But all this contributes to increase the measures of suffering in human life out of all proportion to its pleasures; and the pains of life are made much worse for man by the fact that death is something very real to him. The brute flies from death instinctively without really knowing what it is, and therefore without ever contemplating it in the way natural to a man, who has this prospect always before his eyes. So that even if only a few brutes die a natural death, and most of them live only just long enough to transmit their species, and then, if not earlier, become the prey of some other animal.

(From Goodreads)