PERCHING ON SPRAYS. 65 



other feet, either on the body of the prey, or on some 

 other solid substance ; and even in those climbing 

 mammalia which are the most dexterous leapers, the 

 spring is taken from the hind feet, and the grand use 

 of the fore ones is to catch hold at the end of the 

 leap. In slower climbers the one set of extremities 

 are always fast while the others are extended ; and 

 in the few mammalia (such for instance as the sloths) 

 which may be said to perch, the proper perching 

 apparatus is hooks so formed by the claws and toes 

 that the requisite shape is preserved by stops o/ bone. 

 Thus, as the flexible spine of the mammalia requires 

 to be borne up by two sets of supports, when it is in 

 a horizontal position, so when they are in action 

 they may be said to have always two points of sup- 

 port upon the ground or other surface which bears 

 them up, a fore and hind one, of the opposite sides, 

 alternately, when they walk ; and the two fore and 

 the two hind ones alternately when they bound and 

 leap. The former of these actions is confined almost 

 exclusively to the legs and feet, but in the latter the 

 spine comes more efficiently into play, as may be 

 seen in the leaping of the cat tribe or in the coursing 

 of a greyhound. 



Birds deriving no support upon solid surfaces from 

 their anterior extremities, and having no action of 

 the spine to assist them in leaping, and yet having to 

 perch, and not only poise themselves, but find food, 

 construct nests, and perform other operations upon 

 perches far more unstable than any upon which the 

 mammalia have to rest, must be more sure-footed in 

 proportion. The slender sprays of trees, the flexible 

 stalks of herbaceous plants, and all sorts of sub- 

 stances, not only of a yielding nature in themselves, 

 but exposed to the violent action of the winds, by 



