THE AFTER-LIVES OF TWINS 135 



by their internal mechanism. Necessitarians may derive 

 new arguments from the life histories of twins.' 



Mr. Galton furthermore met with twenty cases of twins 

 (also of the same sex) in whom the differences were 

 greater than those which usually distinguish children of 

 the same family. In such twins the conditions of training, 

 &c, 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 

 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 any- 

 thing at all beyond giving instruction and professional 



training. 



The argument thus leads to the conclusion that nearly 

 everything which is characteristic of an individual is 

 blastogenic, and therefore can be transmitted by the 

 continuity of the germ-plasm. We can thus appreciate 

 Weismann's contention that Natural Selection, while 

 seeming to decide between successful and unsuccessful 

 individuals, is in reality deciding between successful and 

 unsuccessful germs. 



Monstrosities (except such as are produced by external 

 agencies) can be satisfactorily explained, in the same 

 manner as ' identical ' twins, by the occurrence, at some 

 stage of development, of an equivalent division instead of 

 a differentiating division of a cell or a substance which 

 is the precursor of the doubled part. During the vast 

 succession of differentiating divisions which take place 

 in the development of one of the higher animals, the 

 cells which represent parts of less and less importance 

 are gradually told off. Thus the divisions which lead 

 to the doubling of a small part of the body would occur 

 far later in development than those which would lead 

 to the doubling of a large and important part. But 

 early or late the occurrence of an equivalent instead 

 of a differentiating" division at the critical stage was 

 predetermined in the structure of the fertilized egg. 

 We know that supernumerary digits are in a high 



