X PREFACE 



permission to republish the photographs of twins 

 in Figs. 29 to 34. Professor J. A. Piatt has kindly 

 supplied the original reference to Caesar's horse, and 

 I am indebted to Professor F. J. C. Hearnshaw for 

 certain historical references. I am very much in- 

 debted to Professor E. W. MacBride, F.R.S., for 

 kindly reading the proof-sheets and offering many 

 valuable suggestions and criticisms; also to Professor 

 A. Dendy, F.R.S., for certain suggestions. I alone, 

 however, am responsible for the views expressed. 



R. RUGGLES GATES. 



King's College, 



University of London, 



December 27, 1922. 



