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Show 182 DIFFICULTIES ON THEORY. [CHAP. Vl will thus generally succeed in supplanting and exterminating it. We have seen in this chapter how cautious we should 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 selection from an animal which at first could only glide through the air. We have seen that a sp,ecies 1nay under new conditions of life change its habits, or have diversified habits, with some habits very unlike those of its nearest congeners. Hence we can understand, bearing in mind 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 th~rushes, and petrels with the habits of auks. Althouo-h the belief that an organ so perfect as the eye could ~ave 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, then, under changing condit~ons of life, there is no logical impossibility in the acquirement of any conceivable degree of 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 instance, a swimbladder has apparently been converted into an air-breathing lung. The same organ having performed simultaneously very different functions, and then having been specialised for one function ; and two very distinct organs having performed at the same time the same function, the one having been perfected whilst aided by the other, must often have largely facilitated transitions. We are far too ignorant, in almost every case, to be enabled to assert,.that an;:r part or org~n is.so u"!lir~portant for the welfare of a species, that modrficatlons 1n 1ts structure could not have been slowly accumulated by means of natural selection. But we may confidently believe CnAP. VI.] SUMMARY. 183 that many modifications, wholly due to the laws of growth, and at first in no way advantageous to a species, have been subsequently taken advantage of by- the. still further modified descendants of this species. We may, also, believe 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 its present state, have been acquired by natural selection,-a power which acts solely by the preservation of profitable variations in the struggle for life. Natural selection 'vill produce nothing in one species - for the exclusive good or injury of another ; though it may well produce parts, organs, and excretions highly useful or even indispensable, or highly injurious to another species, but in all cases at the same time useful to the owner. Natural selection in each well-stocked country, must act chiefly through the competition of the inhabitants. one with another, and consequently will produce perfection, or strength in the battle . for life, only according to the standard of that country. Hence the inhabitants of one country, generally the smaller one, will often yield, as we see they do yield, to the inhabitants of another and generally larger country. For in the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer, and thus the standard of perfection will have been rendered higher. Natural selection will not necessarily produce absolute perfection; nor, a~ far as we can judge by our limited faculties, can absolute perfection be everywhere found. On the theory of natural selection we can clearly understand the full meaning of that old canon in natural history, "Natura non facit sal tum." This canon, if we look only to the present inhabitants of the world, is not strictly correct, but if we include all those of past times, it must by my theory be strictly true. It is generally acknowleged that all organic beings have been formed on two great laws-Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure, which we see in |