PREFACE V 



flights in this sphere, has left the ground of facts 

 below, and attained to altitudes which put the feats 

 of airmen in the shade. I am not arguing that it 

 is desirable that those writers to whom the ground 

 is distasteful should cease to fly, but merely pointing 

 out that, inasmuch as so much energy is at present 

 devoted to enterprise of this kind, there is room for 

 a book that will enable anyone to look closely at 

 some of the facts which, though they constitute the 

 ground over which a great deal of recent discussion 

 has raged, are seldom known at first hand by those 

 who witness, or even in some cases by those who 

 take part in, these battles of the air. 



My thanks are due, and are readily tendered to 

 Professor de Vries for having taken the photographs 

 which constitute Figs. 1 to 5 inclusive expressly for 

 this book ; and to the trustees of the British Museum 

 for permission to photograph the case of my mice 

 which is reproduced in Plate II. 



I should claim more credit than is properly mine 

 if I did not acknowledge the help which I have received 

 in the actual carrying out of the breeding experi- 

 ments to which such value as this book may possess 

 is in great part due. In connection with the breed- 

 ing experiments with mice, now concluded, my 

 thanks are especially due to Mr. Charles Biddolph 

 * 



