Page - 216 - in The Origin of Species

216 ORIGIN OF SPECIES 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 only glided through the air. W e have seen that a species under new conditions of life may change its habits; or it may have diversified habits, with some verj^ 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 woodpeck- ers, diving thrushes, and petrels with the habits of auks. Although the behef that an organ so perfect as the eye could have been formed by natural selection, is enough to stagger any one; yet in the case of any organ, if we know of a long series of gradations in complexit}-, each good for its possessor, then, under changing conditions of life, there is no logical impossibiHty 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 extremely cautious in concluding that none can have existed, for the metamorphoses of many organs show what wonderful changes in function are at least possible. For instance, a swimbladder has apparently been converted into an air-breathing lung. The same organ having per- formed simultaneously very different functions, and then having been in part or in whole specialised for one function ; and two 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 have seen that in two beings widely remote from each other in the natural scale, organs serving for the same pur- pose and in external appearance closely similar may have been separately and independently formed: but when such organs are closely examined, essential differences in their structure can almost always be detected; and this naturally follows from the principle of natural selection. On the other hand, the common rule throughout nature is infinite diversity of structure for gaining the same end; and this again naturally follows from the same great principle.