32 NATURAL HISTORY OF VERTEBRATE ANIMALS 



that plants are able to maintain themselves upon non-living 

 materials, whereas animals arc dependent for their existence either 

 upon the bodies of other animals or upon plants. 



When the food of animals is traced to its source, the green 

 plants are found to support not only themselves but also the vast 

 majority of animals. Herbivorous animals feed directly upon green 

 vegetation ; carnivorous animals are usually but one step removed, 

 because they feed upon other animals that are herbivorous. 

 Examples will occur to the reader. Since the green plants depend 

 upon sunlight for the energy by which they combine simple chem- 

 ical compounds into the complex ones necessary for a Uving body, 

 the maintenance, from day to day, of every living thing upon the 

 land is dependent upon the light of the sun and the green sub- 

 stance, chlorophyll, in plants. If the green plants should be sud- 

 denly wiped out of existence, the animals would soon perish, for at 

 the best they could do no more than " eat each other up," like the 

 gingham dog and the calico cat. It is true that plants derive much 

 cf their available food from the decomposition of the bodies of 

 animals, but this does not alter the general fact that animals 

 depend upon plants for their food, whereas plants are not so obvi- 

 ously dependent upon animals. Even more fundamental than this 

 difference in the source of their energy is the contrast between the 

 chemical synthetic powers of plants and animals. Green plants 

 make their own nutrients; animals digest and recombine theirs. 

 The colorless plants, or Fwigi, get their energy from organic matter, 

 as do animals, but they are closely related to plants rather than to 

 animals. 



In the foregoing pages, the reader's attention has been directed 

 to the life that is found upon the land. The relationships that 

 exist in the ocean are similar, although at first glance there seems 

 to be nothing there to correspond to the mass of verdure that 

 clothes the fertile portions of the land. 



If we try to call before the mind a picture of the land surface 

 of the earth we see a vast expanse of verdure stretching from high 

 up in the mountains, over hills, valleys, and plains, and through 

 forests and meadows, down to the sea, with only an occasional 

 lake or broad river to break its uniformity. 



Our picture of the ocean is an empty waste, stretching on and 

 on with no break in the monotony except now and then a flying- 

 fish or a wandering sea-bird or a floating tuft of vegetable life. It 

 contains plant-like animals in abundance, but these are true 



