184 SUMMARY. 



an intermediate variety will often be formed, fitted for an 

 intermediate zone ; but from reasons assigned, the interme- 

 diate variety will usually exist in lesser numbers than 

 the two forms which it connects ; consequently the two 

 latter, during the course of further modification, from exist- 

 ing in greater numbers, will have a great advantage over 

 the less numerous 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 lire 

 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 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 per- 

 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 



