x ii PREFACE 



continue this work and to associate with many stimulating colleagues. 

 My particular thanks are due to: Mr. M. S. C. Birbeck with whom I 

 have collaborated in much of the experimental work described here ; to 

 Mrs. Rex Dadd who prepared the typescript; to Mr. K. R. Moreman 

 and Mr. M. Docherty for their careful attention to the photographic 

 material; to Dr. P. Alexander for reading the manuscript and for his 

 encouragement; to Dr. K. M. Rudall for reading the page proofs and 

 for pointing out many shortcomings; to the staff of Pergamon Press 

 Ltd., in particular Mr. B. J. Adams, for their unfailing attention during 

 the preparation of the work for publication. 



E. H. Mercer 

 1961 



