In the same way, to measure the healthiness of a population and ascertain whether it is safely under the level of its food, tending to pass beyond it, or simply rising up to it, and to ascertain by what ways and means the process is going on, we need instead of one single general criterion a whole array of particular tests. It is in the infancy of statistical science that men yield to appearances and “suppose a greater uniformity in things than is actually found there.”[238] This was, for example, the failing of Johann Peter Süssmilch, one of the earliest inquirers into the movements of population. A book like Süssmilch’s had the same relation to the _Essay on Population_ as astrology to astronomy, or alchemy to chemistry; it prepared the way for the more accurate study. Süssmilch first published his researches in 1761, while the Seven Years’ War was still in progress. He dedicated it to Frederick the Great, as became a patriot and Church dignitary; and entitled it, _The Divine Plan in the Changes through which the Human Race passes in Birth, Death, and Marriage_. The Divine plan is the one set forth in the exhortation to Noah in Genesis—the peopling of the earth;[239] and the book tries to show the particular arrangements by which the plan is carried out. One condition is, he says, that fertility be greater than mortality; the births must exceed the deaths. On an average at present each marriage produces four children; and “the present law of death” is on an average, taking town and country together, 1 in 36; out of 36 men now living, 1 must die every year. In the country it is from 1 in 40 to 1 in 45; in the town, from 1 in 38 to 1 in 32. There is a yearly excess of births represented by 1 in 10 and 5 in 10. The increase must have been faster at first than it is now; and the means God took to effect His end in each case was the lengthening and shortening of human life. In the times of Methuselah there must have been a very different law of mortality, perhaps one death in a hundred; the length of life was greater; and probably the power of parentage lasted longer. The average number of children might be about twenty in a family instead of four; and the doubling of population would take place in ten or twenty years, instead of as now in seventy or eighty. Antediluvians were long-lived because their long lives were needed for the replenishment of the earth; and the extreme length was shortened so soon as the time came when the same end could be reached in other ways. When we observe the remarkable adaptiveness of man which enables him alone among the creatures[240] to live in any latitude, and when we observe how he has been preserved while many animals have become extinct, we need have no doubt that the replenishment of the earth was really the Divine purpose. It is remarkable too that, though more sons are born than daughters, death equalizes their numbers before mature life. The “system” which prevails in the increase of man is like the