526 HUMAN BIOLOGY 



variables which have to be taken into account in discussing 

 the problem. 



In most human populations, especially those inhabiting 

 large geographical areas, a third factor may influence 

 directly the size of the population at any given moment, 

 in greater or less degree. This third factor is migration, and 

 it is theoretically to be regarded as a primary variable in 

 determining the growth of such human populations. While 

 theoretically a first-order variable, migration is, in large 

 population aggregates, practically always much less important 

 in its purely quantitative effects upon population size 

 than are the basic variables, natality and mortality. The 

 growth of the population of the United States is a case in 

 point. If one plots the census counts of this population 

 (as in Fig. 5 injra ) from 1790 to 1920 inclusive it is impossible 

 to detect in the curve of growth any separate or disturbing 

 effect of immigration. Unfortunately it is impossible to 

 analyze the effect of immigration upon the population 

 of the United States in detail, for the reason that net immigra- 

 tion figures, which take account of departures as well as 

 arrivals, are available only since 1908. But some general 

 facts are available and illuminating. In the years between 1830, 

 when records began on the point, and 1870, the total number 

 of immigrants into the country was 7.3 millions. The total 

 population of the country, as given by the census in 1870 

 was 38.6 millions. So that if all the immigrants who came into 

 the country had stayed here, and none had died in the 

 meantime (both assumptions being, of course, far from the 

 real facts) the total immigration in the period would have 

 constituted only 19 per cent of the total population. For 

 the census years beginning with 1870 and coming down to 

 1920, numbers of immigrants, in gross (i.e., without deduc- 

 tion of emigrants) in the year, per thousand of population 

 existing in the same year are as follows: 



