It is obvious again that great visitations like pestilence or war diminish population directly, though the effect of such factors is usually {453} temporary.

How much voluntary sterility operates is problematical.

Aegidius Albertinus, writing in 1602, attributed the growth in population of Protestant countries since the Reformation to the abolition of sacerdotal celibacy, and this has also been mentioned as a cause by a recent writer.

Probably the last named forces have a very slight influence; the primary one being, as Malthus stated, the increase of means of subsistence.
As censuses were almost unknown to sixteenth-century Europe outside of a few Italian cities, the student is forced to rely for his data on various other calculations, in some cases tolerably reliable, in others deplorably deficient.

The best of these are the enumerations of hearths made for purposes of taxation in several countries.