XIV . PREFACE 



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. 



No study of human biology should be allowed to leave in 

 the mind of the student the idea that he is merely a chemical 

 engine adapted only for the generation of a certain amount 

 of physical energy. The primary object of all secondary 

 education should be the development of character and effi- 

 ciency, and the true teacher ought to find opportunity again 

 and again to touch the individual life of the young student. 

 Especially should this be true in the study of biology. 

 Growing boys and girls ought to come to feel, as they have 

 never felt, that they have in their keeping a most complex 

 and wonderful piece of living machinery which can be easily 

 put out of order or even wrecked. But, on the other hand, 

 they should see that if the bodily machine is well cared for, 

 it is capable of splendid work which may help to increase 

 the sum total of human efficiency and happiness. 



In the preparation of this book the authors have received 

 a great many suggestions from the teachers in their own 

 departments and those of other schools. Our thanks are due 

 to Miss M. Helen Smith of the Manual Training High School, 

 Brooklyn, N.Y., for several laboratory outlines which formed 

 the basis of corresponding studies in the following pages. 

 The authors have been especially fortunate in securing the 

 constructive criticism of Dr. C. Stuart Gager, Director of 

 the Brooklyn Botanic Garden of the Brooklyn Institute of 



