310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



ages past, no gene very detrimental to viability or fertility can have 

 become common. Each sort of harmful gene is already rare in fre- 

 quency and is maintained in equilibrium between mutation and selec- 

 tion. The combinations of genes which determine complex traits such 

 as intelligence are even more slowly affected by selection. 



Chance changes in gene frequencies are not of as great eugenic 

 significance in modern times as formerly, for they occur significantly 

 only in small populations, such as the hunting groups of prehistoric 

 man or the isolated rural communities of medieval times. In a study 

 my colleagues Milton Sacks, Elsa Jahn, and Charles Hess and I re- 

 cently made of a Dunker community in Franklin County, Pa., we 

 found clear evidence that the genetic composition of such a group can 

 deviate radically and within a span of three generations from its orig- 

 inal makeup as well as from that of the surrounding population. In 

 such communities even harmful genes might become established by 

 chance ; or even a beneficial gene might be eliminated. Furthermore, in 

 a small population there is of necessity a closer degree of relationship 

 between those who marry. This inbreeding does not of itself change 

 gene frequencies, but it brings the recessive genes out into the open 

 and allows selection to be exercised upon them. If a line is free of 

 harmful recessive genes, this inbreeding will do no harm. (Cleopatra 

 herself came of a number of generations of brother-and-sister mar- 

 riages.) But I have already pointed out that most individuals do 

 carry harmful genes. Hence the effect of inbreeding is in general to 

 increase the number of afflicted persons in the population. Actually, 

 since medieval times, the trend has been in quite the opposite direction. 

 This brings us to genetic intermixture. 



Obviously the trend of the modern world is away from the isolating 

 effects of religion, race, and geography, even though not so obviously 

 from those of politics. Little populations have been merging to make 

 large ones, and migi'ations of tremendous magnitude have produced 

 human melting pots the world around. This has had the effect of 

 making a greater proportion of the recessive genes disappear from 

 view and from exposure to selection. They can now increase under 

 cover to a much higher frequency, until such time as by random mating 

 homozygous persons are produced and selection again becomes active 

 upon these genes. The action of the melting pot and the slowness of 

 assimilation are clearly evident in a study of the present-day American 

 Negro made by C. C. Li and myself. On the basis of the frequencies 

 of certain blood groups and other genetic traits in the AVest African 

 Negroes, the North American Negroes, and the North American 

 whites, it was possible to conclude that the genes of the American 

 Negro are now just over 30 percent derived from the white population, 

 nnd that at the average rate of past gene flow from the white into the 



