PREFACE 
THE great interest that has been shown during the last 
fifteen years in the study of Experimental Zodlogy 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 zodlogy 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- 
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