MAN'S CONQUEST OF NATURE 577 



Indirect Economic Value of Plants and Animals 



The Biblical .statement, "All flesh is grass," is literally true of the 

 herbivorous animals, which eat not only grass but also untold masses 

 of weeds that otherwise would crowd out useful plants. Just as 

 plants furnish food for some animals, so do some animals for carnivo- 

 rous species. Protozoa and many kinds of tiny plants form the 

 food supply of forms higher in the scale, especially crustaceans and 

 worms, which in turn are eaten by fishes. Many fishes live on 

 plankton or on smaller fishes that feed on plankton. Thus we see 

 the aquatic world is a great balanced aquarium. Man disturbs this 

 ecological balance when he dumps untreated sewage and factory 

 wastes into a stream near its source, as in the case of the Illinois River. 

 The immediate result of this unsanitary custom was the destruc- 

 tion of fish life for a distance of about 100 miles. It has been esti- 

 mated by Professor Forbes that the Illinois River, before it was 

 polluted by the Chicago drainage canal, produced annually over 

 150,000,000 pounds of fish food. On the other hand, diluted sewage 

 when emptied into a river is utilized by bacteria upon w^hich micro- 

 scopic animals feed, and these in turn furnish food for crustaceans and 

 snails, later eaten by fishes. 



We have already seen the great value of the hymenopterous 

 and lepidopterous insects to the agriculturist. There is yet to be 

 mentioned the indirect value of insects as food for useful animals. 

 Dr. Forbes, for instance, has estimated that over 50 per cent of the 

 food of many fresh-water fishes is made up of insects, mostly aquatic 

 larvae. Nor should we forget the service rendered by parasitic insects, 

 native and imported, in their war upon harmful insects. Ichneumon 

 flies and ladybird beetles stand high in this category. Insects also 

 eat enormous numbers of weeds, often acting as scavengers. Many 

 beetles and some species like the lac insect, which furnishes the basis 

 of shellac ; gall insects, from the galls of which pyrogallic acid is 

 made ; and the cochineal insect, one of the plant scales, produce 

 substances useful to man. 



The toad is of great economic importance to man because of its diet. 

 It is known to eat no less than eighty-three species of insects, mostly 

 injurious. On the whole, our common snakes are beneficial to man. 

 Even the rattlesnake and copperhead feed upon harmful rodents. 



The food of birds makes them of great importance to agriculture. 

 Investigations undertaken by the United States Department of 



