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. 



