258 SUMMARY. [Chap. VI. 



intermediate variety, and will thus generally succeed in 

 supplanting and exterminating it. 



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. 



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 

 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 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 con- 

 ditions of life, there is no logical impossibility in the 

 acquirement of any conceiyable 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 haying performed simultaneously very different 

 functions, and then having been in part or in whole 

 specialised for one function ; and two distinct organs 



