i.] ANTECEDENTS 33 



between close harmony and wide contrast, in 

 which sexual instinct acts most powerfully ? It 

 appears from the facts in this chapter, that the 

 marriages of parents of the scientific men on 

 my list actually tended to produce differentiation 

 and purity of race. My data concerning the 

 parents of men of other groups are insufficient 

 to enable me yet to give comparative results 

 showing how far the selective sexual instincts 

 of the population generally would thwart, be 

 indifferent to, or co-operate with the influences 

 of future social restrictions on unsuitable mar- 

 riages, or encouragement of suitable ones. 



PRIMOGENITURE, &C. 



The following statement shows, in percentages, 

 the position of the scientific men in respect to 

 age among their brothers and sisters : 



Only sons, 22 cases ; eldest sons, 26 cases ; 

 youngest sons, 15 cases. Of those who are 

 neither eldest nor youngest, 13 come in the 

 elder half of the family ; 12 in the younger half ; 

 and 11 are exactly in the middle. Total, 99. 



It further appears that, at the time of the 



