viii PREFACE. 



But above all, such as use this hook are indebted to Prof. 

 Newton, without whose continual encouragement and help it 

 would not have been written. 



The preparation of the book was aided by grants from the 

 Royal Society and the British Association, and its publication was 

 undertaken by the Syndics of the University Press. 



In regard to the classification adopted, I may simply say that 

 I have in all cases thought rather of what would be easy of 

 reference than of what would be most strictly correct according to 

 current views. 



D'ARCY W. THOMPSON. 



