308 AKNTJAL REPORT SMITHSONIAN INSTITUTION, 1955 



tion, produce a desirable effect ; and conversely, a particular gene that 

 in most combinations has a favorable effect may get into a bad com- 

 bination. The geneticist can scarcely appraise the absolute, over-all 

 effect of a gane; it must be judged by the company it keeps. 



Galton was the first person to try to assess the relative power of 

 heredity and environment in determining a person's characteristics. 

 From his studies of the familial occurrence of genius and special 

 talents, and from comparisons of one-egg and two-egg twins, he was 

 convinced that "the power of nature was far stronger than the power of 

 nurture, when the nurtures of the persons being compared were not ex- 

 ceedingly different." There is no reason to alter this judgment today, 

 although one may amplify it. Thus, in general, physical traits are 

 most rigorously determined by heredity, mental traits are more modi- 

 fiable by environment, and social traits and personality, although still 

 clearly affected by heredity, are most readily suppressed, diminislied, 

 or enhanced by past experience and present environment. A similar 

 hereditary nature, as seen in "identicaP' twins, generally leads through 

 similar development to similar characteristics, provided the individu- 

 als are not subjected to radical differences in their environment. 



It is clearly necessary to clarify the meaning of that term "radical" 

 in the present context. A pair of blond one-egg twins may be tempo- 

 rarily separated, one kept indoors, the other sent for a vacation at a 

 beach. In a few weeks one is sunburned and very much darker than 

 his twin. This does not mean that environment determines complexion 

 and that heredity has nothing to do with it. It means that for blonds 

 the amount of sunlight can constitute a "radical" difference of environ- 

 ment. Yet not all characteristics are affected by this particular "radi- 

 cal" difference of environment. It requires careful observation to 

 determine whether a specific difference in environment can affect a 

 characteristic or not, and it must be determined separately for each 

 characteristic. The effects of environment are inextricably interwoven 

 with the effects of the genes in the formation of a person's character- 

 istics. 



Only by mutation do new sorts of genes come into existence, and 

 without it there could be no long-continued process of evolutionary 

 change, eugenic or otherwise. The mutation rates for several human 

 genes have been estimated. Their average is about 1 in 50,000 per gen- 

 eration; that is, to take a specific example, in a population of 50,000 

 persons 1 person has a gene for hemophilia which was not inherited 

 from either parent, but instead arose by mutation from a corres]5ond- 

 ing gene for normal blood clotting. Hence, unless one could somehow 

 prevent mutation to it from occurring, no measures can be successful 

 in eradicating an undesirable gene from the population. At best, its 

 frequency can be reduced to the level of the rate of mutation to it. 



