vi Preface 
over, their consideration would have demanded too much 
space to be included with the present matter in a single 
volume. The fascinating study of the psychical side of living 
phenomena also belongs to the province of experimental 
biology, especially comparative psychology; but this subject 
has quite recently been fully dealt with by Loeb and by 
Jennings, who have themselves been largely instrumental-in 
developing the subject, so that further treatment would be 
more than superfluous. 
The excellent summaries and reviews of some of the topics 
discussed here, that have been published in recent years, 
have greatly facilitated my work. JI need only mention 
Roux’s and Driesch’s analysis of the experimental method, 
Grafin v. Linden’s summary of the experiments on butter- 
flies, Herbst’s excellent treatment of the subjects of “ For- 
mative Reiz,” Phillips’s very full review and literature on sex 
determination, and Cuénot’s, Lenhossek’s, and O. Schultze’s 
treatment of the same subject. I need scarcely add that, 
while using these and other reviews, I have made my own 
compilation almost exclusively from the original sources. 
Covering as extensive a field as I have attempted to cover, 
it is probable that the different subjects have received un- 
equal treatment, and I fear that some omissions may have 
been made. I trust, however, that no serious oversights or 
mistakes will be found. 
It gives me great pleasure to express here my appreciation 
of the generous assistance in the correction of the manuscript 
and proof rendered by my wife, by Professor E. B. Wilson, 
by Professor C. B. Davenport, and by Professor C. E. Castle. 
