PREFACE 



THIS little book is based on a course of eight 

 lectures delivered in November and December, 

 1910, before the Lowell Institute, Boston, as well 

 as on a course of five lectures delivered before 

 the Graduate School of Agriculture held under 

 the auspices of the Association of Agricultural 

 Colleges and Experiment Stations at Ames, 

 Iowa, in July, 1910. The hope is entertained 

 that it may be of service to students and that it 

 will also interest the general reader. 



The writer wishes to express his gratitude to 

 the Carnegie Institution of Washington for per- 

 mission, in its preparation, to draw freely upon 

 published and unpublished material derived from 

 investigations aided by the Institution. 



Acknowledgment is also due to the following 

 persons, or to their publishers, for permission to 

 use figures from their publications, as indicated 

 in the text : Prof. E. B. Wilson and The Mac- 

 millan Co., Prof. H. S. Jennings and The Ameri- 

 can Naturalist, Dr. W. B. Kirkham and The 

 American Book Co. 



W. E. CASTLE 



JUNE, 1911 



