quantity, is in either case a fact standing in complicated relationship with a multitude of other conditions, none of them more than partially apprehended, and many of them doubtless very widely misapprehended. Hence, their apparent testimony must always be closely questioned, and to the last received with many degrees of allowance. Life must give us evidence for itself and of itself before we can even begin to apprehend its nature or its relations in the universe.

But when we can gain an hypothesis distinct and definite as to what life is, and as to how it coëxists and coöperates with the physical forces of nature, here is something positive which can be held up to observation in many new points of view. It can be variously tested as to its agreement or its disagreement with the other facts of the universe. An hypothesis never arises out of nothing. Even the ancients, who tried to master the secrets of nature by bringing them within the compass of their ingenious theorizing, had yet a broad underlying basis of observation for their mental superstructures. When they held that the earth was stable and motionless

and that the sun made its great circuit about it every twenty-four hours, the hypothesis was directly founded upon their too limited observation of the facts; yet it was based all the same upon positive evidence furnished them by nature herself.

The value of any theory must evidently depend upon the intelligent selection, the comprehensiveness, and the consistent unity of the opinions underlying it and grounded with it upon a basis of observation. If it can establish itself firmly upon the plane of accepted science, and can satisfactorily account for all the phenomena of the subject to which it pertains, it must be accepted, at least provisionally, until other facts accumulate and a still broader explanation is necessitated. Hence it becomes apparent that by steadily and judiciously widening the field of observation, in time we are likely to arrive at the actual truth; at the sufficient and positive explanation of the matter of inquiry, whatever this may be, and that no farther modification of theory will be demanded in subsequent investigations.

It seems impossible to call in question our present theory that the earth turns on its axis every day and

that it revolves around the sun every year. This knowledge is so assured to us by many corroborative evidences and is based upon such an exhaustive consideration of the many related facts, that there seems to be no possibility of its future overthrow. Compared with the evidence in favor of the older system, there is all the difference between them that there is between clever guess-work as to a set of complicated perturbations, and the careful, accurate measurement of them.

Yet it is impossible to demonstrate the motions of the earth in the same absolute way in which we can demonstrate that the three angles of a triangle are equal to two right angles; or that five and five are equal to ten. An abstract proposition can be demonstrated; but every question of fact must be left to prove itself by its manifestations of perfect and entire accord with the conditions imposed by the abstract hypothesis.

In a logical proposition, we can show that the conclusion must follow from the premises; but all premises, from the nature of the case, are simply assumptions. We cannot see the earth move. We

must assume that it moves; then we can prove that if it moves so and so, such and such must be the results. Now, on turning to the facts, if we find that they correspond in all respects with those demonstrated results, we inevitably conclude that this earth does move as we suppose. The conclusion

is not so much in the nature of a demonstration as of a more enlightened perception of the actual facts! It is a scientific perception of the complicated, movable relations existing between the earth and the other members of the solar system, extending even to the other less closely related, and more distant worlds.

The greater the number, the variety, and the modifying interaction of the movements supposed, when the facts agree everywhere in detail with the mathematical deductions, the higher must be the evidence that a true explanation of the phenomena has been achieved. The theory involves not only the movements of the earth through space, but also the complex and perpetually variable movements of all the other members of our earth's family-system of worlds. Yet the mathematicians could affirm, by

the accurate working out of the results of the theory, that there ought to be another planet, an unknown. member of the solar group, occupying a certain place in the heavens.

The astronomers were able to point

the telescope as directed, and to find this much needed world just in the place where science decided that it ought to be: such evidence that theory really accords with fact becomes irresistible. And when again and again, without mistake, by calculating the various rates of planetary movement, eclipses and other far off astronomical events are predicted with almost absolute accuracy as to time and place, what shadow of doubt remains that theory and fact are here in close accord?

Yet the earlier astronomical theories were based upon fact, only the facts being so few, so limited in range the earth being taken as the fixed stand-point of observation-no hasty theory arising from them could cover the whole ground. A man sitting in a moving ship would try in vain to estimate the rate at which other vessels were moving around him, so long as he maintained that his own ship was stationary. The greater his scientific skill in exact calculation,