Preface vii 



ology in Cornell University, for many suggestions and for 

 the form of certain sections of the laboratory notes. Some 

 of the illustrations were furnished by others, or borrowed, 

 as credited in the text. Certain of the drawings were pre- 

 pared by Miss Anna M. Keichline ; others by Mrs. B. M. 

 Duggar, of whose constant assistance with manuscript and 

 proof I would express also my appreciation. 



B. M. DUGGAR. 



