VI.] THEIR SIGNIFICANCE IN HEREDITY, 393 



their co-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 fertihzed 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 ^gg^ since such an origin, together with 

 a high degree of similarity, could only be estabhshed 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 spermatozoa, 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 in which 

 differences have been exhibited by twins which were enclosed 

 in the same embryonic membranes ; but nevertheless I believe 

 that two spermatozoa are not necessary to cause the formation 

 of twins by a single &g%. We know, it is true, from the investi- 

 gations of FoP, that multiple impregnation produces the 

 simultaneous beginning of several embryos in the eggs of star- 



nurture is strongly confirmed. Mr. Galton writes, ' I have not a single 

 case in which my correspondents 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 anything at all, beyond giving instruction and profes- 

 sional 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.] 



^ Fol, ' Recherches sur la fecondation et le commencement de Iheno- 

 genie.' Geneve. Bale, Lyon, 1879. 



