Chap. VI. SUMMARY. 199 



unlike those of its ne:u-est conveners. Hence, we can under- 

 stand, bearing in mind that each organic being is trying to Hvc 

 wliercver it can live, how it has arisen that there arc upland 

 geese with Avebbcd feet, ground woodpeckers, diving thrushes, 

 and jx'trels with the hal)its of auks. 



Although the belief that an organ so perfect as the eye 

 could have been formed by natural selection, is more than 

 enouiih to sta^rger any one ; yet in the case of any organ, if we 

 know of a long series of gradations m complexity, each good 

 for its possessor, then, under changing conditions of life, there 

 is no logical impossibilit}'^ 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 

 sliouhl be very cautious in concluding that none have existed, 

 for the homologies of man}' organs and their intermediate states 

 show what wonderful metamorphoses 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 simultaneously very diflerent functions, and then 

 liaving Ixicn 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 perfected while aided by 

 the other, must often have largely-facilitated transitions. 



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

 in the natural scale, that an organ serving in both for the same 

 purpose, and appearing closely similar, may have been sepa- 

 rately and independently formed ; but when such organs are 

 closely examined, essential dilTerences in their structure can 

 almost always be detected; and this naturally follows from the 

 principle of natural selection. On the other hand, the conunon 

 rule throughout Nature is infinite diversity of structure for 

 gaining the same end; and this again naturally, follows on the 

 same great principle. 



In almost every case we are far too ignorant to be enaljled 

 lo assert that any 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. 

 But Ave may confidently believe that many modifications, 

 wholly due to the laws of growth, and at first in no way advanta- 

 geous to a species, have been subsef|uently taken advantage of 

 by the still furtlier modified descendants of this species. We 

 may also believe tliat a part formerly of high importance has 

 often been retained (as tlie tail of an aquatic animal by its ter- 

 restrial descendants), though it has become of such small im- 



