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Show CIIAP. VI. 186 DIFFICULTIES ON THEORY. . f t pe to take the place Creator to cause a being o one Y 1 of one of another type ; but this seems ~Io mel onb Y restating the fact m. dig ni'f ie d languaOo' e : .L e w .1 0 . e-lieves in the struggle for existence and In the pnnmp~e of nat ura1 se1 e c t1. 0n , wi'll acknowledge that ev• ery organic b e1. ng I.S cons t an tl y endeavouring to increalsie I1n nu.m hb ers. ; and that if any one being vary ever so tt e, eit er In h a bI· ts or st rue t u re , and thus gain an .a dva. ntage. over some other inhabitant of the country, I~ will s~Ize on the place of that inhabitant, however different It may b f its own place. Hence it will cause him no e rom d f. b' d surprise that there should be geese an Tigate- u s with webbed feet, either living on the dry land or most rarely alighting on the water; that the::e should ?e long-toed corncrakes living in meadows Instead of In swamps; that there should be woo~p~ckers where not a tree grows; that there should be diving thrushes, and petrels with the habits of auks. Organs of extreme perfe~tion a~d .c~mP_lication.To suppose that the eye, with all I~s Inimita~le contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and co1nplex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist ·' if further, the eye does var.y ev. er so slightly, and the variations be inherited, which. IS c.er-tainly the case ; and if any variation or modrficat~on in the organ be ever useful to an animal unde_r c~anging conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural CHAP. VI. ORGANS OF EXTREME PERFECTION. 187 selection, though insuperable by our imagination can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors ; but this is scarcely ever P?ssible, and we are forced in each case to look to speCies of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition. Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head. In this great class we should probably have to descend far beneath the lowest known fossiliferous stratu1n to discover the earlier stages, by which the eye has been perfected. In the Articulata we can commence a series with an optic nerve merely coated with pigment, and without any other mechanism ; and from this low stage, numerous gradations of structure, branching off in two fundamentally different lines, can be shown to exist, until we r~ach a moderately high stage of perfection. In certain crustaceans, for instance, there is a double cornea, the inner one divided into facets within each of which there is a lens-shaped swellin~. In other c~ustaceans the t::ansparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends |