ADAPTATION TO HAUNTS 149 



habitually on or under the water, Hke the Diving Ducks 

 referred to, have the pocket of feathers well developed, 

 which keeps the wings dry. Such is adaptation 1 



The preening of the feather, so familiar in ducks, prob- 

 ably serves to keep the plumage dry. A thick layer of sub- 

 cutaneous fat serves like the blubber of whales to conserve 

 the animal heat. The air-spaces under the skin of some 

 aquatic birds, like the solan goose, may increase the non- 

 conducting quality. 



§ 3. Adaptations to Swamps 



The long legs of many of the waders, such as redshanks 

 and greenshanks, enable them to go relatively far into the 

 water, in search of food, without getting wetted. The 

 foot of the stork, with the three anterior toes slightly webbed, 

 and the backward-turned first toe well-developed, is as 

 well suited for following the frogs as are its stilt-like legs. 

 What is seen hinted at in the long toes of the water-hen 

 reaches its full expression in the South American Jacana, 

 whose very long four toes with equally exaggerated claws 

 are admirably adapted for walking on the surface of floating 

 water-leaves. 



§ 4. Adaptations to Desert and Steppe 



Hilzheimer notes that it is advantageous to lessen the 

 surface of contact if there is to be swift running over the 

 desert ; so the backward-turned toe tends to disappear, 

 and the African Ostrich has only two toes. Moreover, as 

 in the camel, there is in the Ostrich a soft elastic cushion 

 underneath each toe, which prevents sinking into soft sand. 

 The ostrich, with its great stride of six feet, is a diagrammatic 

 instance of the very long legs that desert birds often have. 

 A notable exception is seen in the sand-grouse, where the 

 legs are unusually short. Hilzheimer explains this by 

 referring to the powerful flight of these birds. The sand- 

 grouse (Pterocles) has the thrse anterior toes united by 



