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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 truc 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 cach other in habits, in food, and in their enormous bills; yet peculiarities in the structure of the fect, 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 difference can have no relation to the habits of the two groups, because cach group exhibits differences 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 distinctness confirmed by other anatomical characters.
On the other hand, peculiarities of organization 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 enalyle 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 soft, while the tongue remains fully developed ; and in another (Weiglyptes) the characteristic tail remains, while the prolonged hyoid muscles have almost entirely disappeared, and the tongue has consequently lost its peculiar extensile power ; yet in both these cases the form of the breast-bone and the character of the fect, the skeleton, and the plumage, show that the birds are really woodpeckers; while even the habits and the food are very little altered. In like manner the bill may undergo great changes; as from the short crow-like bill of the true Lirds-of-paradise to the long slender bills of Epimachina, which latter were on that account long classed apart in the tribe of Tenuirostres, or slenderbilled birds, but whose entire structure shows them to be closely allied to the paradise-birds. So, the long fcathery tongue of the toucans differs from that of every
other bird; yet it is not held to overbalance the weight of anatomical peculiarities which show that these birds are allied to the barbets and the cuckoos.
The skeleton, therefore, and especially the sternum or breast-bone, affords us an almost infallible guide in doubtful cases; because it appears to change its form with extreme slowness, and thus indicates deeper-seated atlinities than those shown by organs which are in direct connection with the outside world, and are readily mollified in accorilance with varying conditions of existence. Another, though less valuable guide is afforilel, in the case of birds, by the eggs. These often have a characteristic form and colour, and a peculiar texture of surface, running unchanged through whole genera and families which are nearly related to each other, however much they may differ in outward form and habits. Another detail of structure which has no (lirert connection with habits and economy, is the manner in which the plumage is arranged on the body. The feathers of birds are by no means set uniformly over their skin, but grow in certain definite lines and patches, which vary considerably in shape and size in the more important orders and tribes, while the mode of arrangement agrees in all which are known to be closely related to each other; and thus the form of the feathertracts or the “pterylography" as it is termed, of a bird, is a valuable aid in doubtful cases of affinity.
Now, if we apply these three tests to the hummingbirds, we find them all pointing in the same direction. The sternum or breast-bone is not notched behind ; and this agrees with the swifts, and not with the sun-birds, whose sternum has two deep notches behind, as in all
the families of the vast order of Passeres to which the latter belong. The eggs of both swifts and hummingbirds are white, only two in number, and resembling each other in texture. And in the arrangement of the feather-tracts the humming-birds approach more nearly to the swifts than they do to any other birds; and altogether differ from the sun-birds, which, in this respect as in so many others, resemble the honey-suckers of Australia and other truc passerine birds.
Resemblances of Swijis and IIumming-birds.--IIaving this clue to their atlinities, we shall find other peculiarities common to these two groups, the swifts and the humming-birils. They have both ten tail-feathers, while the sun-birids have twelve. They have both only sixteen truc quill-feathers, and they are the only birds which have so small a number. The humming-birls are remarkable for having, in almost all the species, the first quill the longest of all, the only other birds resembling them in this respect being a few species of swifts; and, lastly, in both groups the plumage is remarkably compact and closely pressed to the body. Yet, with all these points of agreement, we find an extreme diversity in the bills and tongues of the two groups. The swifts have a short, broad, flat bill, with a flat horny-tipped tongue of the usual character ; while the humming-birds have a very long, narrow, almost cylindrical bil, containing a tubular and highly extensible tongue. The essential point however is, that whereas barilly any of the other characters we have adduced are adaptive, or strictly correlated with habits and cconomy, this character is pre-eminently so; for the swifts are purc aërial insect-hunters, and their short, broad bills, and wide gape, are essential to their mode of life. The humming-birds, on the other hand, arc floral inscet-hunters, and for this purpose their peculiarly long bills and extensile tongues are especially adapted ; while they are at the same time honey-suckers, and for this purpose have acquired the tubular tongue. The formation of such a tubular tongue out of one of the ordinary kind is easily conceivable, as it only requires to be lengthened, and the two lamine of which it is composed curled in at the sides; and these changes it probably goes through in the young birds.
When on the Amazon I once had a nest brought me contining two little unfledged humming-birds, apparently not long hatched. Their beaks were not at all like those of their parents, but short, triangular, and broad at the base ; just the form of the beak of a swallow or swift slightly lengthened. Thinking (erroneously) that the young biruls were fed by their parents on honey, I tried to feed them with a syrup made of honey and water, but though they kept their mouths constantly open as if ravenously hungry, they would not swallow the liquid, but threw it out again and sometimes nearly choked themselves in the effort. At length I caught some minute flies, and on dropping one of these into the open mouth it instantly closed, the fly was gulped down and the mouth opened again for more; and cach took in this way fifteen or twenty little flies in succession before it was satisfied. They lived thus three or four days, but required more constant care than I could give them. These little birds were in the “swift" stage; they were pure insect-caters, with a bill and mouth adapted for insect-cating only. At that time I was not