276 Mr Punnett, On Nutrition etc. 



We have, therefore, good grounds for the conclusion that mothers 

 between the ages of 19—23 tend to produce relatively more ? 

 infants, whilst somewhat older mothers tend to produce relatively 

 more </ infants. Consequently, early marriages would favour the 

 production of a relatively greater number of $ s, whereas marriages 

 subsequent to the age of 23 would result in a production of g 

 infants relatively greater than the normal. Though I can give 

 no figures on this head, I believe that there is reason to suppose 

 that the largest proportion of early marriages occurs amongst the 

 poorest classes, and that with better social conditions there is 

 associated a tendency to defer the age of marriage. Should this 

 supposition be well founded, it affords an additional explanation 

 of the relative preponderance of % births in group A as compared 

 with group B, and again in group B as compared with group G. 



Summary and Conclusion. 



If the population of London be divided into three portions 

 exhibiting graduated poverty it is found that the proportion of 

 ^to $ infants produced is lowest in the poorest portion, highest 

 in the wealthiest portion, and intermediate in the intermediate 

 portion. The proportion of ,/s is highest of all in a number of 

 births taken from Burkes Peerage, where the nutrition may be 

 supposed to be of the best. From this the alternative conclusions 

 may be drawn : that either more favourable conditions of nutrition 

 (1) may result in a large proportion of <$ births, or (2) may have 

 no effect on the proportion of the sexes, or (3) may even result in 

 a relative preponderance of % births, but that in the last two cases 

 the effect is masked by other factors which affect unequally the 

 different strata of society. Such factors are shewn to exist in a 

 differential infant mortality, a differential birth-rate, and probably 

 also in a differential marriage-age. These factors all tend to diminish 

 the proportion of ^s in the poorer portions of the population, and 

 consequently render the first of the above alternative conclusions 

 improbable. Whether the second or third of the other possible 

 conclusions is to be accepted must remain doubtful so long as we 

 are not in a position to estimate the quantitative effect of the 

 factors given above. From the necessarily rough estimate which 

 he has been able to form, the writer's opinion is that their com- 

 bined effect would not be sufficiently great to mask a preponderance 

 of % births due to better nutrition, and consequently he is inclined 

 to believe that in man at any rate the determination of sex is in- 

 dependent of parental nutrition 1 . In any case its influence can 

 be but small. 



1 Mention may here be made of some interesting experiments recently carried 

 out on white mice by 0. Schultze (Archiv f. Mic. Anat. 1903), who has arrived at 

 the conclusion that in these creatures nutrition is without influence on sex- 

 determination. 



