Lyell extends this inference in a remarkable manner to the condition of the globe at a remote period.

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Any naturalist," (he observes,) "will be convinced, on slight reflection, of the justice of this remark. He will also admit that the same species have always retained the same instincts, and therefore that all the strata wherein any of their remains occur must have been formed when the phenomena of inanimate matter were the same as they are in the actual condition of the earth. The same conclusion must also be extended to the extinct animals with which the remains of these living species are associated; and by these means we are enabled to establish the permanence of the existing physical laws throughout the whole period when the tertiary deposits were formed."

Geology, indeed, is full of striking examples of such combined inductions. It calls to its aid the separate resources of many distinct branches of physical inquiry, and from their united testimony, collects the materials of its conclusions.

Thus the naturalist traces not only the invariable characteristics of species, but also the peculiarities which mark the adaptation of species to climate; he finds the characteristics belonging to a warm climate in the fossil remains which the geologist submits to his examination.

The astronomer demonstrates the effects of the change which is taking place with insensible slowGeology, p. 161, 1st ed.

ness in the form of the earth's orbit, to be that of a diminution (however slight,) in the mean temperature arising from the supply of heat from the sun.

While the physical philosopher argues on the theory of central heat for the primitive high temperature of the globe, the meteorological geographer finds in the peculiar distribution of land and water on the earth's surface, a powerful agent in modifying climate. The geologist infers from unequivocal proofs that such changes in the distribution of sea and land have actually occurred. And by successive local operations, extensive and ultimately perhaps universal alterations have been accomplished in the relative position of the oceans and continents. Such effects have gone on in former periods by depositions and inroads, by elevations and subsidences, as they are still continuing to do.

How vastly does the mutually-conspiring testimony of these very different trains of research tend to increase the force of the inductive inference that the fossil animals and plants alluded to existed at incalculably remote periods, when the present land was the bed of the ocean, and when the surface of our planet enjoyed a higher temperature than at present, and that from the changes of temperature and local conditions, causes were brought into action, which occasioned the extinction of some species, and were favourable to the introduction of others of new kinds.

Caution in assuming Analogies.

BUT even if in any case a particular analogy should fail; if the clue should break; if some unexpected and anomalous fact should throw into confusion our previously imagined arrangement, still so powerful is the confidence inspired by what we do satisfactorily know of the permanence of natural order, that we can never really distrust the stability of such a foundation for our reasonings.

If it be true that this particular analogy is overthrown, the only fair conclusion is, that, in this case, our conceptions were too hasty, not that the order of nature is violated. The conviction that some real analogy subsists is in no degree weakened, because we may have failed as yet to light upon it. The certainty that there is a right path is not diminished because we may have taken a wrong one.. The only effect of such an occurrence on a truly philosophic mind, will be the excitement of a still more diligent search after those characteristic circumstances which may indicate the true point of comparison.

We will illustrate this remark by a few examples: (1.) The earth moves from west to east; all the primary planets revolve in the same direction, including Uranus; the same is true of our moon, and of those of Jupiter and Saturn, and the ring of the latter planet. On the discovery of the satellites of Uranus, would it not then have appeared fair to expect that they would move in the same direction?

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Yet, in fact, they revolve in the opposite direction. But why should we thus extend the inference? Perhaps we feel persuaded that there must be something common in the nature of all the planetary motions; nor can it be denied that such an idea is very natural; the only fault is, that the hypothesis is not comprehensive enough. Let us look at one other circumstance; the earth, Jupiter and Saturn have their satellites' orbits very nearly in the plane of the ecliptic. Suppose we were to affirm that those satellites, whose orbits are nearly in the plane of the ecliptic, have their motions from west to east, then our proposition would be completely verified; for the orbits of the satellites of Uranus differ from all the others in being almost perpendicular to the plane of the ecliptic. We should, therefore, now have a very fair ground of inductive inference in supposing some connexion between the inclination of the orbits and the direction of the motion. Again, there may possibly exist a relation of another kind; the peculiarities of this planet may form a connecting link, as it were, with other laws prevailing in a further series of planets beyond it; or if it be the last, in other systems beyond ours. From such instances as this we learn no disparagement to the uniformity of nature, though much caution in forming our conjectures as to its character; no distrust of the existence of order and arrangement, but only the necessity for a just ground of probability in tracing them.

(2.) We may adduce as another instance, the case which has been fully stated, and turned to such important purposes of argument, by Mr. Babbage*, derived from his own calculating engine. Considered here merely with reference to our present subject, it is perhaps one of the most remarkable illustrations which could be selected of the necessity of caution, and the most extended range of induction, before we can satisfactorily establish the absolute generality of any conclusion.

The case is briefly this: the machine being set in a particular manner, will go on producing the series of natural numbers, 1, 2, 3, 4, &c., and this may be carried to several thousands; from induction the observer might then infer that it would go on for as many thousands more. It in fact continues up to 100,000,000; the same induction would seem to render it next to certain that the series would be continued; it produces 100,000,001; but the next term, instead of being 100,000,002, will, in fact, be 100,010,002. Here we might infer analogy is entirely broken, and there is an end of all confidence in induction. But when the calculation is continued it is found that now a new but still perfectly regular law of a different kind begins to prevail. Again, at a further extremely remote period, this ceases, and another law commences; and these all connected by another rule; and so on without limit. The fault then was simply that our first induction was * Ninth Bridgwater Treatise, p. 36.