Chap. VI.] SUMMARY. 165 



We have seen in. this chapter how cautious we should be in con- 

 cluding 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 under- 

 stand, 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 impossibility 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 performed simultaneously 

 very different functions, and then having been in part or in whole 

 specialised for one function ; and two distinct organs having per- 

 formed at the same time the same function, the one having been 

 perfected whilst aided by the other, must often have largely facili- 

 tated transitions. 



We have seen that in two beings widely remote from each other 

 in the natural scale, organs serving for the same purpose 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 fodows from the same great principle. 



In many cases we are far too ignorant to be enabled to assert that 

 a part or organ is so unimportant for the welfare of a species, that 

 modifications in its structure could not have been slowly accumu- 

 lated by means of natural selection. In many other cases, modifi« 



