that may have inhabited this globe at former periods of time; but that the analogies which lead down or lead up to that as yet undiscovered link in the chain are such that it must have existed, and that we may confidently expect that the actual proof of it will be found hereafter. The difficulty with this kind of reasoning is that it borrows from the main hypothesis which one seeks to establish the means of showing the facts from which the hypothesis is to be drawn as an inference. Thus, for example, the hypothesis is that the species called man is a highly developed animal formed by a process of natural selection that went on for unknown ages among the individuals descended from the progenitor of the anthropomorphous apes. The facts in the physical organization and mental manifestations of the animal called man, when viewed historically through all the conditions in which we know anything of this species, lead up to that common supposed ancestor of the apes. The facts in the physical organization and instinctive habits of the ape, when viewed historically through all the conditions in which we know anything of his species, show that he, too, was evolved by the process of natural selection out of that same ancestor. Intermediate, respectively, between the man and the monkey and their primordial natural-selection ancestor or predecessor, there are links in the chain of proof of which we have no evidence, and which must be supplied by inferring their existence from the analogies which we can trace in comparing things of which we have some satisfactory proof. Thus, the main hypothesis, the theory of natural selection as the explanation of the existence of distinct species of animals, is not drawn from a complete chain of established facts, but it is helped out by inferring from facts that are proved other facts that are not proved, but which we have reason to expect will be discovered hereafter. I need not say that this kind of argument will not do in the common affairs of life, and that no good reason can be shown why our beliefs in matters of science should be made to depend upon it. We do not rest our belief in what is called the law of gravitation upon any chain of proof in which it is necessary to supply a link by assuming that it exists. The theory that bodies have a tendency to approach each other, that the larger mass attracts to itself the smaller by a mysterious force that operates through all space, is a deduction from a great multitude of perpetually recurring facts that are open to our observation, no one of which is inferred from any other fact, while the whole excludes the moral probability that any other hypothesis will account for the phenomena which are continually and invariably taking place around us.