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Show 178 DIFFICULTIES ON THEORY (CHAP. VI. eyes of man, or for 1nere variety. This doctrine, if true, would be absolutely fatal to my theory. Yet I fully admit that Inany structures are of no direct use to their possessors. Physical conditions probably have had some little effect on structure, quite independently of any good thus gained. Correlation of growth has no doubt played a most important part, and a useful modification of one part will often have entailed on other parts diversified _changes of no direct use. So again characters which formerly were useful, or which formerly had arisen from correlation of growth, or from other unknown cause, may reappear froin the law of reversion, though now of no direct use. The effects of sexual selection, when displayed in beauty to charm the' females, can be called useful only in rather a forced sense. But by far the most important consideration is that the chief part of the organisation of every being is simply due to inheritance ; and consequently, though each being assqredly is well fitted for its place in nature, many structures now have no direct relation to the habits of life of each species. Thus, we canhardly believe that the webbed feet of the upland goose or of the frigate-bird are of special use to these birds; we cannot believe that the same bones in the arm of the n1onkey, in the fore leg of the horse, in the wing of the bat, and in the flipper of the seal, are of special use to these animals. We may safely attribute these structures to inheritance. But to the progenitor of the upland goose and of the frigate-bird, webbed feet no doubt were as useful as they now are to the most aquatic of existing birds. So we may believe that the progenitor of the seal bad not a flipper, but a foot with five toes fitted for walking or grasping; and we may further venture to believe that the several bones in the limbs of the monkey, horse, and bat, which have been inherited from a common progenitor, were formerly of 1nore special use to that progenitor, or its progenitors, than they now are to these animals having such widely diversified habits. Therefore we may infer that these several bones might have been acquired through natural selection, subjected formerly, as now, to the several laws of inheritance, reversion, correlation of growth, CHAP. VI.] WHAT NATURAL SELECTION CAN DO. 179 &c. l-Ienee e.very detail. of structure in every living cre~ture (mak1ng some httle allowance for the direct actl?n of physical ?onditions) may be viewed, either as haVIng been of special use to some ancestral -form or as being now of special use to the descendants of this form -either directly or indirectly through the complex laws of growth. _N at-.;ral selection can~ot possibly produce any modification 1n all:y one spec1es exclusively for the good of another species ; though throughout nature one species incessantly takes advantage of, and profits by, the structure of another. But natural selection can and does often produce st_ructures for the direct injury of other species, as we see In the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs are deposited in the living bodies of other insects. If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced thraugh natural selection. Although many statements may be found in works on natural -history to this e:ff~ct, I canr;tot fin~ even one which seems to me of any wmght. It 1s admitted that the rattlesnake has a poisonfang for its own defence and for the destruction of its prey ; but some authors suppose that at the same time this snake is furnished with a rattle for its own injury namely, to warn its prey to escape. I would almost a~ soon believe that the cat curls the end of its tail when preparing to spring, in order to warn the doomed mouse. But I have not space here to enter on this and other such cases. Natural selection wHl never produce in a being anything injurious to itself, for natural selection acts soleiy by and for the good of each. No organ will be formed, as Paley has remarked, for the purpose of causinO' pain or for doing an injury to its possessor. If a fair b~lance be struck between the good and evil caused by each part each will be found on the whole advantageous. Afte; the lapse of time, under changing conditions of life, if any part comes to be injurious, it will be modified; or if it be |