Under favorable circumstances, the reproductive power of the human species is very great.[184] Actual cases of doubling of population in from twenty to twenty-five years have been known, and this may be taken as a maximum standard. But man is dependent for his existence on the food supply, and, owing to the actual conditions of production, there is no ground for the hope that the amount of subsistence of the world or of any nation can ever be increased at a rate corresponding to the possible increase of mankind. Consequently, the growth of the species is always limited by the possibilities of the increase of the food supply, and as the strength of the reproductive instinct is very great, population will always be pressing hard on the limits of subsistence. The only means of providing for a greater population is by increasing the amount of productive land, or, by improvements in the arts, by making the land already under cultivation produce more food. Briefly stated, in any society, population tends to increase up to the supporting power of the soil. The forces which retard the growth of population, however, are something more than starvation in the strictest sense of the word. They are enumerated by Malthus in a list of what he calls checks. These naturally fall under two heads: First, the positive checks, which increase the death rate, viz. war, famine, pestilence, vice, etc.; these all produce misery and arise whenever population becomes too dense. Second, the preventive checks, which limit the birth rate, such as deferred marriage, celibacy, and voluntary restriction of births, vicious or otherwise; these are under the control of the human reason and will, and while they too entail a degree of suffering, it is not comparable to that caused by the other class of checks. All civilized societies have come more and more to employ the preventive checks, particularly that which is known as moral restraint.