412 GENERAL PRINCIPLES 



and all the vital processes are at a low ebb. This sleep is 

 called hibernation. It permits the animal to tide over the sea- 

 son when food is difficult to find. The small amount of energy 

 required to maintain life in the hibernating condition is fur- 

 nished by the reserve store in the form of fat which the animal 

 possesses at the beginning of winter. 



843. Most Birds have adapted themselves to the changing 

 seasons in another way. At the approach of the winter season 

 they move southward in easy flights of twenty-five to fifty 

 miles a day and thus keep south of the region of ice. In the 

 spring this migration is repeated in the opposite direction. We 

 do not know what impels the birds to begin their migration, 

 for they do not wait until the season has advanced far enough 

 to make conditions uncomfortable. Nor do we know by what 

 means the bird is informed in which direction to fly. We call 

 such actions instinctive, which, however, does not explain them. 

 It may be that they should be classed with such rhythmical 

 physiological processes as the fall of the leaves of deciduous 

 trees, and tropisms like geotropism and heliotropism. But 

 whether the fact is explained or not the real fact remains, 

 and if a bird failed to migrate, that bird would probably not 

 survive the winter and its eccentricities would not be 

 perpetuated. 



844. Adaptations for securing food are exceedingly manifold. 

 Under this class would fall most of the peculiarities connected 

 with saprophy tic and parasitic habits. Insectivorous plants, the 

 teeth and digestive tract of the carnivorous and herbivorous 

 animals, the claws, beak and digestive tract of the birds of prey, 

 and the digestive tract of grammivorous birds may be cited 

 in this connection. The great baleen whale feeds on minute 

 pelagic organisms, which it secures by filling its mouth with the 

 water containing the food and then straining out the food by 

 allowing the water to flow out through the fringe of horny 

 baleen fibres which hangs down from the upper jaw. Many 



