ECONOMIC ZOOLOGY AND ENTOMOLOGY 



PART I 



CHAPTER I 

 ANIMALS, AND THE STUDY OF ANIMALS 



Many books about animals begin with a definition of an 

 animal. As a matter of fact animals cannot be precisely 

 defined. At the bottom of the animal scale are many very 

 small, very simple creatures that are so like certain other very 

 small and very simple creatures which are usually classified as 

 plants, that one cannot state in precise words just what dis- 

 tinguishes the so-called animals from the so-called plants. 

 But the experience that everyone of us has from knowing 

 dogs, horses, birds and butterflies, and trees, bushes, flowers 

 and weeds, will serve to make us recognize the major and usual 

 distinctions between most animals and plants. The free 

 locomotion, the sense organs, nervous system and sensitive- 

 ness, the need of already living or once living substances for 

 food, the large intake of oxygen and out-pouring of carbon 

 dioxide of most animals, contrasted with the fixity, the lack of 

 sense organs and nervous system, the use of inorganic sub- 

 stances for food, and the large intake of carbon dioxide and out- 

 pouring of oxygen of most plants, are distinctions made familiar 

 by our common experiences. 



This common observation and experience enables us, also, 

 to distinguish with practical certainty between all living and 

 all non-living things, although the modern study of biology is 

 moving along lines that make it more and more nearly impos- 

 sible to tell accurately in words just what distinguishes so- 

 called living or organic nature from so-called non-living or 

 inorganic nature. In fact almost the only remaining positive 

 criterion of living matter is the inevitable presence in it of 

 certain complex chemical compounds called proteins. These 



