Science as yet has no answer to this most important of all questions, if we wish to understand human nature: namely, How is the body, and how are its senses and passions, determined to develop as they do? We may reply: Because God wills it so; or Because such is the character of the human species; or Because mechanical causes necessitate it. These answers do not increase our scientific understanding in the least; nevertheless they are not wholly vain: for the first tells us that we must be satisfied with ignorance; the second that we must be satisfied with the facts; and the third, which is the most significant, that these facts are analogous in every province of nature. But how dose are these analogies? Mechanism is one habit of matter, and life is another habit of matter; the first we can measure mathematically and forecast accurately, the second we can only express in moral terms, and anticipate vaguely; but that the mechanical habit runs through the vital habit, and conditions it, is made obvious by the dependence of life on food, on time, on temperature, by its routine in health and by its diseases, by its end, and above all by its origin; for it is a habit of matter continuous with other inorganic habits, and (if evolution is true) arising out of them. In any case, life comes from a seed in which it lies apparently dormant and arrested, and from which it is elicited by purely mechanical agencies. On the other hand, the seed reacts on those agencies in a manner as yet inexplicable by what we know of its structure; and its development closely repeats (though perhaps with some spontaneous variation) the phases proper to the species.