In Plingy the Elder's Natural History (23-79 AD), he tells a tale about beavers. He reports that beavers castrated themselves in order to escape hunters who pursued them for their testicles, which could be used to produce an analgesic medication. This of course is not true. Explain why Darwin would have considered it reasonable that lizards should drop their tails, but implausible that beavers should self-castrate even to spare their own lives.