The Beasts of Tarzan by Edgar Rice Burroughs

Slowly Tarzan unfolded the note the sailor had thrust into his hand, and read it. At first it made little impression on his sorrow-numbed senses, but finally the full purport of the hideous plot of revenge unfolded itself before his imagination.

This will explain to you [the note read] the exact nature of my intentions relative to your offspring and to you. / You were born an ape. You lived naked in the jungles -- to your own we have returned you; but your son shall rise a step above his sire. It is the immutable law of evolution. / The father was a beast, but the son shall be a man -- he shall take the next ascending step in the scale of progress. He shall be no naked beast of the jungle, but shall wear a loincloth and copper anklets, and, perchance, a ring in his nose, for he is to be reared by men -- a tribe of savage cannibals. / I might have killed you, but that would have curtailed the full measure of the punishment you have earned at my hands. / Dead, you could not have suffered in the knowledge of your son's plight; but living and in a place from which you may not escape to seek or succour your child, you shall suffer worse than death for all the years of your life in contemplation of the horrors of your son's existence. / This, then, is to be a part of your punishment for having dared to pit yourself against / N. R. / P. S. -- The balance of your punishment has to do with what shall presently befall your wife -- that I shall leave to your imagination.