NATURAL HISTORY. 307 



1 'Tis reigning- noon ; and, vertical, the sun 

 Darts on the head direct his forceful rays. 

 O'er heaven and earth, far as the ranging eye 

 Can sweep, a dazzling deluge reigns." THOMSON. 



980. How is the varied colouring of the plumage of birds 

 accounted for? 



The variation is supposed to depend upon the sun, the colour 

 being gay and glossy according as birds are exposed to the action of 

 that luminary. Whether the bright colours of tropical birds are 

 less sentient to the sun than the more sober hues of the birds of 

 cold climates it is difficult to say ; but the smooth surface and 

 metallic lustre must reflect the light, as well as decompose it by 

 that refraction which shows the colours ; we may, therefore, 

 conclude that the variegated plumage of birds answers as a sort of 

 protection against the ardour of the sun in those climates where 

 such protection is most needed.* 



981. The hrighter colours of male birds may be in some degree accounted for 

 upon this hypothesis : the male is more exposed to the sun than the female ; nis 

 more active life and greater daring, her seasons of retirement and incubation, at 

 once explain this. That light has great influence, not only in illuminating, but in 

 developing the colours of bodies, is borne out by the fact that all night-fly ing birds 

 and insects are of sombre hues, while those which are active by day are, with only 

 a few exceptions, more brilliantly arrayed. 







982. Upon what mechanical principles do birds maintain a 

 standing position ? 



In standing, a base of sustentation must be given to birds of 

 sufficient magnitude, and in such a position as to keep the line of 

 direction of the centre of gravity within it, without too fatiguing 

 exertion of the muscles. 



983. This is accomplished partly by giving the leg such a structure that the tarsal 

 bones, which extend from the foot to the body, have a sufficient length, and are in- 

 clined to the leg bonei?, so as to direct the foot forwards ; while the body, on the other 

 hand, assumes such a position that the spinal column is inclined more or less upwards. 

 The flexibility of the neck, which enables the animal to throw the head more or 

 less backwards, and in bringing the centre of gravity into the desired position, as 

 Eh* ini in Jiff. 1, which represents the ibis. In the position of the bird here shown, 



Partington's " Cyclopaedia." 



