IV 
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 
