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of them wear at first sight a redoubtable aspect. For example, it may be said that since I compare man's power of guessing at the truth with the instincts of animals, I ought to have noticed that these are entirely explained by the action of natural selection in endowing animals with such powers as contribute to the preservation of their different stocks; and that there is evidence that man's power of penetrating the secrets of nature depends upon this, in the fact that all the successful sciences have been either mechanical in respect to their theories or psychological. Now, some notions of mechanics are needed by all animals to enable them to get food, and are needed most by man; while correct ideas of what passes in his neighbours' minds are needed for the existence of society, and therefore for the propagation of his kind. Metaphysics, however, cannot adapt the human race to maintaining itself, and therefore the presumption is that man has no such genius for discoveries about God, Freedom, and Immortality, as he has for physical and psychical science; and the history of science supports