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Show 168 DIFFICULTIES ON THEORY. [CHAP. VI. of gradation in the structure of the eye, and fro~ fossil species we can learn nothing on this bead. In this great class we should probably have to desce:r;d far beneath ~he lowest known fossiliferous stratum to discover the earher sta es, by which the eye has been perfected. . . ~n the Articulata we can commence a seri~S With an optic nerve merely coated with. pigment, and Without any other mechanism · and from this low stage nuinerous gradations of struct~re, branching off i~ two f~ndamentally different lines, can be shown to. exist, until w~ reach a moderately high stage of perfection. In certai;n crustaceans, for instance, there is a double c~rnea, the .Inner one divided into facets, within each of which there IS a lensshaped swelling. In other ~rustaceans the. transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends and must act by co~ vergence ; ~nd at their lower ends there seems to be an Impe~fect vitre?us substance. With these facts, here far too bnefiy and Imperfectly given, which s~o:V that there is much gra~uat~d diversity 1n the eyes of hving cr~s~aceans? and ~eaying 1n mind how small the number of hving animals IS In proportion to those which have become .extinct, I can see no very great difficulty (not more than In the case o~ many other structures) in believing that natur~l selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transp~rent membrane into an optical instrument as perfect as IS possessed by any member of the great Articulate class. . . . He who will go thus far, if he finds ~n ~nishu~g thiS treatise that large bodies of facts, otherwise Inexplicable, can be explained by the theory of descent, ought not to hesitate to go further, and to admit. that a structure even as perfect as the eye of an eagle might be formed by natural selection, although in this case he does not know any of the transitional grades. llis reason ou~ht to conquer his imagination; though I have felt the di~cu~ty ~ar too keenly to be surprised at any degree ?f hesitation In ~xtending the principle of natural selection to such starthng lengths. 0RAP. VI.] ORGANS OF EXTREME PERFECTION. 169 It is scarcely possible to avoid comparino- the eye to a telescope. We know that this instrume~t has been perfe~ted by the long-continued efforts of the highest human Intellects; and we naturally infer that the eye has been f?rmed by a somewhat analogous _process. But may not this inference be presumptuous? Have we any right to assume that the Creator works by intellectual powers lik~ tho~e of man ? If we must compare the eye to an op.tiCal Instrument, we ou~ht in imagination to take a t~nck layer of transparent tissue, with a nerve sensitive to hght benea~h, and then s.uppose every part of this layer to be co~1tinually chan~Ing slo·wly in density, so as to separate Into layers of· dlfi'erent densities and thicknesses placed at different distances from each other, and with th~ surfaces of each laye~ slowly ~hanging in form. Further we m~st suppose .that th~re IS a power always intently watching each shght accidental alteration in the transpar. ent layers; ~nd c.arefully selecting each alteration :vhwh, under varied cncumstances, may in any wa.y, or In any degree, tend to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million ; and each to be preserved till a bett~r.l)e pro?uced, ~n~ the~ the old ones to be destroyed. In hving bodies, variation will cause the slio-ht alterations generation :Will :r;nult~ply the~ almost infi~itely, and na~ tural selection w1ll piCk out With unerring skill each improvmnent. Let this :process go on for n1illions on milli? ns of years; an~ during each year on 1nillions of indi': I~uals of. ma~y lnnds ; an~ may we not believe that a hving optical Instrument m1ght thns be formed as superior to one of glass, as the works of the Creator are to those of man? If it could be demonstrated that any complex oro-an existed, which could not possibly have been formed5by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case. No doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, round which, according to my theory, there has been much extinction. Or again, if s |