PREFACE TO THE THIiil) EDITION 



Much ecological research has been carried on in the interim since 

 the appearance of the second edition of this book. There is now 

 a much closer agreement on many ecological principles between 

 various workers than there was some years ago. At the same time 

 there still are many disagreements and there probably will be for 

 a long time to come, which is inevitably true of a healthy, grow- 

 ing science. 



In this, the third, edition the same general order of presentation 

 has been followed as in previous editions, because the author's 

 many years of teaching experience indicated that this was the most 

 acceptable order. As in previous editions, too, a rather large 

 amount of space is devoted to a discussion of symbiotic phenomena 

 since the author still believes that, in general, too little attention 

 is given to the living part of the envu-onment as compared to the 

 non-living. Every effort has been made to keep this edition brief, 

 concise and easily readable. It is largely because of these qualities 

 that it is believed the book will continue to fill a distinct need. 



The author's greatest source of help in preparing this revision, 



aside from the published papers of numerous workers, has come 



from criticisms of the previous editions. To all who offered such 



criticisms he is most grateful and he again solicits unrestrained 



criticisms of the present edition. 



W. B.McD. 



Santa Fk, New Mexico. 



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