168 THE ORIGIN OF SPECIES 



process of natural selection is always very slow, and at any one 

 time acts only on a few forms; and partly because the very proc- 

 > ess of natural selection implies the continual supplanting-ano-ex- 

 tinction of preceding and intermediate gradations. Closely allied 

 species, now living on a continuous area, must often have been 

 formed when the area was not continuous, and when the condi- 

 tions of life did not insensibly graduate away from one part to 

 another. When two varieties are formed in two districts of a con- 

 tinuous area, an intermediate variety will often be formed, fitted 

 for an intermediate zone; but from reasons assigned, the inter- 

 mediate 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 existing in greater numbers, 

 will have a great advantage over the less numerous intermediate 

 variety, and will thus generally succeed in supplanting and exter- 

 minating it. 

 f 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 pet- 

 rels 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 



I changing conditions of life, there is no logical impossibility in the 

 acquirement of any conceivable degree of perfection through natu- 



\ ral selection. In the cases in which we know cf 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 swim-bladder has apparently been converted into an 

 air-breathing lung. The same organ having performed simultane- 

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

 whole specialized for one function ; and two distinct organs having 

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



