AEFINITIES OF HUMMING-BIRDS. 149 



preference those parts of an animal which have little or 

 no direct influence on its habits and general economy. 

 The value of an organ, or of any detail of structure, 

 for purposes of classification, is generally in inverse 

 proportion to its adaptability to special uses. And the 

 reason of this is apparent, when we consider that 

 similarities of food and habits are often accompanied by 

 similarities of external form or of special organs, in 

 totally distinct animals. Porpoises, for example, are 

 modified externally so as to resemble fishes ; yet they 

 are really mammalia. Some marsupials are carnivorous, 

 and are so like true carnivora that it is only by minute 

 peculiarities of structure that the skeleton of the one 

 can be distinguished from that of the other. Many of 

 the hornbills and toucans have the same general form, 

 and resemble each other in habits, in food, and in their 

 enormous bills ; yet peculiarities in the structure of the 

 feet, in the form of the breast-bone, in the cranium, and 

 in the texture and arrangement of the plumage, show 

 that they have no real affinity, the former approaching 

 the kingfishers, the latter the cuckoos. Such structural 

 peculiarities as these have no direct relation to habits ; 

 and they are therefore little liable to change, when from 

 any cause a portion of the group may have been driven 

 to adopt a new mode of life. Thus all the Old World 

 apes, however much they may differ in size or habits, 

 and whether we class them as baboons, monkeys, or 

 gorillas, have the same number of teeth ; while the 

 American monkeys all have an additional premolar 

 tooth. This difierence can have no relation to the 

 habits of the two groups, because each group exhibits 

 differences of habits greater than often occur between 



