342 AN INTRODUCTION TO MODERN GENETICS 



usually isolated communities living under fairly primitive conditions 

 and have a high degree of inbreeding. We might attach a definite 

 meaning to the term race if we defined it with reference to such groups, 

 but the concept would not be very useful. We should in the first place 

 have to make sure that the characters concerned were hereditarily and 

 not environmentally determined. This is not always certain; it is 

 probable that even the cranial index, that sheet-anchor of the physical 

 anthropologist, can be modified by the environment, since it is less 

 extreme in the American-bom children of immigrants than it was in 

 their parents.^ Even when this point had been disposed of, the races 

 would only be definitely characterized in respect of the points mentioned 

 in their definition; there is no reason to suppose that because a popula- 

 tion is homogeneous for long-headedness it is therefore homogeneous 

 for other characters such as intelligence. Finally, such a concept of race 

 would be a purely statistical one, defined in terms of populations, and 

 could not be applied to individuals. In the first place, different races 

 overlap, so that the longest-headed members of a short-headed group 

 have longer heads than the shortest variants of a long-headed group. 

 Even if we knew an individual belonged to one of the pure races, we 

 could not tell, looking at him alone, which of the races it was. And, 

 further, as we have said, only a few small populations do consist of 

 pure races. The vast majority of the population is made up of indi- 

 viduals who would have to be considered as very complex hybrids 

 showing all sorts of segregation and recombination. 



Thus the attempt to classify mankind into genetically homogeneous 

 groups becomes progressively more difficult as we take account of more 

 genes. If we try to derive a concept of race which will be relevant to all 

 characters of a man, the attempt is a complete failure. Segregation and 

 recombination has gone so far in most sections of the human population 

 that it is impossible to summarize an individual by any description less 

 complete than a specification of his whole genotype. 



2. Gene Differences Between Races 



Nearly all attempts to define races are based on examination of 

 phenotypic characters, since we know much too little about the genes. 

 It is certain, however, that there are differences in the frequencies of 

 some genes in different populations. For instance, the blood groups of 

 many races have been tested.^ Very few "races" are homogeneous, 

 though some American Indian tribes possess no B and A is fairly rare 

 1 Boas 1928. ^ Cf. Davis 1938. 



