PREFACE 



The great interest that has been shown during the last 

 fifteen years in the study of Experimental Zoology has led 

 to the rapid development of this branch of biology. An 

 attempt is made in the following pages to bring together the 

 results of this work. A series of about thirty-five lectures 

 formed the basis for my treatment of the subject, and this 

 will account, in part, for the way in which the matter has 

 been handled ; many details have been omitted that an 

 exhaustive treatment would demand ; and the plan has been 

 to select the most typical and most instructive cases for 

 presentation, when such a choice was possible. Neverthe- 

 less, I believe that the reader will find a fairly full account 

 of the subjects considered. 



Physiology has from the beginning made use of the method 

 of experiment, and with notable success. Morphology has, 

 up to the present time, followed mainly the historical and 

 descriptive methods, although striking exceptions could be 

 cited. While the historical study of zoology must always 

 remain a legitimate field for activity, as human history has 

 been a time-honored study, yet there can be little doubt that 

 the more promising and searching method of zoological study 

 in the future will be found in experiment. 



The central problem of morphology — the causes of the 

 changes in form, or at least the determination of the condi- 

 tions under which changes in form occur — will furnish the 

 main theme of the present treatise. Two fields of study that 

 properly fall under this head are, however, not considered, 

 viz. experimental embryology and the experimental study 

 of regeneration. Both of these subjects have in recent years 

 received comprehensive treatment in book form, so that it 

 did not seem desirable to go over the ground again. More- 



