POPULATION. About two-thirds of the inhabitants of the world are now periodically enumerated by means of a census. Whenever a census is taken a number of questions is asked con cerning each person. Thus with respect to two-thirds of the inhabitants of the world there is a considerable volume of in formation of a fair degree of accuracy. The questions asked vary considerably from country to country. Information is nearly al ways required regarding age, sex and marital condition and there fore the inhabitants of different countries of which the popula tion is periodically enumerated can be compared in respect of these matters. But with regard to the asking of questions con cerning other matters much variation exists, and comparisons are often rendered impossible. There is no question regarding religion in the English census, for example, and comparisons in this respect between England and other countries where such information is collected are therefore not possible. For the remaining third of the world population it is necessary to rely upon estimates. No great degree of confidence can be placed upon these estimates, and a wide margin of error must be allowed for. Furthermore, estimates can only be made for the total population. It is im possible to estimate the distribution of the population in respect to age or sex, and as to these matters, therefore, for the remaining third of the world population there is no information.
The information obtained by the taking of a census can be analysed and tabulated. The results of so doing can be compared with the results of analysing former censuses. It will almost cer tainly be found that important changes have occurred since the taking of the last census. The population may have increased or diminished; the sex ratio may have altered. The taking of a census brings such changes to light but does not explain them. A census is, as it were, a snapshot of the population, and for the study of population changes more is required than a series of snapshots taken at different dates. An increase in the population of a country, such as a census may show, might be due to the immigration of people into the country, to an increase in the birth-rate or to a decrease in the death-rate. In those countries
where a census is taken, data are also collected regarding births, deaths, marriages, movement into and out of the country and other matters. There is considerable variation in the methods of taking these data as there is in the taking of a census from coun try to country. These data are not only of interest in themselves but they can be used to throw light upon the changes which occur between one census and another.
These registration statistics of births, marriages and deaths may provide immediate explanations of some of the changes ex hibited by the taking of a census. They may show, for instance, that the slackening of the rate of increase of the population of a country is due to a decline in the birth-rate. We are in that case led on to enquire into the causes of the decline of the birth rate, or it may be in other cases into the causes of an increase in the death-rate or in the immigration rate. It is likely that to some extent an examination of the census figures and of the registration statistics will assist us in any effort we may make to explore these more distant matters. A detailed examination, for instance, of changes in the birth-rate over a number of years will almost certainly contribute towards the solution of the prob lem of the decline in the birth-rate. But it will seldom be found that the evidence of census and registration figures will be con clusive regarding these more distant problems. Biological ques tions are frequently involved. It may, for example, be shown that the statistics of the decline in the birth-rate are compatible with the increasing use of contraceptive methods. But before any con clusion can be come to on the point, it is necessary to enquire into the possible influence of diet upon fertility and other purely biological problems. Evidently the pursuit of explanations as to how the population of any country has come to be what it now is can lead us very far afield. It is not possible in this ar ticle to follow up these matters further than the point at which census and registration data cease to throw light upon them; nor can more than incidental reference be made to the results of population changes.