THEIR SIGNIFICANCE IN HEREDITY. 381 



operation controlled the construction of the organism were the 

 same, and consequently the results were also the same. Twins 

 derived from a single egg- are identical : this is a statement which, 

 although not mathematically proved, may be looked upon as nearly 

 certain. But there are also twins which do not possess this high 

 degree of similarity, and these are even far commoner than the 

 others. The explanation is to be found in the fact that the latter 

 were derived from two egg-cells which were fertilized at the same 

 time. In most cases, indeed, each twin is enclosed in its own 

 embryonic membranes, while much less frequently both twins are 

 enclosed in the same membranes. In one point only the proof is 

 incomplete ; for it has not yet been shown that identical twins are 

 always derived from a single egg, since such an origin, together with 

 a high degree of similarity, could only be established as occurring 

 together in a small proportion of the cases. We therefore see that 

 under conditions of nutriment which are as identical as possible, 

 two egg-cells develope into unlike twins, one into identical twins ; 

 although we cannot yet affirm that the latter result invariably 

 follows. It is conceivable that the stimulus for the production of 

 two eggs from one may be afforded by the entrance of two sper- 

 matozoa, but these latter, as was shown above, could hardly contain 

 identical hereditary tendencies, and thus two identical twins would 

 not arise. It appears indeed that some cases have been observed 



to the latter.' And again, ' where the maladies of twins are continually alike, the 

 clocks of their two lives move regularly on, and at the same rate, governed by their 

 internal mechanism. Necessitarians may derive new arguments from the life 

 histories of twins.' 



The above facts and conclusions held for twins of the same sex, of which at 

 any rate the majority are shown by Kleinwachter's observations to have been 

 enclosed in the same embryonic membranes, and therefore presumably to have 

 been derived from a single ovum ; but in rarer cases the twins, although also 

 invariably of the same sex, were marked by remarkable differences, greater than 

 those which usually distinguish children of the same family. Mr. Galton met with 

 twenty of these cases. In such twins the conditions of training, etc. had been 

 as similar as possible, so that the evidence of the power of nature over nurture is 

 strongly confirmed. Mr. Galton writes, ' I have not a single case in which my cor- 

 respondents speak of originally dissimilar characters having become assimilated 

 through identity of nurture. The impression that all this evidence leaves on the 

 mind is one of wonder whether nurture can do anybhing at all, beyond giving in- 

 struction and professional training.' 



The fact that twins produced from a single ovum seem to be invariably of the 

 same sex is in itself extremely interesting, for it proves that the sex of the individual 

 is predetermined in the fertilized ovum. E. B. P.] 



