8 AUTHOR'S PREFACE 



other groups of sense organs, and these principles will be 

 found to be of an essentially dynamic character as con- 

 trasted with the older conceptions in which function has 

 been brought into relation less intimately with structure. 



The author is greatly indebted to the editors of this 

 series of monographs for many suggestions that have led 

 to improvements in the text. He is also under obligations 

 to his wife for a careful revision of the manuscript. He 

 wishes to extend his thanks to numerous persons who 

 have permitted him to copy and use figures contained in 

 their publications. In all such instances the sources of 

 such figures are acknowledged in the text. Where a 

 figure is given without reference, it is an original. The 

 drawings for all figures were made by Mr. E. N. Fisher. 



G. H. P. 

 Harvard University, Cambridge, Mass. 



January, 1922. 



