child, the known mother and both 

 of the putative fathers can be seen, and some 

 inquiry be made as to family stock of the three 

 adults a decision can generally be rendered 

 with a high degree of certainty ranging from 

 75 to 99 per cent. For usually there will not 

 be one critical trait merely but several traits 

 whose combined evidence will be overwhelm- 

 ing. Already the Eugenics Record Office has 

 been asked to answer certain questions about 

 the inheritance of traits in a case of a claimant 

 who maintained that he was the son of a 

 wealthy man who died without known heirs. 

 As lawyers get more used to the idea of utiliz- 

 ing the advances of knowledge for evidence, it 

 is probable that eugenical knowledge will be 

 more and more called upon. 



Not only of the physical traits referred to 

 above but also of those of behavior we are 

 learning the hereditary basis. It appears prob- 

 able, from extensive pedigrees that have been 

 analyzed, that feeble-mindness of the middle 

 and higher grades is inherited as a simple re- 

 cessive, or approximately so. It follows that 

 two parents who are feeble-minded shall have 

 only feeble-minded children and this is what is 

 empirically found. It has been urged against 

 this finding that it is improbable that so com- 

 plicated a thing as full mentality depends upon 

 only one factor. On the other hand, a consid- 

 eration of the effect of internal secretions, of 

 thyroid, of hypophysis and others leads to the 

 conclusion that a brain with well differentiated 

 intellectual centers may fail of complete de- 

 velopment because of the absence of proper 

 developmental impulses of glandular origin. 



