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.