TROPICAL NATURE 



soft, while the tongue remains fully developed ; and in another 

 (Meiglyptes) 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 feet, 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 birds-of-paradise to the long slender bills of Epimachinse, 

 which latter were on that account long classed apart in the 

 tribe of Tenuirostres, or slender-billed birds, but whose entire 

 structure shows them to be closely allied to the paradise-birds. 

 So, the long feathery 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 affinities than those 

 shown by organs which are in direct connection with the out- 

 side world, and are readily modified in accordance with varying 

 conditions of existence. Another, though less valuable guide 

 is afforded, 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 direct 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 uni- 

 formly 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 feather -tracts 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 humming-birds, 



