332 TROPICAL NATURE Iv 
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 Epimachina, 
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, 
