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Show 166 DIFFICULTIES ON TllEORY. [CTI.AP. VI. petrel, but with many parts of its organisation pr9foundly 1nodified. On the other hand, the acutest observer by examining the dead body of the water-ouzel :vould never have suspected its s~b-aquatic h~b~ts ; yet tlus ~nomalous member of the strictly terrestnal thrush. fa~Ily wholly subsists by diving,-grasping the stones w1th 1ts feet and using its wings under water. He who believes that each being has been created as we now see it, must occasionally have felt surprised when he has met with an animal having habits and structure not at all in agreement. vVhat can be plainer than that the webbed feet of ducks and geese are formed for swilnming? yet there are upland geese with web bed feet which rarely or never go ne~:· the 'Yater; ~nd no one. except Audubon has seen the fngate bud, whiCh has all 1ts four toes web bed, alight on the surface of the sea. On the other hand, grebes and coots are eminently aquatic, althouO'h their toes are only bordered by 1nembrane. What seem~ plainer than that the long toes of grallatores are fonned for walking over swamps and floating plants, yet the water-hen is nearly as aquatic as the coot; and the landrail nearly as terrestrial as the quail or partridge. In such cases and many others could be given, habits have chanO'ed ~ithout a corresponding change of structure. The ~ebbed feet of the upland goose may be said to have been rudimentary in function, thou~h not in structure. In the frigate-bird, the deeply scooped membrane between the toes shows that structure has begun to change. He who believes in separate and innumerable acts of creation will say, that in these cases it has pleased the Creator to cause a being of one type to take the place of one of another type ; but this seems to me o1:ly res.tating the fact in dignified language. He who believes 1n the struggle for existence and in the principle. of ~atu:al selection, will acknowledge that ev~ry organic bmng 1s co~stantly endeavouring to incr~ase 1n. num.bers ; .and that If any one being vary ever so httle, mther 1n habits or. struc· ture, and thus gain an advantage over some other Inh~bitant of the country, it will seize on the place of that Inhabitant, however different it may be from its own place. 0HAP. VI.] ORGANS OF EXTREME PERFECTION. 16'7 Hence it will cause him no surprise that there should be geese and frigate-birds with webbed feet either living on the dry land or most rarely alighting o~ the water· that ~here shou~d be long-toed corncrakes living in 1ne~dows Instead of In swamps; that there should be woodpeckers where not a tree grows; that there should be diving thrushes, and petrels with the habits of auks. Organs of extreme perfection and oomplioation.-To suppo~e t~at the eye, with all its inimitable contrivances for adJusting the focus to different distances for admitin~ d~fferent amounts of light, and for the c~rrection 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 ~e that if numerous. gradations fi·orr;· a perfect and comple~ eye to one. very Imperfect and simple, each grade being useful to Its possessor, can be shown to exist · if further the eye does vary ever so slightly, and the v~ri~tions b~ inherited~ whi?h i~ certainly the case ; and if any variation or modification In the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and co1nplex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us In ore 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 a?y speci~s ha_s been perfected, we ought to look exclusive~ y to Its hneal ancesto~s; but this is scarcely ever possible, and we are forced 1n 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 gra~ations ~re possible, and for the chance of some gradations hav1?g been transmitted from the earlier stages of descent, In an unaltered or little altered condition. Amongst existing Vertebrata, wo find but a small amount |