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. INDEX. Aberration, perhaps indicated universally, 16, 17, 18. Aborigines, insect, unimportant for climatal modifications, 25, 26, 27. _Acalles_, the Canarian type of, apparent on the Salvages and Dezertas, 124. _---- Neptunus_, Woll., perhaps a state of _A. argillosus_, 124. _Achatina Eulima_, Lowe, its extinction in Porto Santo, 131. _Achenium Hartungii_, Heer, a form of _A. depressum_, 65. _Acherontia Atropos_, Linn., its introduction into Madeira perhaps recent, 74. _Adimonia_, the capture of, out at sea, 150. _Aepus marinus_, Stroem., pallid hue of, 64. _---- Robinii_, Lab., pallid hue of, 64. _Agabus bipustulatus_, Linn., unaffected by climate, 31.