CHAP, viz THE INDUSTRIES OF ANIMALS 135 



heavy tread, the swift gallop of Mammals. All their 

 gambollings and playful frolics, their travels in search of 

 food, and their wanderings over land and sea, should be 

 reckoned up. 



Most marvellous is the winged flight of birds. As a 

 boat is borne along when the wind fills the sails, or when 

 the oars strike the water, and as a swimmer beats the 

 water with his hands, so the bird beating the air back- 

 wards with its wings is borne onward in swift flight. 

 But the air is not so resistent as the water, and no bird 

 can float in the air as a boat floats in the water. Thus 

 the stroke has a downward as well as a backward direc- 

 tion. When there is more of the downward direction 

 the bird rises, when there is more of the backward direc- 

 tion it speeds forward ; but usually the stroke is both 

 downwards and backwards, for the lightest bird has to 

 keep itself from falling as it flies. The hollo wness and 

 sponginess of many of the bones combine strength of 

 material with lightness, and the balloon-like air-sacs 

 connected with the lungs help indirectly in the rapid 

 breathing ; but, buoyant as many birds are, all have 

 to keep themselves up by an effort. But the possi- 

 bility of flight also depends upon the fact that the raising 

 of the wing in preparation for each stroke can be accom- 

 plished with very little effort ; the whole wing and its 

 individual feathers are adjusted to present a maximum 

 surface during the down-stroke, a minimum surface 

 during the elevation of the wing. There are many 

 different kinds of flight, which require special explana- 

 tion the fluttering of humming-birds, the soaring of the 

 lark, the masterful hovering of the kestrel, the sailing 

 of the albatross. The effortless sailing motion of many 

 birds is comparable to that of a kite, ' ' the weight of the 

 bird corresponding to the tail of the kite " ; it is possible 

 only when there is wind or when great velocity has been 

 previously attained. 



In connection with the movements of animals there 

 arises the interesting problem of finding the way, which 

 is illustrated by homing-pigeons, hive-bees, nesting-birds, 

 and many other creatures. For hive-bees it has been 



