184 ON THE ORIGIN OF SPECIES 



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 impossi- 

 bility 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 very cautious in concluding that none 

 could have existed, for the homologies of many organs 

 and their intermediate states show that wonderful meta- 

 morphoses 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 different functions, and then having 

 been specialised for one function ; and two very distinct 

 organs having performed at the same time the same 

 function, the one having been perfected whilst aided 

 by the other, must often have largely facilitated 

 transitions. 



We are far too ignorant, in almost every case, to be 

 enabled to assert that any part or organ is so unim- 

 portant for the welfare of a species, that modifications 

 in its structure could not have been slowly accumulated 

 by means of natural selection. But we may confidently 

 believe that many modifications, wholly due to the laws 

 of growth, and at first in no way advantageous to a 

 species, have been subsequently taken advantage of by 

 the still further modified descendants of this species. We 

 may, also, believe that a part formerly of high import- 

 ance has often been retained (as the tail of an aquatic 

 animal by its terrestrial descendants), though it has 

 become of such small importance that it could not, in 

 its present state, have been acquired by natural selec- 

 tion, — a power which acts solely by the preservation of 

 profitable variations in the struggle for life. 



Natural selection will produce nothing in one species 

 for the exclusive good or injury of another ; though it 

 may well produce parts, organs, and excretions highly 

 useful or even indispensable, or highly injurious to 

 another species, but in all cases at the same time useful 

 to the owner. Natural selection in each well-stocked 



