HUMAN LONGEVITY 77 



generations characterizes human populations to an extent per- 

 haps not equalled in any other living form. It is a factor of 

 profound importance in population biology. This tremendous 

 burden is borne by mankind for reasons in large part emotional. 

 We (in a statistical sense) care for our parents beyond the time 

 limits of biological necessity in good part only because we want 

 to. But for this emotional satisfaction we pay a high price. 



In these facts is to be found unquestionably one of the basic 

 reasons for the practice of contraception or birth control in coun- 

 tries having what we are pleased to regard as a "high" state 

 of civilization. As the burden becomes more and more clearly 

 recognized the natural tendency is to attempt to reduce it by 

 limiting the number of children, since the emotional conflict 

 over the care of unborn babies versus provision for our own old 

 age and that of our parents is the easiest one to resolve out 

 of the whole assortment presented to us. But the optimism thus 

 engendered is in some degree illusory, because what happens is 

 merely that the distribution of the burden is altered, not its 

 total drag. For it turns out that the proportionate number of 

 persons falling in the middle portion of the life span — the 

 workers — is nearly the same in all populations of the world 

 today. The proportion of the infants and of the old vary widely 

 from country to country. But the proportion of the workers stays 

 relatively constant at approximately 50 per cent. 



A tabulation demonstrating this point for the populations of 

 fifty countries, as they existed at an average date of about the 

 middle of the year 1 929, is too long to be shown in full here, but 

 Table 2 gives a condensation of this table down to workable 

 limits. 



While this condensed table is riot as impressive as the com- 

 plete one, it serves to show clearly that no matter how the work- 

 ers' burden is shifted between the young and the old its total 

 remains about constant, as does the proportionate number of 

 workers who must bear it. Thus whether the numbers in the in- 

 fantile period of life are high and those in the senescent period 

 are low, as in the populations of Brazil and India, or whether 



