PREFATORY NOTE. 



The experiments on which the following paper is based were begun in 

 the Zoological Laboratory of Harvard University at Cambridge, Massachu- 

 setts, in November 1907. Since that time more than 10,500 young have 

 been recorded. In 1909 the animals were moved to the Laboratory of Gen- 

 etics at the Bussey Institution, Forest Hills, Massachusetts, at which place the 

 experiments are still in progress. 



Throughout the course of the experiments the writer has been encouraged, 

 aided, and advised by Dr. William E. Castle, to whom any merit that this 

 paper may possess is chiefly due. 



The skillful and accurate representations of the various color varieties are 

 from water-color paintings from life, by Mr. Eugene N. Fischer, whose interest 

 has shown itself in the excellent work that he has done. 



The experiments have been supported in part by an annual grant from the 



Carnegie Institution of Washington made to Dr. Castle "for the continuation 



of the study of heredity in small mammals." 



Clarence C. Little. 



May 1912. 



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