CONTENTS. 



Page. 



Introduction 5 



Material and methods 7 



Plus selection series 9 



Minus selection series 12 



Return selection 13 



Crosses with wild rats 10 



Crosses with black "Irish " rats 18 



Plus selection of "extracted hooded" rats 20 



Crosses of the plus race with the minus race 22 



Summary of results 22 



Discussion 23 



The "mutant" series 25 



Bibliography 31 



Tables . 32-54 



Explanation of plates 56 



