vi Preface 



societies. To each of these classes of students a quarter 

 of this volume is dedicated ; perhaps they will correct 

 their partiality by reading the whole. 



As to the two Appendixes, I may explain that instead 

 of giving references at the end of each chapter, I have 

 combined these in a connected bibliography ; the other 

 Appendix on " Animal Life and Ours " may show how my 

 subject is related to some of the others usually discussed 

 in University Extension Courses. 



My friend Mr. Norman Wyld has written the three 

 chapters on " The Powers of Life," pp. 125-166, and I am 

 also indebted to him for helpful suggestions in regard to 

 other parts of the book. I have to thank Mr. Murray, 

 Messrs. Chambers, and Mr. Walter Scott, for many of 

 the illustrations; while several original drawings have 

 been made for me by my friend Mr. William Smith. 



Professor Knight and Mr. John Murray have given 

 me many useful hints while the book was passing through 

 the press, and Mr. Ricardo Stephens was good enough 

 to read the proof sheets. 



J. A. T. 



SCHOOL OF MEDICINE, 



EDINBURGH, May 1892 



