PREFACE 



ALL the activities of a plant, of an animal, or of man may 

 be grouped in three classes. One class embraces the func- 

 tions relating to the life of the individual organism. These 

 functions have to do with the processes of eating, digest- 

 ing, assimilating, taking in of oxygen, producing of energy, 

 and excreting of waste matters. These may be called the 

 nutritive functions, if the term is used in its broadest sense. 

 To the second group of activities belong the functions that 

 have to do with the perpetuation of the animal or plant 

 species, and these are known as the reproductive functions. 

 Living organisms, whether plant, animal, or human, may, in 

 the third place, be considered in their relations to one another 

 and especially to the general welfare of mankind. Thus we 

 may discuss the beneficial or injurious effects, so far as man 

 is concerned, of different kinds of insects or of various types 

 of bacteria ; we may learn of the activities of individual men 

 or of groups of individuals which promote or retard the 

 advance of human society ; or we might, if we were to carry 

 the study still farther, even seek to learn the ways by which 

 the higher thoughts of mankind, as expressed in poetry, 

 music, and religion, affect the development of the human 

 race. 



In the preparation of this text, the authors have sought 

 to keep continually in mind these three classes of activities, 

 and to unify the study of plant, animal, and human biology 

 by choosing those topics for laboratory work or text descrip- 

 tion that have to do in a broad sense with one or the other of 

 the three great groups of functions of living things to which 



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