CONTENTS.

	Page.
Introduction	[5]
Material and methods	[7]
Plus selection series	[9]
Minus selection series	[12]
Return selection	[13]
Crosses with wild rats	[16]
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]

PIEBALD RATS AND SELECTION.

INTRODUCTION.

The fundamental importance of Mendel’s law of heredity is generally recognized among biologists. It is a working hypothesis whose utility is fully substantiated by abundant results daily increasing in amount. But biologists are not in agreement as to how much this law includes. All perhaps would agree that it implies the existence in the germ-cell of specific determiners essential for the production of particular characteristics in the offspring. Further, no one probably will object to the statement that it implies a dual or duplex condition of the zygote as regards determiners and a simple or simplex condition of the gamete. Thirdly, the fact will be admitted by all that most mendelizing characters are wholly independent of each other in heredity, for which reason we are forced to suppose that their determiners are distinct within the germ-cell.