88 MAN THE ANIMAL 



and used" than in rural habitats. These examples sufficiently 

 indicate that long before the world population began to slow 

 down in its growth rate, enough contraceptive effort was being 

 made in the western world, at least, to worry some students 

 of the matter. 



Ill 



In 1924 I published a logistic curve fitted to the data as- 

 sembled by Knibbs to indicate the growth of population of the 

 world, from the estimate of Riccioli in 1 660 up to and including 

 that of Knibbs himself as of 1914. 



It became apparent some time ago that this original world 

 logistic needed revision badly, because by 1930 the population 

 of the world had already exceeded the upper limit set by the 

 curve. Consequently we have recently brought the old curve 

 up to date, adding five more estimates of world population since 

 1914, the latest being our own of mean date 1931.5. The new 

 logistic curve is shown in Figure 16. 



As might reasonably be expected, the estimates of world 

 population have been strewn irregularly about a trend line that 

 may perhaps be fairly regarded, taking all the circumstances 

 into account, as reasonably represented by the logistic curve. 

 The nature of figures that have to be used in lieu of "observa- 

 tions," which even at the present date represent only about 

 75 per cent of complete actual observation, precludes the pos- 

 sibility of ever getting anything like a really smooth fit with a 

 four-constant curve. But taking the material for what it is, with 

 all its inherent disabilities, a number of interesting points are 

 presented for discussion. 



In the first place the upper limit of the current cycle of 

 growth is raised by about 31 per cent of the former figure, to 

 a new value of 2645.5 millions. According to the curve this 

 colossal total will be closely approached around 2100 a.d., pro- 

 vided nothing happens in the meantime to alter seriously the 

 present trends of reproductivity. No formal prediction is made, 

 however, that this will represent the ultimate size of the world's 



