6 INTRODUCTION. 



The investigatioD which we are about to describe was started six 

 years ago to test the validity of the theory of pure gametes which was 

 then current. Pure "genes" had not yet been invented. The inves- 

 t igat ion lias been in continuous progress ever since, and while we expect 

 to continue it further, it seems to us desirable that the results already 

 obtained be presented for criticism. 



Some conception of the work entailed in the investigation may be 

 gathered from the statement that we have during its progress reared 

 and studied the color pattern of over 25,000 rats. A long and arduous 

 investigation of this kind has been made possible by a series of grants 

 from the Carnegie Institution of Washington made to the senior author, 

 for which he here makes grateful acknowledgment. Thanks are also 

 due to Dean W. C. Sabine, of Harvard University, for encouraging and 

 supporting the work in a variety of ways. 



