CH. vi FORM AND FUNCTION 61 



must be large and pretty constant if much work is to 

 be done. Little appetite, little energy, is a rule that 

 holds throughout nature. In his book on the Crayfish, 

 Professor Huxley has a very instructive illustration of 

 what life is. Pie compares a living creature to a wave 

 in a river which remains always in the same place, 

 being caused by a rock, or something of the kind, 

 near the surface. A still more striking illustration of 

 the same thing is a jet of water in a cataract which, 

 except for slight variations, always keeps the same 

 shape. The wave and the jet of water arc at no two 

 moments that you look at them made up of the same 

 materials. Every moment a fresh supply of water as 

 it reaches the same point assumes the same shape and 

 appearance. So it is with the living creature : he may 

 look the same from year to year, but the atoms of 

 which he is built up are not the same. And if he is 

 to be vigorous, an animal must change his constituent 

 atoms rapidly. The large appetite, therefore, of a 

 bird is to be looked upon as a proof of strength and 

 energy. Of course, the appetite alone, without pro- 

 portionate digestive power, would be worse than 

 useless. The apparatus of digestion must be first-rate, 

 and to the investigation of this apparatus we must 

 now proceed. 



In man the saliva plays an important part. In 

 birds, however, the glands which secrete it are small, 

 and the secretion from them, probably, has but little 

 chemical effect upon the food, only helping to 

 soften it. Though the small development of the 

 saliva glands is their chief feature, they vary in 

 size in different birds, those of the Woodpecker being 



