rying the line back to the Eocene formations, and connecting the present Equus or horse-tribe with an early Eocene animal known as the Orohippus. In his lecture Prof. Huxley traced the relationship of these six ancestral forms of the existing horse, and based his argument for the demonstrative evidence of evolution on the continuity and extent of the series. But he went further, and stated what the characteristics of a still earlier form would be if it were ever discovered; and, within a month from his departure from the country, Prof. Marsh announces that fossils of the predicted animal have been actually found in the lowest Tertiary deposits of the West, giving the Eohippus as the seventh term of paleontological ancestry of the Equine group.
We pointed out last month that proof is a thing of degrees, and that demonstration may be cumulative; and the very case we were considering now furnishes further illustration of it. Prof. Huxley says that the doctrine of evolution and the Copernican theory of the motions of the heavenly bodies have precisely the same basis, that is, "the coincidence of observed facts with theoretical requirements;" and that "an inductive hypothesis is said to be demonstrated when the facts are shown to be in entire accordance with it." But the demonstration becomes still stronger when the requirements of theory lead to the prediction of what must follow from it, and Nature subsequently furnishes the facts that vindicate the prophecy. It is one of the highest tests of the truth of a theory, that it leads to new discoveries, as was conspicuously the case with the wave-theory, of light. A scientific professor is reported to have said that the proof of the evolution theory is far less strong than that of the undulatory theory, while nobody regards that as demonstrated. On the contrary, it is so regarded, and with abundant reason. The objective existence of the ether may not be proved, but this conception is not essential to the theory, and is held by many as nothing more than a convenient assumption or hypothetical artifice to aid the imagination in picturing wave-actions. The essence of the theory, whether the medium assumed be ethereal or material, is that light originates in some kind of undulatory motion, and a rational optical science is now only possible on this view. In the sense in which Huxley uses the term demonstration, as "the coincidence of observed facts with theoretical requirements," it is an established demonstration, and evolution stands exactly on the same ground. The facts are what the theory requires them to be, and what it predicts them to be; it explains them by the operations of real causes, and offers the only explanation we can have without going outside of Nature to get it.
Prof. Huxley has done us great service by going over the question of evidence in his three lectures, and bringing out the full force of the proof for this doctrine, and to make any less claim than he has made is to be wanting in fidelity to the truth.
And from this point of view we must think that Prof. Martin, in his admirable introductory discourse, did not fairly represent the case in giving the scientific status of the principles of the conservation of energy and of natural selection. He said: "These ideas may or may not be true; increase of knowledge may confirm or may possibly upset them." So sharp an alternative as true or false, determinable only in a contingency of the future, certainly does injustice to the logical validity of these great ideas. They can no more be subverted or abandoned in the future than any other truths of experiment and observation. They may disappear by absorption into larger truths, may change aspects, but they are basal and permanent factors of science, and we see not why the professors
