SOME ASPECTS OF THE BIOLOGY OF HUMAN POPULATIONS 533 



for more than thirty years, and then even more decisively 

 fell. But the concurrent changes in the death rates were such 

 that no sensible alteration in the general trend upward of 

 the vital index was produced by this change in the course 

 of the birth rate. 



The second example of the self-regulation of human pop- 

 ulation size is afforded by the vital index of the population 

 of France. Figure 3 shows a plot of this index for the years 

 1900 to 1926 inclusive. The abscissal points of plotting 

 are taken as mid-year points. 



What the diagram shows is that during the period from 

 1900 to the outbreak of the World War in 1914 the birth- 

 death ratio in France had maintained a level somewhere 

 between 100 and no, with fluctuations which occasionally, 

 brought it a little below the dead line of survival at 100 

 per cent. With the onset of the war the vital index fell to 

 unprecedentedly low values. But immediately upon the cessa- 

 tion of hostilities the vital index rose at an even more rapid 

 rate than that of its previous fall, reaching in 1920 a higher 

 value than it had attained at any time during the century. 



Again the magnitude and speed of action of the size- 

 regulating powers of a human population are strikingly 

 demonstrated. 



THE GROWTH OF HUMAN POPULATIONS 



It is observed that the growth of populations of the most 

 diverse organisms follows a regular and characteristic course. 

 The population at first grows slowly, but gains impetus 

 as it grows, passing gradually into a stage of rapid growth, 

 which finally reaches a maximum of rapidity. After this 

 stage of most rapid growth the population increases ever 

 more and more slowly, until finally there is no perceptible 

 growth at all. In short, the populations of various forms 

 of life first wax in this speed of growing and then wane. 



The equation to the curve which has been found by exper- 

 iment and observation to be descriptive of population 

 growth in a wide variety of organisms as first discovered by 

 the Belgian mathematician Verhulst in 1838. His pioneer 

 work was forgotten, and consequently overlooked by most 

 subsequent students of the population problem. In 1920 th 



