Brown-Sequard's discovery that an epileptic tendency artificially produced by mutilating the nervous system of a guinea-pig is occasionally inherited may be a fact of "considerable weight," or on the other hand it may be entirely irrelevant. Cases of this kind strike one as peculiar exceptions rather than as examples of a general rule or law. They seem to show that certain morbid conditions may occasionally affect both the individual and the reproductive elements or transmissible type in a similar manner; but then we also know that such prompt and complete transmission of an artificial modification is widely different from the usual rule. Exceptional cases require exceptional explanations, and are scarcely good examples of the effect of a general tendency which in almost all other cases is so inconspicuous in its immediate effects. Further remarks on this inherited epilepsy can be most conveniently introduced later on in connection with Darwin's explanation of the inherited mutilation which it usually accompanies, but which Mr. Spencer does not mention. INHERITED INSANITY AND NERVOUS DISORDERS. Mr. Spencer infers that, because insanity is usually hereditary, and insanity can be artificially produced by various excesses, therefore this artificially-produced insanity must also be hereditary (p. 28). Direct evidence of this conclusion would be better than a mere inference which may beg the very question at issue. That the liability to insanity commonly runs in families is no proof that strictly non-inherited insanity will subsequently become hereditary. I think that theories should be based on facts rather than facts on theories, especially when those facts are to be the basis or proof of a further theory. Mr. Spencer also points out that he finds among physicians "the belief that nervous disorders of a less severe kind are inheritable"--a general belief which does not necessarily include the transmission of purely artificially-produced disorders, and so misses the point which is really at issue. He proceeds, however, to state more definitely that "men who have prostrated their nervous systems by prolonged overwork or in some other way, have children more or less prone to nervousness." The following observations will, I think, warrant at least a suspension of judgment concerning this particular form of use-inheritance.