THE FALL OF THE BIRTH-RATE 41 



ing the successive intercensal increases of population 

 in England and Wales, and another giving the i i-year 

 average levels of prices (as indicated by the index- 

 numbers of Jevons and Sauerbeck), the latter having its 

 base and its scale so adjusted as to fit the population- 

 increase curve as closely as may be and also being 

 shifted by ten years: i.e. the point in the centre of the 

 decade 1801-1811 gives the price level of 1791-1801 

 and so on. The two curves seem to me to be too 

 similar for the similarity to be accidental. The only 

 time when the two are positively discordant is in 

 1881-1901 when the rate of increase rose but the 

 price-level (of a decade earlier) fell*. Considering 

 how complex a quantity the rate of increase of a 

 population is, and how dependent nowadays on the 

 rates of emigration and of immigration as well as 

 mortality, it may well seem astonishing that any 

 similarity at all should be evident. That there is such 

 resemblance is, I venture to think, due to the fact 

 that migration, marriage-rate, and fertility are only 

 three forms of response to demand for population: 

 if part of the demand is unexpectedly met by a 

 lowering of the death-rate fertility may well be 

 checked. But let me again enforce a point which 

 seems to me of importance. The demand is for adults. 

 A rise in the birth-rate responds by the production 

 of infants who are not available for economic pro- 

 duction for some twenty years or more. Population 

 workers are demanded now: workers turn up 

 some twenty years later, when they may not be 

 wanted. And, if there is any truth in this economic 

 theory of population, this lag in response must tend 



>r those who are accustomed to the use of the coefficient of cor- 

 relation as a measure of resemblance, it may be added that the corn 

 between the intercensal increase- rate and the index-number ot 

 years preceding the decade is 0-91 : the correlation between the first 

 differences (movements) of the two quantities is 0-72. 



