284 THE ORIGIN OF SPECIES 



he in concluding that the most different habits of life 

 could not graduate into each other; that a bat, for in- 

 stance, could not have been formed by natural selection 

 from an animal which at first only glided through the air. 



We have seen that a species under new conditions of 

 life may change its habits; or it may have diversified 

 habits, with some very unlike those of its nearest con- 

 geners. 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 thrushes, and petrels 

 with the habits of auks. 



Although the belief 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 

 complexity, each good for its possessor, then, under 

 changing conditions of life, there is no logical impossi- 

 bility 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 pos- 

 sible. 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 in part or in whole specialized for 

 one function, and two distinct organs having performed j; 

 at the same time the same function, the one having beea n 

 perfected while aided by the other, must often have ': 

 largely facilitated transitions. 



