^ PREFACE 



largely to whatever value the book may have. It is a 

 pleasure to acknowledge also the willing help of two 

 friendly colleagues in the work of illustration: Miss 

 R. A. Barr with her careful draughtsmanship and Mr. H. 

 Britten with his taste and skill in nature-photography. 

 Finally hearty thanks are due to the publishers for their 

 patience and consideration through the prolonged fulfil- 

 ment of the author's promise to take this part in their 



scientific enterprise. 



G. H. C. 



The University, 

 Manchester, 



October, 1927- 



