PREFACE TO THE SECOND EDITION 



In the present edition, although there is no change in the 

 method by which the subject of Biology is developed, there are 

 many changes in the text, some parts being condensed, others 

 elaborated, in the interest of clearness. Apart from verbal 

 improvements throughout the book, the most important altera- 

 tions and additions have been made in connection with the sub- 

 jects of fermentation and enzyme activities; the significance of 

 conjugation; plants, the food of animals; photosynthesis; circu- 

 lation in the earthworm; and immunity. Three figures in the 

 first edition (numbers 6,21, and 39) have been replaced by more 

 instructive illustrations, and in all cases where necessary, the 

 legends have been amplified. The glossary, \vhich was intro- 

 duced with the second printing of the first edition, is consider- 

 ably enlarged, and a bibliography added. For kindly criticisms 

 and many valuable suggestions in connection with the chapters 

 dealing with plant forms, I gratefully acknowledge here my 

 indebtedness to my friend Professor H. M. Richards of Barnard 

 College, Columbia University. 



GAT r< 

 J.N . ^. 



April, 1917. 



