376 EVOLUTION OF PHYSICAL CHAKACTERS 



on who will leave most descendants. It is thus of interest to 

 investigate the frequency of polygamy among the races of the 

 first two groups. This point has been inquired into by Professor 

 Hobhouse and his fellow authors. Polygamy was distinguished 

 by them into general and occasional polygamy, and though it is 

 very difficult to arrive at any exact figures their general conclusions 

 may be summed up as showing that, in all the subdivisions con- 

 sidered by them, general and occasional polygamy was found in 

 from 80-95 per cent, of the cases. General polygamy increases 

 among the races here placed in the second group, though there 

 is no strong correlation between the degree of polygamy and the 

 economic stage. We observe in fact a wide prevalence of polygamy 

 among all races of "any economic stage, and upon the basis of these 

 figures we must regard the differential fertility arising from 

 polygamy as a factor of great importance. 



It seems, therefore, that within the first and second periods 

 there were two tendencies at work towards the evolution of 

 varieties and towards the maintenance of the various types. 

 The evolution of varieties was due in the first place to the spread- 

 ing of man. As soon as spreading was complete, a tendency 

 towards a cessation of progress must have set in. That further 

 evolution took place was due chiefly to migration, with the results 

 of which we have yet to deal. Once varieties had been evolved, 

 apart from migration, and apart from changes in climate and 

 progress in skill, which only slightly and at long intervals modified 

 the surroundings, there was little basis for further evolution. Man 

 had mastered his surroundings up to a point and had made his 

 position secure in the circumstances under which he lived and the 

 tendency was towards the preservation of the types which had 

 achieved these results in different places. 



5. In the third of our three periods there has been a remarkable 

 change. We find that there has been a gradual moving away 

 from the conditions under which in normal times there is in the 

 races of the first and second groups a rigorous elimination of those 

 types which depart from the mean. Lethal selection has come 

 largely to take the form of selection through disease. 1 Though 

 polygamy gradually ceases to be a factor of importance, other 

 forms of differential fertility become prominent. 



1 For evidence that disease leads to a selective death-rate see Popenoe and 

 Johnson, loc. cit., pp. 124 ff. 



