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utmost possible dissimilarity in every respect, yet there was found the closest analogy; in every creature of that class could be pointed out by the skilful comparative anatomist, an exact correspondence in the nature and office of every part relative to the nature of the animal, though bearing not the slightest resemblance. When, however, these great classes themselves were compared together, such analogy seemed to be no further traceable. Each class seemed to include a distinct plan of organization, possessing an unity within itself, but not apparently related to that of the other classes. But these views have been pushed further; another school of physiologists, among whom Geoffrey St. Hilaire stands most conspicuous, have contended that even these breaks between the four classes may be filled up, and new principles of analogy developed and traced out, by which these apparently distinct plans of organization may be shown to have a common system of relation; and thus throughout all classes, a principle of still more recondite connexion be perceived.
This principle carried out to its full extent, has been named the “ unity of plan or of composition,” or the “ theory of analogues,” and the particular views of structure by which it is upheld, have been designated as the “ equilibrization of organs,” or other similar names. While, however, the main principle of the reality of some such system of primitive types is universally recognised by naturalists,
yet considerable difference of opinion prevails as to the details of its application, and as to the extent to which it may be followed out. And the question which has thus arisen has occasioned considerable controversy between the partisans of the two rival schools; principally, perhaps, in consequence of its being mixed up with other topics, with which it has at least no necessary connexion; and to which we shall recur in another place. With reference to our present purpose, it will be sufficient to remark simply, that the question between these theories, or rather the question whether the extension given to the great principle of uniformity by the second of them, is true, must be decided merely by physiological evidence: it is purely a question of fact, and must be investigated by careful examination and enlightened comparison of facts and analogies, without any reference to the speculative topics with which it has been mixed up; a distinction which, however plain, seems to have been too much overlooked.
But however this question may be determined, our chief consideration should be to observe, either way, how beautiful an extension of the great principles of natural order and harmonious arrangement is opened to our view. And it is the increasing assurance of this which is continually strengthening the foundation of all our inductive reasonings; of all consistent and profitable search into the laws of the material creation.
Induction in Geology.
We meet with perhaps the most remarkable instances of the application of the inductive conclusions of comparative anatomy in deciphering the history of fossil remains. The whole subject is, in in fact, only a continued series of such inferences. The entire structure, nature, and habits of animals once inhabiting the earth, are deduced from the occurrence of a few bones, or perhaps a single tooth, or scale, embedded and mineralized in some now solid rock; and yet with the full force of inductive evidence.
It is upon the study of fossil organic remains that the conclusions of geology mainly depend. And in a more general point of view, the researches of this science afford some of the best examples of the vast range of inference to which strict induction may be extended; of conclusions apparently the most remote, the most inconceivable, the most startling; yet all evinced by the same rigidly inductive process by which the most palpable laws of mechanical action are established. The observer finds at various elevations above the sea, beds of shells, and remains of marine animals, buried beneath the soil, or imbedded in the solid rock; he comes most legitimately to the conclusion that these relics were buried when the beds of soil or rock were in the process of formation, in a soft state; when they formed, in fact, the bottom of the waters in which, or on the margin, the animals lived and died; and that these beds have since been laid dry, and elevated into their present position.
Again he finds one sort of earth or rock extending over a considerable tract, from beneath which, at the edge of that tract, another of a widely different species comes up, and appears at the surface, showing its relative position wherever a section may be made, and proved to exist under the other wherever a well is sunk to a sufficient depth. He infers that the lower was deposited before the upper. And this simply on the same evidence as that on which we believe that the roof of a house was built at a later time than the foundation, though we did not witness it.
Again, the geologist calls in the aid of the naturalist; in the upper strata, the fossil remains are found nearly agreeing with existing species; in the lower, some perhaps the same, but many totally different; belonging, that is, to species not now known to exist. He carries on the research, and in another still lower stratum, of a different kind of material, finds fewer, or none, of existing species, and many, or all, extinct species. Nay, whole genera, and even orders of animals, successively disappearing, and new ones taking their place, as he comes into deeper deposits of other kinds of rock. His inference, that in successive periods, different classes of beings inhabited the earth; that their races successively became extinct; that new species were successively produced and destroyed in like manner; until the present order of animated beings was thus gradually introduced, is a conclusion simply dependent on the very same grounds of induction, guided by analogy, as those by which any of the most certain physical truths are established.
The appearances, in some instances, of the imbedding of organic remains may be those of a sudden destruction; in others, of a gradual deposition and covering up of the remains of many generations of animals, dying in the course of nature, and in various stages of growth. The beds containing these deposits are superposed, to a vast number; and often alternate with others in which no organic remains occur. It is from the consideration of all these and the like circumstances, on every ground of analogy, and by the most strict induction, that the geologist arrives at his conclusion, that to bring about these results involved a series of events which required a long succession of ages for their accomplishment.
Again, when it is remarked that this applies not merely to the destruction of a vast multitude of individual creatures, but to the disappearance of entire species, classes, and orders; and that this disappearance is every where gradual, the proportion of the numbers of one species discovered co-existing with another, being found to go through a regular course of diminution as we recede to older formations; the same inference is further confirmed and extended.