As regards that most obscure of questions, what the limits of species really are, observation alone can decide the point. It frequently happens indeed that even observation itself is insufficient to render the lines of demarcation intelligible,—therefore, how much more mere dialectics! To attempt to argue such a subject on abstract principles, would be simply absurd; for, as Lord Bacon has remarked, "the subtilty of Nature far exceeds the subtilty of reasoning:" but if, by a careful collation of facts, and the sifting of minute particulars gathered from without, the problem be fairly and deliberately surveyed, the various disturbing elements which the creatures have been severally exposed to having been duly taken into account, the boundaries will not often be difficult to define. Albeit, we must except those races of animals and plants which, through a long course of centuries, have become modified by man,—the starting-points of which will perhaps continue to the last shrouded in mystery and doubt. It would be scarcely consistent indeed to weigh tribes which have been thus unnaturally tampered with by the same standard of evidence as we require for those which have remained for ever untouched and free,—especially so, since (as we have already observed) it does absolutely appear, that those species, the external aspects of which have been thus artificially controlled, are by constitution more tractile (and possess, therefore, more decided powers for aberration) than the rest. Whether traces of design may be recognized in this circumstance, or whether those forms were originally selected by man on account of their pliability, it is not for me to conjecture; nevertheless, the first of these inferences is the one which I should, myself, be à priori inclined to subscribe to.
In examining, however, this enigma, of the limits within which variation is (as such) to be recognized; it should never be forgotten, that it is possible for those boundaries to be absolutely and critically marked out even where we are not able to discern them: so that the difficulty which a few domesticated creatures of a singularly flexible organization present, should not unnecessarily predispose us to dispute the question in its larger and more general bearings. Nor should we be unmindful that (as Sir Charles Lyell has aptly suggested) "some mere varieties present greater differences, inter se, than do many individuals of distinct species;" for it is a truth of considerable importance, and one which may help us out of many an apparent dilemma.
But, whatever be the several ranges within which the members of the organic creation are free to vary; we are positively certain that, unless the definition of a species, as involving relationship, be more than a delusion or romance, their circumferences are of necessity real, and must be indicated somewhere,—as strictly, moreover, and rigidly, as it is possible for anything in Nature to be chalked out. The whole problem, in that case, does in effect resolve itself to this,—Where, and how, are the lines of demarcation to be drawn? No amount of inconstancy, provided its limits be fixed, is irreconcilable with the doctrine of specific similitudes. Like the ever-shifting curves which the white foam of the untiring tide describes upon the shore, races may ebb and flow; but they have their boundaries, in either direction, beyond which they can never pass. And thus in every species we may detect, to a greater or less extent, the emblem of instability and permanence combined: although perceived, when inquired into, to be fickle and fluctuating in their component parts, in their general outline they remain steadfast and unaltered, as of old,—
"Still changing, yet unchanged; still doom'd to feel
Endless mutation, in perpetual rest."
FOOTNOTES:
[84] Religion of Nature Delineated, p. 103.
[85] Vide supra, p. 128.
[86] Principles of Geology, 9th edition, pp. 583, 584.
[87] Vide supra, p. 121.
[88] Indications of the Creator (London, 1845), p. 163.