Passage 5.12.2
πάρεστι δὲ ἀναδιδάσκεσθαι καὶ τοῖσδε ἔτι· κέρατα γὰρ κατὰ ἐτῶν περίοδον ἀπογίνεται καὶ αὖθις ἐκβλαστάνει ζῴοις, καὶ τοῦτο ἔλαφοί τε καὶ δορκάδες, ὡσαύτως δὲ καὶ οἱ ἐλέφαντες πεπόνθασιν. ὀδοὺς δὲ οὐκ ἔστιν ὅτῳ δεύτερα παρέσται ζῴῳ τῶν γε ἤδη τελείων· εἰ δὲ ὀδόντες τὰ διὰ στόματος ἐξίσχοντα καὶ μὴ κέρατα ἦσαν, πῶς ἂν καὶ ἀνεφύοντο αὖθις; οὐ μὴν οὐδὲ εἴκειν πυρὶ ἔχουσιν ὀδόντες φύσιν· κέρατα δὲ καὶ βοῶν καὶ ἐλεφάντων ἐς ὁμαλές τε ἐκ περιφεροῦς καὶ ἐς ἄλλα ὑπὸ πυρὸς ἄγεται σχήματα. ποταμίοις γε μὴν ἵπποις καὶ ὑσὶν ἡ κάτωθεν γένυς τοὺς χαυλιόδοντας φέρει, κέρατα δὲ ἀναφυόμενα οὐχ ὁρῶμεν ἐκ γενύων·
Further evidence may also be drawn from these examples: horns are shed by certain animals annually and subsequently grow again—this occurs among deer and gazelles, and likewise elephants experience the same. Yet no mature animal regenerates its teeth a second time. If the protrusions from elephants' mouths were teeth, not horns, how could they regrow once more? Moreover, teeth by nature do not soften under fire, whereas horns of both cattle and elephants can be heated and shaped from round forms into flat or other contoured shapes. And indeed, the lower jaw of river-horses and boars supports tusks, but we never see horns growing from the jaws.