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. He compares a living creature toa 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 are 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 
