AFFINITIES OF HUMMING-BIRDS. 151 



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 outside world, and are readily 

 modified in accordance with varying conditions oi 

 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 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 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, 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 



