156 TROPICAL NATURE, AND OTHER ESSAYS. 



Here then we have an extensive group of birds, 

 considerably varied in external form, yet undoubtedly 

 closely allied to each other, one division of which is 

 specially adapted to feed on the juices secreted by 

 flowers and the minute insects that harbour in them ; 

 and these alone have a lengthened bill and double 

 tubular tongue, just as in the humming-birds. We can 

 hardly have a more striking example of the necessity 

 of discriminating between adaptive and purely structural 

 characters. The same adaptive character may coexist 

 in two groups which have a similar mode of life, with- 

 out indicating any affinity between them, because it 

 may have been acquired by each independently, to enable 

 it to fill a similar place in nature. In such cases it is 

 found to be an almost isolated character, apparently 

 connecting two groups which otherwise differ radically. 

 Non-adaptive, or purely structural characters, on the 

 other hand, are such as have probably been transmitted 

 from a remote ancestor ; and thus indicate fundamental 

 peculiarities of growth and development. The changes 

 of structure rendered necessary by modifications of the 

 habits or instincts of the different species, have been 

 made, to a great extent, independently of such characters ; 

 and as several of these may always be found in the 

 same animal their value becomes cumulative. We thus 

 arrive at the seeming paradox, that the less of direct 

 use is apparent in any peculiarity of structure, the 

 greater is its value in indicating true, though perhaps 

 remote, affinities ; while any peculiarity of an organ 

 which seems essential to its possessor's well-being is 

 often of very little value in indicating its affinity for 

 other creatures. 



