44 THE EVOLUTION THEORY 



parents). If we should succeed in penetrating more deeply into 

 these processes we should probably also understand why in certain 

 human families the hereditary characters are transmitted more purely 

 and more tenaciously than those of other families with which they 

 have mingled, and so on. It may well be that the persistence of 

 character is due to the fact that ids which have once combined into 

 rods hold firmly together, for it seems to me in no way impossible 

 that individual differences should occur even in these most delicate 

 processes. 



But let us leave these more intimate questions out of account 

 altogether, and turn our attention to the more obvious and less delicate 

 phenomena, and we find that the re -arrangement of ids (Neotaxis) 

 which we have just discussed afibrds a simple explanation of the 

 generally observed phenomenon of the difierences between individuals ! 

 Each individual is different from every other, not in the case of Man 

 alone, but in all species in which we can judge of differences, and this 

 is true not only of descendants of different parents, but even of those 

 of the same parents. 



Of course the differences between two brothers or two sisters do 

 not depend entirely on the hereditary basis, but in part also on external 

 conditions which have affected them from embryonic development 

 onwards. Let us suppose that of two brothers who have sprung from 

 identical germ-cells one becomes a sailor, the other a tailor ; it would 

 not surprise us to find them very different in their fiftieth year, one 

 weather-beaten and tanned, the other pale ; one muscular, straight, 

 and vigorous, the other weakly and of bent carriage. The same 

 primary constituents develop differently according to the con- 

 ditions to which they are exposed. But the two brothers will still 

 resemble each other in the features of the face, colour of hair, form 

 of eyes, stature and proportion of limbs, perhaps even in a birth- 

 mark, more than any other human beings of their own or any other 

 family, and this resemblance will depend upon the identity of the 

 hereditary primary constituents, on the similar id- combination of the 

 germ-plasm. 



Man himself affords a particularly good example in favour of this 

 interpretation in the case of so-called ' identical twins.' It is well 

 known that there are two kinds of twins, those that are not strikingly 

 alike, and often very different, and those that are alike to the extent 

 of being mistakable for one another. Among the latter the resem- 

 blance may go so far that the parents find it necessary to mark the 

 children by some outward sign, so that they may not be continually 

 confused. We have now every reason to believe that twins of the 



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