Laudator Temporis Acti

Augustine, Sermons 51.23 (Patrologia Latina, vol. 38, col. 346; tr. Edmund Hill):

In this respect the human race, as it says somewhere [Ecclesiasticus 14:18], is rather like the leaves of a tree; of an olive tree, though, or bay tree, or any other evergreen, which is never without foliage, but yet doesn't always have the same leaves. As it says in that place, the tree sheds some while it produces others; the ones that are opening succeed the ones that are falling. It's always shedding leaves, its always clothed with leaves. So with the human race: it doesn't feel the loss of those who die every day, because they are being made up for by those who are being born. Thus the whole species of the human race continues in its proper manner, and just as there are always leaves to be seen on a tree of that sort, so the earth is always evidently full of human beings. If however they only died and weren't born, then the earth would be stripped of all people, as some kinds of trees are stripped of all their leaves.

Quia ita est genus humanum, sicut scriptum est, quomodo folia in arbore: sed in arbore olea, vel lauro, vel aliqua huiusmodi, quae nunquam sine coma est; sed tamen non eadem semper habet folia. Nam quomodo scriptum est, alia generat, et alia deicit: quia ea quae suboriuntur, succedunt ruentibus aliis. Semper enim deicit folia, semper foliis vestita est. Sic et genus humanum quotidie morientium detrimenta non sentit, per supplementa nascentium: et sic pro modo suo stat universa species generis humani; et sicut folia in arbore semper videntur, ita plena hominibus terra conspicitur. Si autem morerentur tantum, et non nascerentur; velut arbores quaedam omnibus foliis, ita terra omnibus hominibus nudaretur.