THE FLIGHT OF BIRDS 295 



versing considerable spaces of sea, since two North American 

 species either regularly or occasionally visit the Bermudas, a 

 distance of 600 miles from the mainland. 



THE FLIGHT OF BIRDS (AvES) 



In spite of the many elaborate investigations which have been 

 made on this subject, there are numerous points in regard to 

 which we are comparatively ignorant, and the whole subject is 

 so exceedingly complicated that the space here available will only 

 serve for a brief survey of the more salient facts. 



It will be convenient to deal in the first place with the 

 structure of Birds in relation to flight, and afterwards to con- 

 sider the flying mechanism in action. 



STRUCTURE OF BIRDS IN RELATION TO FLIGHT. The entire 

 organism of Flying Birds has undergone very great specializa- 

 tion as a result of very perfect adaptation to aerial locomotion. 

 The compact boat-shaped body and pointed head are well-suited 

 to cleave the air with a minimum resistance, strongly reminding 

 us of the " lines " of Fishes and Whales, which fit them to pass 

 with ease through the water (see pp. 41 and 83). 



The fore-limbs have undergone remarkable modification by 

 which they have been converted into the chief agents of flight, 

 but in spite of this they are merely one more variation on the 

 common plan of structure that can be made out in the long 

 series of land Vertebrates, from Amphibians to Mammals. If 

 we look at the right-hand side of a Bird the partly-extended 

 wing may be diagrammatically represented by a capital Z, the 

 lower horizontal line corresponding to the upper-arm, the slant- 

 ing stroke to the forearm, and the upper horizontal line to the 

 hand. The digits have been reduced to three, which are usually 

 considered to be the thumb, first finger, and second finger. The 

 thumb may bear a claw, and this is occasionally the case with the 

 first finger as well. Although a Bird's wing differs in many ways 

 from that of a Bat, e.g. in the fact that the digits are not elongated, 

 there are nevertheless certain resemblances between the two. For 

 stretching between upper -arm and forearm there is a fold which 

 may be called the front wing-membrane, while a much smaller 

 hind wing-membrane extends between upper-arm and body (see 

 fig. 834). The former is quite comparable to the front para- 



