528 HUMAN BIOLOGY 



eflect they may have upon the final result, namely the size 

 of the population at any given moment, by acting, more or 

 less powerfully as the case may be, upon one or more of the 

 three primary biological variables, natahty, mortality and 

 migration. Thus an economic depression in a particular 

 country may affect adversely the birth rate of that country, 

 or even the death rate if the degree of the depression is 

 sufficiently great or its duration sufficiently prolonged. 

 These effects will, in greater or smaller degree, reflect 

 themselves finally in the size of the population. This final 

 effect upon the growth of the population may, however, 

 be extremely slight, and difficult or even impossible of 

 separate statistical recognition or measurement, because 

 of compensating influences at work at the same time. 

 Logically, however, the operation of these secondary factors 

 must always be recognized. But from the point of view of 

 the theory of population growth their influence is always a 

 second order one. They can produce any effects upon 

 population only by operating upon the primary biological 

 forces of natality, mortality and migration. 



The net effect of the two important variables, natality and 

 mortality, upon population may be studied in various ways. 

 One of the most illuminating is by the use of a constant 

 which has been called the "vital index" of a population. 

 It has this form: 



,,. , . , Births X 100 



Vital index = t^ ^f 



Ueatris 



The vital index gives an accurate picture of the net 

 biological status of a population as a whole at the moment of 

 its calculation. If the ratio 100 births: deaths is greater than 

 100 the population if growing naturally, and is in so far 

 biologically healthy. If the ratio is less than 100 the popula- 

 tion is not exhibiting natural growth, however sound it 

 may be in other respects. There may be a sufficient amount 

 of immigration to compensate the deficiency in births, 

 so that there is no actual depopulation. But the condition is 

 fundamentally unsound biologically. 



The vital indices of the population of the United States 

 have been extensively studied by the writer (1924, Chaps. 



