INTRODUCTION ii 



be able to evaluate them and to determine whether they should be 

 treated as friends and encouraged to increase, or even artificially- 

 propagated, or whether they should be attacked by chemical and 

 mechanical agencies or by protecting their natural enemies. 



The most practical way within our reach of studying that 

 adjustment between the organism and the external world — the 

 fitness — which constitutes life is to learn all we can about the 

 physical basis, and all we can about its fitness. 



To study life, we must consider three things: (i) the orderly 

 sequence of external nature; (2) the living organism and the changes 

 that take place in it; and (3) that continuous adjustment between 

 the two sets of phenomena which constitutes life. 



Plants and Animals. — It might appear that, since man is an 

 animal, Zoology is the more important of the two studies. But we 

 must remember our thesis — the proper relation of the individual to 

 his environment — and we will see that plants and animals alike make 

 up the environment. The study of the relation of animals and 

 plants to disease is at present demanding the attention of some 

 of the world's greatest scientists. 



To Linnaeus (1707-78) we owe the classification of plants and 

 animals. He stated that "plants grow and live, while animals 

 grow, live, and feed." Owen (1803-93) declared that a definition 

 of plants excluding all animals, or of animals excluding all plants, 

 is impossible. As we go down to the simplest forms we find dif- 

 ficulty in distinguishing between plant and animal. No one can 

 tell. We call these lowest types Zoophytes (Protista) and Phytozoa. 



Origin. — Every organism kiiown in nature arises as a simple cell. 

 In the plant we call it the ovule; in the animal the ovum. 



Composition. — So far as their chemical nature is concerned, the 

 plant and animal cell are the same. This has been repeatedly 

 proved. All contain carbon, hydrogen, oxygen, and nitrogen. 

 The skeletons, however, differ widely. The skeleton is largely a 

 cell wall modified in many ways. In general, plants exhibit a linear 

 aggregation of cells, while animals show a mass aggregation. 



Morphology. — No outline can be drawn which will be common 

 to all plants and animals. 



Physiology. — Plants and animals stand widest apart in the 

 mode of nutrition, and here we have the chief distinctions: 



