viii PREFACE 



mits, are representative mammals, reptiles, amphibia, arthro- 

 pods, molluscs, worms, and ccelenterates. In many classes 

 there are students who can work faster than the others, or 

 who are interested in pursuing further their biological stud- 

 ies. Such students may be directed in carrying on some of 

 these studies either in class or outside of school hours. In 

 any case, students are likely to acquire considerable infor- 

 mation by redding these textbook descriptions and studying 

 the illustrations. 



All the work of the year should lead up to and culminate 

 in human biology. Here, too, however, many important top- 

 ics must be treated only superficially, or altogether omitted, 

 on account of lack of time. The authors beUeve that in 

 this, the most important part of the course, practical hygiene 

 should be taught as effectively as possible, and that the 

 necessity for good food, pure air, varied exercise, and sufiB- 

 cient sleep should be continually emphasized. If boys and 

 girls can be led to conform their daily habits to the princi- 

 ples of healthy living, the course in biology will have its 

 highest justification. 



In the treatment of Stimulants and Narcotics, the authors 

 have tried to state in simple language the conclusions of 

 experts regarding the effect of tobacco and alcohol, and to 

 present the strongest scientific arguments against the use of 

 these substances which are so injurious to growing youths. 



In our judgment there are few, if any, biological topics 

 that are more important in their practical bearing than is 

 that of bacteria. As commonly studied, the disease-pro- 

 ducing effects of these organisms are emphasized so much 

 that boys and girls do not appreciate that all the work of 

 the higher organisms depends ultimately upon the activity 

 of these low forms of plant life. In order to bring out this 

 aspect of the work of bacteria, we have called special atten- 



