The causes of shrinkage in population are the opposites of those which we have found to promote its increase. The production of food may be diminished by the exhaustion of the soil, or by the progressive aridity caused by cutting down woods. The manufacture of goods to be exchanged for food may fall off owing to foreign competition, a result which is likely to follow from a rise in the standard of living, for the labourer then demands higher wages, and consumes more food per head, which of itself must check fertility, since the same amount of food will now support a smaller number. The delusion shared by the whole working class that they can make work for each other, at wages fixed by themselves, is ludicrous; a community cannot subsist 'by taking in each other's washing.' Or the supply of importable food may fail by the peopling up of the countries which grow it. Any conditions which make it no longer worth while to invest capital in business, or which destroy credit, have the same effect. One of the causes of the decay of the Roman Empire was the drain of specie to the East in exchange for perishable commodities. When trade is declining a general listlessness comes over the industrial world, and the output falls still further. There have been alleged instances of peoples which have dwindled and even disappeared from _taedium vitae_. This is said to have been the cause of the extinction of the Guanches of the Canary Islands; but the symptoms described rather suggest an outbreak of sleeping-sickness.
Paradoxical as it may seem, neither voluntary restriction of births, nor famine, nor pestilence, nor war, has much effect in reducing numbers. Birth-control instead of diminishing the population, may only lower the death-rate. France in 1781, with a birth-rate of 39, had much the same net increase as in the years before the war with a birth-rate of 20. The parallel lines of the births and deaths in this country have already been mentioned. Famine and pestilence are followed at once by an increased number of births. India and China, though frequently ravaged by both these scourges, remain super-saturated. Of course, if the famine is chronic, the population must fall to the point where the food is sufficient; and a zymotic disease which has become endemic may be too strong for the natural fertility of the nation attacked, as has happened to several barbarous races; but an invasion of plague, cholera, or influenza has no permanent effect on the numbers of Europeans. War resembles plague in its action upon population. When, as in the late war, nearly the whole of the able-bodied men are on active service, the loss of population caused by cessation of births is greater than all the fatal casualties of the battle-field. A rough calculation gives the result that twelve million lives have been lost to the belligerent nations by the separation of husbands and wives during the war. And yet it may be predicted that these losses, added to the eight millions or so who have been killed, would be made good in a very few years but for the destruction of capital and credit which the war has caused. If we study the vital statistics of a country like Germany, which has engaged in several severe wars since births and deaths began to be registered, we shall find that the contour-line representing the fluctuations of the birth-rate indicates a steep ravine in the year or years while the war lasted, followed by a hump or high table-land for several years after. In a short time, as far as numbers are concerned, the war is as if it had never been. When we remember that the number of possible fathers is much reduced by casualties, this rise in the birth-rate after a war offers a strong confirmation of the thesis which we have been maintaining, that the ebb and flow of population are not affected by conscious intention, but by increased or diminished pressure of numbers upon subsistence. If the German people, who before the war consumed more food than was good for them, have been habituated by our blockade to a reasonable abstemiousness, we shall have contributed to the eventual increase of the German people, in spite of all their soldiers whom we killed in France, and the civilians whom we starved in Germany. And if our success leads to a greater consumption by our working class, our population will show a corresponding decline. Emigration, as we have seen, does not diminish the home population by a single unit; and so, while there are empty lands available for colonisation, it is by far the best method of adding to the numbers of our race.