Data for the ProbJern. of Ecolution in Man. 



29 



classes, and marrying within the district or set cannot, I think, intro- 

 duce much local race influence. 



I accordingly tm^ned the problem round, and asked if there appeared 

 any reason why, in collecting my material, I should have selected uncon- 

 sciously homogamous marriages. Xow, clearly, I was much more likely 

 to get a return from a large than a small famil}^ ; one member of a 

 family of eight was more likely to take interest in the matter than one 

 member of a family of two or three. It seemed to me that out of 10,000' 

 families I was clearly more likely to get returns from the larger families 

 than the smaller ones, remembering that 26 per cent, of the families 

 correspond to 50 per cent, of the ofl"spring. Hence arose the im- 

 portant problem is fertility associated with homogamy ? When like 

 mates with like, is the number of progenj^ greater than when like and 

 unlike mate ? Put in this way the problem appears to be of first class 

 importance for the theory of evolution. If homogamy rather than 

 heterogamy results in fertility, then we get a first gleam of light on 

 what may be ultimately of -sital significance for the differentiation 

 of species. ^Mien any form of life breaks up into two groups under the 

 influence of natural selection, what is to prevent them intercrossing, 

 and so destropng the differentiation at each fresh reproductive stage '2 

 Various hjrpotheses — isolation, recognition marks, physiological selec- 

 tion — have been propoimded. But if like mating with like connotes 

 greater fertility, the answer to the problem of diff'erentiation would be 

 simply summed up in differential fertility. AVe should have merel}^ a 

 case of genetic selection arising from the correlation of fertility and 

 homogamy. 



AVe must be careful, however, not to rush to any conclusions without 

 ample data. In particular we must not confuse homogamy with endo- 

 gamy. Nor must we argue that relatives being closel}' alike, kin-mar- 

 riages would mean increased fertility. DarAvin has shown statistically 

 that, as a rule, self-fertilised flowers are more sterile than cross-fertilised ; 

 kinship, sameness of stock, means likeness of characters, but likeness of 

 characters does not necessarily indicate sameness of stock. It is quite 

 possible for like indi^-icluals of diff'erent stocks to be fertile inter se, 

 and like indiA-iduals of the same stock to be in part or wholly sterile inter 

 se. In fact, that homogamy means fertility may in all or certain forms 

 of life be dominated by a more potent rule, namely, that endogamy 

 means sterility. The two statements are not contradictory if we 

 interpret homogamy to mean the mating of two like individuals, not 

 in the first place like because they come of the same stock. In fact, if 

 a man seeks a ^viie of statm-e corresponding to his own, he will as a 

 rule have a larger field of suitable mates in the general population than 

 within his own limited kin. Bearing this point in mind, I novr turn to 

 the somewhat narrow data available at present for the influence of 

 homogamy in the matter of stature on fertility in man. 



