PREFACE ix 



me the use of a number of figures which have done duty in other 

 books of mine. My thanks are also due to Mr. Murray, who 

 has encouraged me in a work which I was often tempted to 

 abandon, whose good-humoured patience over many delays I 

 should long since have exhausted had he been as many men 

 are. 



J. A. T. 



The University of Aberdeen, 

 August 1907. 



PREFACE TO SECOND EDITION 



The demand for a second edition has given me the opportunity 

 of correcting some errors — for a knowledge of which I am in 

 part indebted to my critics — and of inserting references to 

 some of the new discoveries that have been made in the last 

 five years in this rapidly progressive department of Biology. 



J. A. T. 



The University of Aberdeen, 

 May 1912. 



