THE UNIQUENESS OF THE INDIVIDUAL 



what is the fate of tissue grafted from old animals into young- 

 sters? If ordinary laboratory mice are used for such experiments, 

 as very likely they have been, or even what are sometimes with 

 undue optimism called ''pure strains', then the evidence is 

 falsified at the outset; for the transplantation of tissues be- 

 tween animals very little dissimilar genetically simply provokes 

 an immunity reaction, not different in principle from that which 

 governs the outcome of certain blood transfusions, as a con- 

 sequence of which the grafted tissue is destroyed."'^ But if suit- 

 able genetic precautions are taken, these problems and others 

 of equally wide compass are capable of solution. Only when 

 they are solved can the physiologist begin to ask more specific 

 questions, such as whether the determinative factors of ageing 

 are humoral in nature or of some other more complex type. 



It is rather urgent that research of this type should be under- 

 taken. Man''s mean expectation of life at birth has increased 

 very dramatically over the last 100 years, but chiefly as a 

 consequence of reduced mortality in infancy and childhood. 

 The mean expectation of life at the age of forty has increased 

 hardly at all. But because of this reservation for life of many 

 who would otherwise have died, the age-spectrum of the popu- 

 lation, i.e. the proportion of its members within each age-group 

 of life, is in many civilized countries shifting slowly towards old 

 age.f In forty years'* time we are to be the victims of at least a 



* [See The Uniqueness of the Individual herein.] 



t For the population of the U.S., see W. S. Thompson and P. K. Whelp- 

 ton, Estimates of Future Population of the United States, 1940-2000, 

 National Resources Planning Board, Washington, 1943. F. W. Notestein 

 et al. The Future Population of Europe and the Soviet Union, Population 

 Projections 1940-1970, Office of Population Research, Princeton University, 

 1944. [For the population of the U.K., see Reports and Selected Papers of 

 the Statistics Committee (\^ol. 2 of Papers of the Royal Commission on 

 Population), London, H.M.S.O., 1950. Secular changes in the age-structure 

 of populations since demographic data first began to be compiled are 

 summarized by L. I. Dublin, A. J. Lotka and M. Spiegebnan, Length of 

 Life: a Study of the Life Table, New York, 1949.] 



42 



