OCR Text |
Show CH.\P. VI. 188 DIFFICULTIES ON THEORY. . d t their lower ends and must act by convergence' an .a W'th there seems to be an I•m pen.ce ct VI' tr e ous. substance. I 1 . b . fir nd lillperfect y given, these facts, here far too rie Y a . . . . h. h I th t there is much graduated diveisity In W IC SlOW' a · · ' d h the eyes of living crustaceans, an~ be~ri~g In min. ow small the number of living animals ISm proportion to those which have become extinct, I can see no very great difficulty (not more than in the case of m~ny other structures) in believing that natural ~electwn has converted the simple apparatus of an optw nerve merely coated with pigment and invested by transpare~t membrane into an optical instrument as perfect as IS possessed by any member of the great Articul~t~ class .. He who will go thus far, if he find on fi~Ish~ng th~s treatise that large bodies of factA, 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. His reason ought to conquer his imagination ; thong~ I have felt the difficulty far too keenly to be surpnsed at any degree of hesitation in extending the principle of natural selection to such startling lengths. It is scarcely possible to avoid comparing the eye to a telescope. We know tha.t this instrument has. been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous? Hav~ we any right to assume that the Creator works by Intellectual powers like those of man? If we must compare the eye to an optical instrument, we ought_ in ima?ination to take a thick layer of transparent tissue, with a nerve sensitive to light beneath, and then suppose every CHAP. VI. ORGANS OF EXTRElVIE PERFECTION. 189 part. of this layer to be c~ntinually changing slowly in d_ensity, so _as to separate Into layers of different densities and thicknesses, placed at different distances fro each ~the:, and with the surfaces of each layer slow~ ?hanging In form. Further we must suppose that there IS a power always intently watching each slight accidenta~ alteration in the_transp~rent layers; and carefully selecting each alteratwn whwh, under varied circum .. stances, ~a~ in any way, or in any degree, tend to produce a dist~ncter image. We must suppose each new state of the Instrument to be multiplied by the million. and each to be preserved till a better be produced, and the~ ~he ol~ ones to be destroyed. In living bodies, v~riatwn _will cause the slight alterations, generation ":Ill ~ult1ply them almost infinitely, and natural selectwn w~l pick out with unerring skill each improvement. Let this. process go on for ~i~lions on millions of years ; a~d during each year on millions of individuals of many ~nnds ; and ~ay we not believe that a living optical Instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man? !fit coul.d be demonstrated that any complex org~n ensted, which cou~d not .possibly have beeh formed by numerous, successive, shght 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 ;ve look to much-isolated species, round which, accordIng .to ~y theory, there has been much extinction. Or again, if we look to an organ common to all the members of a large class, for in tllis latter case the oraan mu~t ha;e been first formed at an extremely rem 5 ote period, smce which all the many members of the class have been developed ; and in order to discover the early transitional grades through which the organ has |