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Show 200 DIFFIOULTIES ON 'fHEORY. CHAP. VI. goose or of the frigate-bird are of special use to these birds · we cannot believe that the same bones in the arm o' f the monkey, 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 had 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 more 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, &c. Hence every detail of structure in every living creature (making some little allowance for the direct action of physical conditions) 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. Natural selection cannot possibly produce any modifi-cation in any one species 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 structures 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 depo- CHAP. VI. WHAT . . NATURAL SELECTION CAN DO. 201 sited In the living b od "I es of oth · be proved that any part f th er Insects. If it could species had been formedo-" ehstructure of any one h . 10r t e excl · anot er species ' it would a nni' hI'l a te my thu sive good o£ c?uld not have been produced h eory, for such twn. Alt'hough many t t t rough natural selec-k sa ements may b £ . wor s on natural history t th' ffi e ound In even one which seems t o , Is fe ect, I cannot find d . o me o any w . ht I . a mitted that the rattlesnake has . mg . t .Is own defence and for the d t ~ poison-fang for Its J es ruction of · t some authors suppose that at th . I s prey ; but is furnished with a rattle -" 't e sam~ ~Ime this snake . lOr I s own InJur 1 warn Its prey to escape I uld y, name y, to that the cat curls the ~nd wfo't a~most as soon believe · · 0 I s tall when · spring, In order to warn the d d prepanng to not space here to enter on th~o:~ mouse. But I have Natural selection will n dother. sueh cases. th . . . ever pro uce In b . Ing InJurious to itself, for nat I I . a mng anyby and for the good of ea h ;ra se ecti?n acts solely as Paley has remarked £ c th o organ Will be formed, or for doing an injury t ?: e purpose of causing pain be struck between the o Io~Jo:sessor.. If a fair balance part, each will be -" dg nd evil caused by each lOUn on the h I d After the lapse of ti d w o .e a vantageous. life, if any part come:~' ~n ~r. c~anging conditions of fled; or if it be not so ~h eb I~uno~s, it will be modi-as myriads have b ' e. mng Will become extinct ecome extinct. ' ~ atural selection tends onl bemg as perfect as or . y to make each organic other inhabitants of th slightly more p~rfect than, the to struggle for exist e sam1 country With which it has degree of perfectionence. . nd we see that this is the demic productions fa;ained under nature. The enperfect one com a od ~w Zealand, for instance, are rapidly yielding 1e~~re ~th anothe.r ; but they are now t e advanCing legions of plants K3 |