Karl Marx Quote
So far as living instruments of labour are concerned, for instance horses, their reproduction is timed by nature itself. Their average lifetime as instruments of labour is determined by the laws of nature. As soon as this term has expired they must be replaced by new ones. A horse cannot be replaced piecemeal; it must be replaced by another horse.
Vol. II, Ch. VIII, p. 174. - Das Kapital (Buch II) (1893)