196 SUMMABT. 



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 or- 

 ganic 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 anyone; 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 con- 

 ceivable degree of perfection through natural selection. 

 In the cases in which we know of no intermediate or transi- 

 tional 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 swim-bladder has appar- 

 ently 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 special- 

 ized for one function; and two distinct organs having pei'- 

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

 been perfected while 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. 



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

 assert that a part or organ is so unimportant for the wel- 

 fare of a species, that modifications in its structure could 

 not have been slowly accumulated by means of natural 

 selection. In many other cases, modifications are probably 



