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To these views the advocates of theory of "unity of composition," or "analogues," have been strongly opposed. Geoffroy St. Hilaire, while he confesses that" it is very difficult for the most cautious man never to have recourse to final causes in his explanations," yet agrees with Bacon in considering them as" sterile;" and expressly observes, "I take care not to ascribe any intention to God; for I mistrust the feeble powers of my reason. I observe facts. merely, and go no further. I only pretend to the character of the historian of what IS. I cannot make nature an intelligent agent who does nothing in vain, who acts by the shortest mode, who does all for the best*." And again remarks upon the before-quoted expression of Cuvier, "I know nothing of animals who have to play a part in naturet." He has also illustrated his ideas still further by the remark, that when it is stated, for example, that fishes have certain locomotive powers because they are destined to live in the water," by the same reasoning you would say of a man who makes use of crutches, that he was originally destined to the misfortune of having a leg paralyzed or amputated."
Let us merely, in these cases, as in others, look to the correctness of our reasoning, if we be really aiming at a sound analysis of the evidences of truth. Let us but guard against the manifest error of taking
Principes de Philosophie Zoologique, p. 10.
+ Ib. p. 65.
for granted the thing to be proved. If in physiological, as in any other inquiry, we set out by assuming design, intention, or in a word, a moral cause, then it is clear that any inference we may wish to draw in support of the truth of natural theology, is vitiated and nullified. Considered, therefore, as taken in the correct order of reasoning, the caution expressed by Geoffroy St. Hilaire, appears to me strictly just, and the censure which has been cast upon it undeserved. If he did set out by "ascribing intention to the Deity," he could not arrive at any proof of such intention. The reason, indeed, which he assigns of mistrusting his feeble powers," appears to me to be a wrong one; it should rather be that all reasoning would thus be lost, and all its steps confounded together. But as a physiologist, as supplying the materials for the conclusions of natural theology, he is undoubtedly right in assuming solely the character of the historian, and seeking only bare facts and laws, that he may from them elicit the great inferences to which they lead. To assume the "economy" of causes, to "make nature an intelligent agent" in the outset, would be to anticipate the conclusion; if the idea itself do nof involve an entire confusion of thought. To assign to animals "a part to play" before we have traced their analogies, is surely premature in the order ot just reasoning, if by that expression anything more be meant than the mere guiding conjecture which the structure of their organs may suggest.
It is equally true that the habit of making such assumptions, even tacitly, is difficult to avoid. Because when proofs have already largely accumulated, we are of course prepossessed with the conclusion which we suppose the further instances must also substantiate. But when our avowed object is the exact discrimination of evidence as such, and the strict analysis of our impressions into their logical elements, the distinctions must be studiously kept in view.
The illustration of the man on crutches has been blamed as idle and unbecoming. It does not appear to me deserving of censure in this way, though I think it inapplicable, as being an argument upon an isolated and accidental case; the essence of such reasoning consists in its extension to entire species and classes of beings in their natural relations. At all events, whatever may be thought of the particular illustration, the same remark must be still applied with regard to any assumption of designing moral agency in the adaptation.
Let us, in such reasoning, only bear in mind these obvious distinctions. Let us not allow physical investigation to borrow from natural theology, if it is to furnish the very means of support to divine truth. Let not natural theology be made to minister to science, if science is to be the handmaid to natural theology.
In physical laws we must keep clear of assuming moral causation. But the general laws and analogies
already established may fairly be taken as our guides to others yet to be made out. They, indeed, must be so appealed to; for, as we endeavoured to show at the outset, they are recognised, and bear an essential part, even in the strictest logical analysis of inductive reasoning*.
In the particular instance, then, of the physiological topics referred to, we may fairly adopt (as the evidence may seem to warrant,) either the principle of " analogues," or of the "use of organs," (this latter being carefully distinguished in meaning from any idea of moral causation.) But these or other equivalent physiological theories must be taken simply as such, and barely as far as they are warranted by inductive laws; and in this way they may be most correctly adopted, and even become indispensable as our guides in considering any new case, or carrying on any further research.
Comparison of Arguments from Order and from Mechanism.
THE examination of the question last referred to may suggest some further reflections not unimportant with reference to the evidences of natural theology.
The two theories of comparative physiology to which we have been referring, and which have been
See Section I., page 25, and note,
considered as so much opposed, especially in their bearing upon the conclusions of natural theology, do not, upon closer examination, appear to present anything really at variance. Without pretending to form an opinion on their respective merits, considered physiologically, I will merely observe this much: Both systems tend equally to establish the existence of profoundly adjusted order and uniformity in nature, though of somewhat different kinds. The one system contemplates the entire range of animated beings, and unveils to the well-directed eye of the physiologist the most marvellous preservation of exact analogy, even throughout the most apparently trackless mazes of dissimilarity. other, without taking this wide range, yet establishes the same truth, within more limited and detached portions of the field of inquiry, and within those several limited portions enlarges greatly our perception of the combination of adjustments in which the same great principles are manifested.
The one theory dwells on the relations and adaptations of whole classes and orders of animated structures to each other; the other, on the relations and adaptations of the several parts of one animated structure to each other, and to the purpose of the preservation and well-being of the whole. The one leads us to consider the more abstract analogies which connect together the various parts of animated nature by general laws; the other, the more practical relations and mutual subserviency of the parts of