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examine their minute internal structure, you 

 will probably still find many small points of 

 deep-seated difference, underlying their ex- 

 ternal adaptive similarity ; and these points 

 are the important clues which aid us in 

 discovering their real relationship to unlike 

 groups elsewhere. Thus the humming-birds 

 of America and the sun-birds of India are 

 extremely similar in outer appearance, be- 

 cause they are both highly adapted to a 

 flower-feeding existence ; but their minute 

 anatomy shows that the one family are 

 modified swifts or swallows, while the other 

 family are modified tropical fruit-eaters. 

 Hence we are landed at last in the apparent 

 paradox so ingeniously pointed out by Mr. 

 A. R, Wallace, that the less functionally 

 useful any structure may be, the greater 

 becomes its value as a test of relationship by 

 descent. 



