34<^ AN INTRODUCTION TO MODERN GENETICS 



The parts played by heredity and environment in the development of 

 several other characters of man have been studied in the same way as 

 has just been described for I.Q. In particular the study of twins has 

 proved extremely revealing in several other connections.^ A very im- 

 portant part in this study has been played by the Medico-Genetical 

 Institute in Moscow/ where a large team of doaors collaborated with 

 scientists and statisticians in the study of "normal" or medically 

 important characteristics. They found, for instance, that identicals 

 show greater similarity than fraternals in age at teething, age at learn- 

 ing to sit or to walk, in susceptibility for certain diseases (e.g. scarlet 

 fever) but not for others (e.g. tuberculosis).^ Again, twins are the ideal 

 human experimental material, since it is possible to treat one twin and 

 keep the other as a control which is known to be exactly comparable. 

 This method has as yet been very little employed. One of the most 

 promising experiments dealt with different methods of teaching. One 

 twin is allowed to build models with wooden blocks, imitating a struc- 

 ture which is in front of him. The other does the same, but paper is 

 glued over the structure so that he cannot see how it was made. It is 

 reported that after two months (half an hour's play a day) the twins of 

 the second group were considerably superior to those of the first, both 

 in model making and in other activities whose connection with model 

 making was not at all obvious. 



B. THE GENETIC STRUCTURE OF HUMAN POPULATIONS 



I. Race^ 



Man is a very variable animal. An Australian aborigine, a Chinaman 

 and a West European differ as much from each other as do many 

 related species of monkeys. But all the living types of man are mutually 

 fertile, and it is usual to classify them all in a single species Homo 

 sapiens. 



The local varieties of man are spoken of as races. But, because we 

 know more, or want to know more, about man than about, say, Lyman- 

 tria, and because there are very many more specimens available for 

 examination, the concept of race breaks down when we try to apply it 



^ Rev. Verschuer 193 1. ^ Cf. Muller 1935^. 



^ Evidence for inherited susceptibility to this disease has been found in other 

 cases. 



* General references: Castle 1930, Fisher 1930, Freeman 1934, Haldane 1934, 

 1938, Hogben 1938, Holmes 1934, Huxley and Haddon 1936. 



