vi PEE FACE 



the earth's history, and to appreciate the importance of the fact 

 that he derives his existence from the same ultimate sources and 

 is subject to the same natural laws as all the other living things 

 with which he shares the earth, we shall perhaps see the 

 necessity for making Biology, in the widest sense of the term, 

 one of the foundation stones of our educational system. 



In the meantime those who wish to familiarize themselves 

 with the rapidly accumulating results of biological investigation 

 and the bearing of these results upon human problems ought 

 not to be debarred from so doing by want of the necessary 

 knowledge of fundamental facts and principles, and it is largely 

 with a view to the requirements of such students that the present 

 work is offered to the public. 



That even the elementary study of biological theory should, 

 wherever possible, be preceded by a systematic course in Zoology 

 and Botany, based upon the type-system and including laboratory 

 work, is, no doubt, indisputable. Unfortunately, under existing 

 conditions, regular laboratory work is, for most people, im- 

 possible. We are apt to forget, however, that in reality we all 

 of us spend our lives in a biological laboratory, where we are 

 surrounded by living organisms which we can hardly avoid 

 studying. In this way we learn much of the nature of living- 

 things and are to some extent prepared for the study of biological 

 principles. 



The problems of life, however, cannot be satisfactorily solved 

 if we confine our attention to the higher and more familiar forms 

 of plants and animals. Man, in particular, is far too complex a 

 type to begin with in a philosophical treatment of the subject. 

 The logical method of study is, no doubt, to follow as closely as 

 possible the course which we believe to have been taken in the 

 actual evolution of living things, beginning with the simple and 

 ending with the complex. This method, of course, is attended 

 with certain practical difficulties, mainly due to the microscopic 

 size of the more primitive organisms, but these difficulties are 

 not insurmountable and need not be considered in relation to the 

 present work. 



As I wish this book to be of use to those who have had no 

 special biological training, as well as to students who have taken 

 the ordinary first year's course, I have, in the earlier chapters, 

 dealt in a very elementary manner with the structure and 

 functions of both plants and animals. I have described Amoeba 



