Nietzsche considered it to be important for philosophers to be scientifically informed in their philosophical thinking. I suggest that this conviction led him to embrace a biological idea that seemed at the time to be scientifically sound but subsequently turned out not to be. (He then compounded the problem by making much of what he took to be its social and political implications.) I refer to the now-discredited (Lamarckian) idea that it is possible for characteristics acquired by individual creatures of some type (through intensive application, however prompted) to be biologically transmitted in some degree to their progeny. I make my case for this reading of him by drawing attention to a variety of texts that would seem to commit him to this idea, and I consider how we might best deal with and even benefit interpretively from recognizing this to have been an instance of his having gone astray.