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Show 204 DIFFICULTI~S ON THEORY. CHAP. VI. the two forms which it connects ; conseq~entl~ the two latter during the course of further modification, from . t.' . t numbers will have a great advantage exis Ing m grea er ' . . d T ·11 over the less numerous intermediate. vanety, an 1\ ~. thus generally succeed in supplanting and extermi-nating it. . We have seen in this chapter how cautious v;e sho~ld be in concluding that the most different habits of life could not graduate into each other ; that a bat, for instance, could not have been formed by natural sel~ction from an animal which at first could only ghde through the air. We have seen that a species may under new condi-tions of life change its habits, or have diversified habits, with some habits very unlike those of its nearest congeners. Hence we can understand, b~aring in min~ that each organic being is trying to live wherever It can live, how it has arisen that there are upland geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of auks. Although the belief that an organ so perfect as the eye could have been formed by natural selection, is more than enough to stagger any one ; yet in the case of any organ, if we know of a long series of gradations in complexity, each good for its possessor,. the~, unde.r changing conditions of life, there is no logical Impossibility in the acquirement of any conceivable degree ?f perfection through natural selection. In the cases In which we know of no intermediate or transitional states, we should be very cautious in concluding that none could have existed, for the homologies of many organs and their intermediate states show that wonderful metamorphoses in function are at least possible. For i~stance, a s"rim-bladder has apparently been converted Into an air-breathing lung. The same organ having performed CHAP. VI. SUMMARY. 205 simultane?u~ly very different ~unctions, and then having been speCia~Ised for one functwn ; and two very distinct organs having performed at the same time the same function, the one having been perfected whilst aided by t~1~ other, must often have largely facilitated transitiOns. We are far too ignorant, in almost every case, to be enabled to assert that any part or organ is so unimportant for the welfare of a species, that modifications in its structure could not have been slowly accumulated by means of natural selection. But we may confidently believe that many modifications, wholly due to the laws of growth, and at first in no way advantageous to a speci~ s, have been s~bsequently taken advantage of by the still further ~odified descendants of this species. We may, also, beheve that a part formerly of high importance has often been retained (as the tail of an aquatic animal by its terrestrial descendants), though it has become of such small importance that it could not in i~s present state, have been acquired by natural seiectiOn,- a power which acts solely by the preservation of profitable variations in the struggle for life. Natural selection will produce nothing in one species for the exclusive good or injury of another; though it may well produc~ p~rts, organs, and excretions highly useful or e:en Ind~spensable, or highly injurious to another species, but In all cases at the same time useful to the owner. Natural selection in each well-stocked cou~try, ~ust act chiefly through the competition of the Inhabitants. one with another, and consequently will produc.e perfection, or strength in the battle for life, only ~ccording to the standard of that country. Hence the ~abitants ?f one country, generally the smaller one, ~ill often y~eld, as we see they do yield, to the inhabitants of another and generally larger country. For in |
