“The preventive check,” says Malthus, “is perhaps best measured by the smallness of the proportion of yearly births to the whole population. The proportion of yearly marriages to the population is only a just criterion in countries similarly circumstanced, but is incorrect where there is a difference in the prolificness of marriages or in the proportion of the population under the age of puberty, and in the rate of increase. If all the marriages of a country, be they few or many, take place young, and be consequently prolific, it is evident that to produce the same proportion of births a smaller number of marriages will be necessary, or, with the same proportion of marriages, a greater proportion will be produced.” Curiously enough, in his day Malthus mentions that in France both the births and deaths were greater than they were in Sweden, although the proportion of marriages was then rather less in France. “And when,” he adds, “in two countries compared, one of them has a much greater part of its population under the age of puberty than the other, it is evident that any general proportion of the yearly marriages to the whole population will not imply the same operation of the preventive check among those of a marriageable age.” One of the most interesting chapters in the second volume of Malthus’ essay is that which relates to the rapid increase of births after the plagues. According to Sussmilch, very few countries had hitherto been exempt from plagues, which every now and then would sweep away one-fourth or one-third of their population. That writer calculated that above one-third of the people in Prussia were destroyed by the plague of 1711; and yet, notwithstanding this great diminution of the population, it appeared that the number of marriages in 1711 was very nearly double the average of the six years preceding the plague. Hence the proportion of births to deaths was prodigious—320 to 100—an excess of births as great, perhaps, as has ever been known in America. In the four years succeeding the plague the births were to the deaths in the proportion of above 22 to 10, which, calculating the mortality at 1 in 36, would double the population in 21 years.