THE UNIQUENESS OF THE INDIVIDUAL 



has a unique combination of endowments. The number of 

 hereditary factors from which these combinations can be built 

 up, though large, is finite, but the combinations themselves 

 are far more numerous than the individuals who can enjoy 

 them, so that for each man actually on stage there are hundreds 

 of possible men still waiting for a cue behind the scenes. 



It follows that although the mechanism of heredity may be 

 ultimately atomic — though, for reasons I shall explain below, 

 I should prefer to describe it as a matter of chemical si7igu- 

 larities rather than of physical particles — yet the relationship 

 between human beings is defined by a virtually continuous 

 spectrum of affinities, bounded at one end by identical twins 

 and extending the other end far beyond the genetically visible 

 region into the affinities between animals which are not 

 members of the same species or even of the same order or class. 



The technique of skin grafting is particularly well qualified 

 to demonstrate these propositions, for it can reveal {a) that all 

 individuals, with the exceptions already noticed, are immuno- 

 logically unique; (h) that the immunological differences between 

 individuals are combinational in character; (c) that the com- 

 binations are so diverse that there is an almost continuous 

 range of variation in the acceptability of foreign tissues to their 

 hosts. It is bounded at one end by grafts exchanged between 

 identical twins, grafts which survive as long as the animals 

 which bear them; and close to this end lie, for example, the 

 grafts which have been transplanted from males to females of 

 the same inbred line and which may survive their transplanta- 

 tion by as much as fifty days. Grafts transplanted between 

 ordinary members of the same species normally survive for 

 little longer than a week; and when donors and hosts are 

 members of different species, the grafts ('"heterografts'') never 

 heal in properly, and it is only in a narrowly technical sense 

 that they can be said to survive for any length of time at all. 

 The range therefore extends from something near zero to 



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