They have taught us that for the deciphering of the past and the predicting of the future, no hypotheses are admissible which are not based upon the actual behaviour of things in the present. Once there was unlimited facility for guessing as to how the solar system might have come into existence; now the origin of the sun and planets is adequately explained when we have unfolded all that is implied in the processes which are still going on in the solar system. Formerly appeals were made to all manner of violent agencies to account for the changes which the earth's surface has undergone since our planet began its independent career; now it is seen that the same slow working of rain and tide, of wind and wave and frost, of secular contraction and of earthquake pulse, which is visible to-day, will account for the whole. It is not long since it was supposed that a species of animals or plants could be swept away only by some unusual catastrophe, while for the origination of new species something called an act of "special creation" was necessary; and as to the nature of such extraordinary events there was endless room for guesswork; but the discovery of natural selection was the discovery of a process, going on perpetually under our very eyes, which must inevitably of itself extinguish some species and bring new ones into being. In these and countless other ways we have learned that all the rich variety of nature is pervaded by unity of action, such as we might expect to find if nature is the manifestation of an infinite God who is without variableness or shadow of turning, but quite incompatible with the fitful behaviour of the anthropomorphic deities of the old mythologies. By thus abstaining from all appeal to agencies that are extra-cosmic, or not involved in the orderly system of events that we see occurring around us, we have at last succeeded in eliminating from philosophic speculation the character of random guesswork which at first of necessity belonged to it. Modern scientific hypothesis is so far from being a haphazard mental proceeding that it is perhaps hardly fair to classify it with guesses. It is lifted out of the plane of guesswork, in so far as it has acquired the character of inevitable inference from that which now is to that which has been or will be. Instead of the innumerable particular assumptions which were once admitted into cosmic philosophy, we are now reduced to the one universal assumption which has been variously described as the "principle of continuity," the "uniformity of nature," the "persistence of force," or the "law of causation," and which has been variously explained as a necessary datum for scientific thinking or as a net result of all induction. I am not unwilling, however, to adopt the language of a book which has furnished the occasion for the present discussion, and to say that this grand assumption is a supreme act of faith, the definite expression of a