SEASIDE WEEDS. ' 165 



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 ai y structure may be, the greater 

 becomes its value as a test of relationship by 

 descent. 



