INHERITANCE IN MAN 319 



all the tendencies, limitations, and qualities that are bequeathed to the 

 individual as maternal and paternal factors at the time of conception. 

 Once a given sperm and egg are fused in fertilization, the full complement 

 of factors is fixed, and the new individual's "nature" is irrevocably 

 determined. From this time on, through the 9 months of embryonic 

 development, through the early years of babyhood and childhood, with 

 their psychological conditioning and training by example, through youth 

 and schooling, many types of environmental influence are shaping the 

 development and phenotypic expression of the particular set of factors 

 that constitutes the individual's "nature." The sum of all these environ- 

 mental influences is nurture. 



Since both nature and nurture are absolutely necessary, there is no 

 point in asking which is the more important for the existence of the in- 

 dividual. Their real antithesis lies in the different and complementary 

 ways in which they determine the characters of the individual and in 

 which they are or may be manipulated to modify the race. All the evi- 

 dence from experiment, from checked observations, and from a detailed 

 knowledge of embryonic development strongly supports the conclusion 

 that only the individual's "nature" can be biologically inherited by his 

 offspring. Not any of the influences of nurture, however much they 

 modify, dwarf, or develop the individual, can in any appropriate way 

 modify the factors in his germ cells. This is, of course, a declaration that 

 no characters, qualities, or modifications that are due to the environment 

 or that are acquired by training, practice, or injury can be biologically in- 

 herited. The only way an individual's nature can be determined is by 

 controlling what factors shall be brought together when the zygote is 

 formed, and this, of course, can be done only by selecting parents with 

 the requisite genotypes. 



It has just been stated that there is "no direct way" in which the 

 environment can influence or change the nature of an individual or of a 

 racial stock. There is, however, an indirect way in which various environ- 

 mental influences can tremendously affect the genotypic qualities (average 

 "nature") of a race. This is by selection. Whenever any environmental 

 influence causes some group of individuals to reproduce more or less 

 than its proportionate share of offspring, then that influence will 

 increase or decrease, for the race as a whole, the proportion of whatever 

 factors are peculiar to or are more concentrated in that group of 

 individuals. 



Among the numerous influences that civilization and social organiza- 

 tion introduce and that indirectly and unconsciously cause selection in 

 human stocks and in national or cultural groups are war, immigration, 

 social and sanitary legislation, and a differential birth rate. We shall 

 look at two of them very briefly. 



