ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 379 



are free to move about within certain, often narrow, limits, 

 to find new pasture or to change their domicile. In a small 

 society of green plants it is comparatively easy to find all 

 the species, for they are fixed in place, and come out into 

 the light, and into view; but so different is the case with 

 animals, so small are many of them and so secretive and 

 elusive of habit, that there is not an acre of the earth's sur- 

 face of which the entire animal population is known. Even 

 of that class of large animals to which we ourselves belong, 

 there are many mammals living in our own immediate 

 environs that we seldom or never see. 



As already stated in the opening chapter, herbivores and 

 carnivores, parasites and scavengers are everywhere, be- 

 cause they fulfill permanent functions in natural society. 



The herbivores are, 

 among animals, the pro- 

 ducing class; all the 

 others are consumers. 

 The food of animals is 

 not to be found every- 

 where, even that of the 

 most omnivorous 

 species. The deer that 

 roams the forest, crop- 

 ping the leaves and 

 twigs of a great variety 

 of plants, leaves a much 

 greater number of spe- 

 cies untouched. The 

 caterpillars of the gypsy 

 moth will eat the leaves 

 of almost .every green 

 FIO. 221. Photograph from life of a tree, but most caterpffl- 



young a nd * iy weevil larva arg W JU; ea ^ o f a single 



