PREFACE ix 



which an elementary knowledge of biology ought to help him 

 to answer. Some of these questions may be the following : 

 Whence comes the food and oxygen supply used by man? 

 Why are food and oxygen needed in our bodies? Why are 

 some substances beneficial to the body and others injurious? 

 What is the cause of disease, and how is disease transmitted? 

 And if we were to tabulate the biological questions that occur 

 spontaneously to the average pupil in the first year in the 

 high school, we should doubtless find that a great proportion 

 of these questions had to do with the relation of the living 

 world to human life. Is it not clear, therefore, if we are to 

 outline a course in biology that will best fit the interests of 

 the " live material/' i.e. the boy or girl who is to take the 

 course, that the central idea or factor must be man ; that all 

 the various functions considered must have some relation to 

 human life ; and that the course, to be of practical importance, 

 must suggest to the youth better ways of carrying on his own 

 life and of helping to improve the surroundings in which he 

 lives ? 



In order, however, to treat intelligently such a function, for 

 example, as respiration or digestion, it is of course necessary 

 to know something of the machinery by which each of these 

 processes is carried on, and so there must be at least a mini- 

 mum consideration of the structure of plants, animals, and 

 the human body. In every case, however, the authors have 

 called attention only to those details which seem to be abso- 

 lutely essential for an interpretation of the function under 

 consideration. Whenever names in common use are suffi- 

 ciently accurate for descriptions, these are chosen in prefer- 

 ence to scientific terms. Frequently the latter are neces- 

 sarily used, and so, whenever their meaning is made clearer 

 by referring to their derivation from Latin or Greek, these 

 derivations are indicated in parentheses. 



