615. Serpens et Filius Eius. Serpens filium monebat ut sibi adesset dum  in alium serpentem rueret ac mordicus corriperet et glutiret. “Facinus  tam nefarium, pater,” inquit, “tune ut perpetres? De gente nostra is  unus est.” “Tace, inepte, tace; sic agere expedit. Serpens enim, si  serpentem non devoret, draco non fit.” Fabula ad hos potentes pertinet,  qui crescere solent ex alienis cladibus.   
M0615 (not in Perry). Source: Desbillons 10.9 (adapted into prose). This fable is not in Perry’s catalog; it appears original to Desbillons, as he cites no source. Compare the Latin proverb, Serpens ni edat serpentem, draco non erit (Erasmus, Adagia 3.3.61).