MATERIAL AND METHODS. 



MATERIAL AND METHODS. 



In June 1906 Dr. Hansford MacCurdy completed, under the direc- 

 tion of the senior author, a study of the inheritance of color in rats. 

 His studies had shown that the piebald pattern of "hooded" rats 

 behaves as a mendelian recessive character in relation to the uniform 

 or nearly uniform coloration of wild rats, but that the hooded pattern, 

 when extracted from a cross with wild stock, shows a different vari- 

 ability, the pigmentation of the extracted recessives being increased in 

 extent. This result was interpreted as showing the unsoundness of the 

 current doctrine of "purity of the gametes" in mendelian crosses. 



Upon the conclusion of Dr. MacCurdy's experiments, the pedigreed 

 stock which he had used was not entirely discarded. A certain portion 

 of it was utilized for new experiments designed to show whether the 

 "hooded" coat-pattern can be modified by selection unattended by 

 cross-breeding. 



Two series of selections were started in October 1907, in one of which 

 animals were chosen as parents which had pigmentation as extensive as 

 possible. This we may call the plus series. In the other series animals 

 were chosen as parents which had pigmentation as restricted as pos- 

 sible. This we may call the minus series. 



During the academic year 1906-7, the experiments were in immediate 

 charge of Mr. W. G. Vinal; during 1907-8 the plus series was in charge 

 of Mr. H. S. Rand, while the minus series was in charge of Mr. F. C. 

 Bradford. Throughout this time the experiments were closely super- 

 vised by the senior author, who assisted in the "grading" of every litter 

 of young. In October 1908 the junior author began his association in 

 the experiments, which has continued up to the present time. Through- 

 out these five years he has looked after the details of the experiments 

 almost continuously, but both authors have in most cases taken part 

 together in the grading of the young, and in no case has the grading 

 been done except under the immediate supervision of one or the other 

 of the authors. This fact is stated to show that the personal element 

 in the grading has been kept as constant as possible. In the tabulation 

 of results and computation of statistical constants, the authors have 

 worked together. This statement of results is written by the senior 

 author. 



During the year 1906-7 the young rats were graded by the method 

 used by MacCurdy and Castle (1907) that is, the back-stripe was 

 measured and a calculation made of the percentage of the dorsal sur- 

 face posterior to the hood which was pigmented. But on account of the 

 irregular outline of the back-stripe in many individuals the method of 

 measurement was found to be at best a rough one, as well as extremely 

 laborious. Accordingly in the summer of 1907 a set of arbitrary grades 



