128 L\f€ and Letters of Francis Gait on 



and we hope that this may be forthcoming. At the same time his general 

 coaclusion that the differential effects of nurture within the range of exist- 

 ing social or possible political conditions are extremely small has oeen amply 

 confirmed by other methotis ot'approivching the problem. 



Galton lelt that this conclusion was very essential in its bearing on his 

 theory of the heredity of the mental charactei-s. The two principles to- 

 gether — i.e. inheritiince of ability, and the relatively small influence of nurture 

 as compared to nature — form the basis of his scheme for racial betterment. 

 While Galton always acknowledged the importance of a good pliysicjue, and 

 suggested that marks should be allotted to it in competitive examinations, 

 he was deeply impressed with the fact that the future lies rather with the 

 man of brains than with the man of muscles ; he repeatedly asserted, however, 

 that the man of ability was very often of good physique, and the stock 

 wherein the two were combined was the ideal stock for race-betterment. 



In his second paper on twins, entitled "Short Notes on Heredity, etc., 

 in Twins'," Galton turns somewhat more to the numerical and physiological 

 sides of the subject. 



"The word 'twin' covers different classes of events — those in which each twin is derived 

 from a separate ovum, and those in which they come from two germinal spots in the same ovum. 

 In the former case the_v are enveloped, previously to their birth, in separate membranes; and 

 in the latter in the siime membrane. Now it appears that twins enveloped in the same raenibrHne 

 are invariably of the same sex, and these according to the cases of Spaeth, who has evidently 

 taken great pains to secure reliable data', are 24 per cent, of the whole numlier." (p. 329.) 



Galton finds great variation in the .stati.stics a.s to twins of various types, 

 but concludes that about twice as many twins are born of the same sex as 

 of opposite sex'. This would mean that Spaeth's percentage should have 

 been 33°/^ instead of 24°/^,. It seems probable that the divergence arises 

 from CJalton's "twice" being a rough approximation. He considers that 

 twins do not marry as often as other people and are less fertile. He con- 

 tradicts, however, tne popular belief that both twins whether of the same or 

 opposite sexes never have children, for he had many instances to the contrary. 

 He says that as far as he is aware nothing corresponding to the 'free-martin' 

 of cattle occurs with the human twin. Still the infertility of twins Galton 

 considers to be so great that it renders the question of direct inheritance 

 of twinning difficult. He accordingly asks whether the number of twins 

 among the uncles and aunts of twins is in excess of the probable on the 

 basis of the non-inheritsince of the twinning tendency. Galton shows from 

 Ansell's Statistics of Families that there is 1 twin born to every 100 births, 



• Journal of the Royal Anihropologieal ItislUute (187.5), Vol v, pp. 324-29, 1876. 



' "Studien flber Zwillingen," Zeitschrifl der Wiener Gesdlgchaft der Aerzle, I860, No8. 15 

 aod 16. 



* Let x = number of single membrane or 'like' twins, y = number of double membrane or 

 'unlike' (Le. only as like as normal siblings). Then according to Spaeth, x-\(x ^ y), nearly. 

 Now in X all twins are of the same sex, and in y only half will be of the same sex; hence total 

 number of all twins of same sex = a; -t- J y and of different sexes = \y, but x=^y, and accordingly 



'•^ly 



J, or there are 1*7 times as many twins of same sex as of unlike sex. 



