INHERITANCE IN MAN 



323 



on the other are a very good source of data because uniovular twins should 

 have identical genotypes. Biovular twins should differ in genotypes as 

 much as do ordinary brothers and sisters, and both types of twins should 

 have the same degree of similarity of environment for the two individuals 

 concerned. The evidence from these data very strongly emphasizes that 

 most of an individual's capacities and qualities are determined by his 

 nature ; but this method gives little information as to the particular genes 

 that are concerned or their mode of interaction. 



3. Results obtained by the experimental breeding of other animals may 

 be applied to the interpretation of human heredity. In all organisms in which 

 experimental breeding and selection are possible there is positive and 



Fig. 22.2. Inheritance of ear size and eyelash length. Large ears dominate small ears, and 

 long lashes dominate short ones. 



unmistakable evidence of the role and importance of the genetic constitu- 

 tion (nature). Selection is demonstrably able to increase or decrease the 

 learning ability of rats, the resistance or susceptibility of mice to cancer, 

 and the longevity of Drosophila. It has been shown in these same animals 

 that, so long as the genotype is unmodified, the environment can produce 

 no permanent change in the stock. When we reflect upon how closely 

 and invariably man's structure and functioning conform to the same laws 

 and principles that apply to other organisms, it seems extremely unlikely 

 that man would show a different relationship in the interaction of his 

 nature and nurture. 



The eugenics movement. Because of the promise for racial better- 

 ment that an application of human genetics would appear to hold out 

 for society, many people have become actively interested in the effort 

 to make such an application on the basis of our present knowledge. This 

 practical program of improving the human stock through an application 

 of the findings of human genetics has been termed eugenics — the science 

 of being well born. 



Theoretically, there is little reason to question the fact that if man- 

 kind could be subjected to the same procedures of rigid selection and 



