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Dr. Gregory says:

Hypothesis is often confounded with theory; bat hypothesis properly means the supposition of a prinCope of whose existence there is no proof from expenence, but which may be rendered more or less

* Hamilton's Metaphysics, p. 117.

4 Bystem of Logic, 4th ed., bk. iii., c. iv.

probable by facts which are neither numerous enough nor adequate to infer its existence.*

Have we any criteria of legitimate hypothesis? Almost all writers on the subject have furnished what they consider criteria, and we may therefore compile a code upon which there will be an approach to unanimity of acceptance.

Hamilton, quoted by Rev. J. W. Flinn (Southern Presbyterian Review, April, 1885), gives several criteria of a good hypothesis in the tenth lecture of his "Metaphysics," and in his discussion of the "Representative Theory of Perception" (Lecture XXVI.). They are in substance as follows: 1. The facts to be explained must really exist. Prove ghosts before explaining them. Establish an sit before cur sit. 2. The phenomena cannot be explained by any known cause or principle. 3. The hypothesis must involve no internal or external contradiction. It must be consistent with its parts, and not contradict other known truth. 4. It must explain the phenomena

* Fleming's Vocabulary of Philosophy.

better than any known or supposed law or cause. 5. It must explain the phenomena simply and fully, independently of subsidiary hypotheses to help it out. 6. It must save the facts to be explained and not subvert, distort, or mutilate them. Professor Jevons,* in giving the requisites of a good hypothesis, considers "agreement with fact the sole and sufficient test of a true hypothesis."

Professor Huxley ("Origin of Species," Lecture VI.) says:

We must, in the first place, be prepared to prove that the supposed causes of the phenomena exist in nature; that they are what the logicians call vera causæ true causes; in the next place, we should be prepared to show that the assumed causes of the phenomena are competent to produce such phenomena as those which we wish to explain by them; and in the last place, we ought to be able to show that no other known causes are competent to produce these phenomena. If we can succeed in satisfying these three conditions, we shall have demonstrated our hypothesis; or rather, I ought to say, we shall have proved it as far as certainty is possible for us; for, after all, there is no one of our

* Jevons's Principles of Science, bk. iv., c. xxiii.

surest convictions which may not be upset, or at any rate modified, by a further accession of knowledge.

The criterion given by Boyle of a legiti mate hypothesis is that it should not be inconsistent with any other truth or phenomena of nature.

Professor Clifford says:

In order to make out that your supposition is true, it is necessary to show, not merely that that particular supposition will explain the facts, but also that no other will.*

The question is whether there be any hypothesis of evolution which can satisfy these criteria.

Davy (said Sir Lyon Playfair in his late presidential address to the British Association for the Advancement of Science) described hypothesis as the mere scaffolding of science, useful to build up true knowledge, but capable of being put up or taken down at pleasure. Undoubtedly a theory is only temporary, and the reason is, as Bacon has said, that the man of science "loveth truth more than theory." The changing theories which the

* Conditions of Mental Development, Humboldt, Lib. edition, P. 44.

world despises are the leaves of the tree of science drawing nutriment to the parent stems, and enabling it to put forth new branches and to produce fruit; and though the leaves fall and decay, the very products of decay nourish the roots of the tree, and reappear in the new leaves or theories which succeed. When the questioning of nature by intelligent experiment has raised a system of science, then those men who desire to apply it to industrial invention proceed by the same methods to make rapid progress in the arts.

But no hypothesis can claim the dignity of a theory, and be maintained as a scientific doctrine, so long as there are very many well ascertained facts which cannot be accounted for by the hypothesis. This is admitted. To this unquestioned canon let us bring the hypothesis of evolution.

VI. TESTIMONY OF THE VEGETABLE WORLD.

ANY theory of evolution demands that there shall have been a gradual but steady development from rudest and simplest forms to most complete and complex forms, as a rule, and not as an exception. Is that a