THE UNIQUENESS OF THE INDIVIDUAL 



The third exception, the surprising one, turns out on analysis 

 to be a variant of the second. In some animals it is sometimes 

 possible to exchange skin homografts between non- identical 

 twins, that is between ordinary litter mates, which resemble 

 each other in hereditary make-up no more closely than ordinary 

 sibs. This dispensation applies to about 90 per cent of twin 

 cattle — I must be understood to mean non-identical twins; that 

 tissue homografts can always be exchanged between identical 

 tAvins has already been conceded — and, so far as our present 

 meagre evidence goes, to all twin chickens; and it can be 

 assumed to be true of a certain very small proportion of twin 

 sheep and a still smaller proportion, surely less than 0*1 per 

 cent, of human twins. (It was for this reason that I put Victor ""s 

 and Eric''s chances of being identical twins no higher than 

 99*9 per cent.) Twin chickens, I should explain, are those that 

 hatch from a two-yolked egg, each yolk being a separate egg 

 as the embryologist understands that word. They could there- 

 fore be described with equal propriety as uniovular or bin- 

 ovular: it just depends on what one means by 'egg\ 



The non-identical twins between which it is possible to 

 exchange skin homografts are among the most remarkable 

 animals in nature, for they are graft-hybrids or chimeras; each 

 twin is a mixture of cells of two genetic origins, most of its cells 

 being its own, the remainder having been at one time the 

 property of its partner. The exchange of homografts between 

 them in later life does not therefore make them chimeras; it 

 merely makes them more so. Chimerism of natural origin was 

 first described by the American biologist R. D. Owen in 1945; 

 nearly all cattle twins, so he found, contain a mixture, not 

 necessarily a fifty-fifty mixture, of each other's red blood 

 corpuscles. (This is known to be true of non-identical twins in 

 cattle; it can be assumed to be true of identical twins, but it 

 cannot be proved because their red blood corpuscles cannot be 

 told apart.) How does this come about? The origin of the 



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