PREFACE TO THE THIRD EDITION 



With the ninety-third adoption of this volume as a classroom text, 

 the need for another new edition arises. A number of changes have 

 been made, not only in the interest of later scientific findings, but also 

 where the text or illustration lent itself to some misinterpretation. Sev- 

 eral illustrations have been redrawn so as to make them more usable 

 by the student. 



Again, the author and the publisher welcome any suggestions or 

 criticisms which will make further editions of this book of still greater 

 service. 



EDWARD J. MENGE. 

 July 1, 1928. 



