HUMMING-BIRDS 331 



classification, is generally in inverse proportion to its adapt- 

 ability 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 approach- 

 ing 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 anthropoids, have the same number 

 of teeth ; while the American monkeys all have an additional 

 premolar tooth. This difference can have no relation to the 

 habits of the two groups, because each group exhibits differ- 

 ences of habits greater than often occur between American 

 and Asiatic species.; and it thus becomes a valuable character 

 indicating the radical distinctness of the two groups, a distinct- 

 ness confirmed by other anatomical characters. 



On the other hand, peculiarities of organisation which seem 

 specially adapted to certain modes of life are often diminished 

 or altogether lost in a few species of the group, showing their 

 essential unimportance to the type, as well as their small 

 value for classification. Thus the woodpeckers are most 

 strikingly characterised by a very long and highly extensible 

 tongue, with the muscles attached to the tongue-bone prolonged 

 backward over the head so as to enable the tongue to be 

 suddenly darted out ; and also by the rigid and pointed tail, 

 which is a great help in climbing up the vertical trunks of 

 trees. But in one group (the Picumni) the tail becomes quite 



