THE NUMBERS OF MEN 83 



deliberately made in his own intrinsic pattern of reproductivity, 

 particularly through such things as the practice of contraception, 

 postponement of marriage until long after puberty, and other 

 factors of a comparable sort. 



5. In the population of the United States, at least, differ- 

 ential fertility relative to race (Negro versus white) and to the 

 three most important social class differentiations (economic, 

 educational and religious) are due primarily to differences in 

 the relative prevalence and effectiveness of the efforts made to 

 prevent conception, correlatively aided by relative frequency 

 and postponement of marriage, and the practice of criminal 

 abortion, and to practically nothing else. 



6. But in this same United States population it appears in 

 high degree probable that the general level of fertility, and 

 particularly the steady decline of total fertility that is now going 

 on and has been in the recent past, both in the population as a 

 whole and in differentiated classes within it, cannot be justly 

 regarded as due solely, or possibly even primarily, to the opera- 

 tion of the forces that have just been enumerated. Other, and 

 more obscure factors are involved, about which literally almost 

 nothing of a precise and definite character is now known. 



While the evidence that has been offered here in support of 

 the six general points that have been enumerated has come 

 largely from one population only, that of the United States, 

 it seems plain that the same principles and forces are operating 

 in all populations, in widely different and varying degrees and 

 extents, to be sure, but always and everywhere in some degree. 

 if this be granted, as seems reasonable, it then follows that in 

 the population of the world as a whole we are presented with 

 the objective record, at any particular time, of the net integrated 

 effect of the operation of all these complexly interrelated factors 

 and forces that have been enumerated. So then let us now turn to 

 an examination of the population of the world. 



II 



It should be emphasized that it is as impossible now as it has 

 always been in the past, and will be for a long time in the future, 



