4 MAINE AGRICULTURAL EXPERIMENT STATION. I9I5- 



evidence he submits the results of six years (13 generations) 

 of breeding hooded rats. Attempts were made both to increase 

 and to decrease the hooded pattern. He was able to do both of 

 these things and then by return selections to bring the character 

 back to its original condition. 



This work is open to the criticism that his rats may not have 

 been homozygous for all the factors concerned in the pattern 

 .determination. The stock with which he started was derived 

 from pedigreed animals used by MacCurdy in a Mendelian study 

 of coat color." If, as is imphed in the text, these animals were 

 extracted recessives from Mendelian crosses it is quite possible 

 that they were heterozygous in respect to many characters. 

 Castle maintains that the hooded pattern is a simple unit 

 character and hence if the rats breed true to this they must be 

 homozygous. However, it is entirely possible that there are 

 various modifying factors closely associated with the hooded pat- 

 tern for which the animals were not homozygous. Indeed the 

 work of MacCurdy and Castle shows some evidence of this in 

 that "the hooded pattern, when extracted from a cross with 

 wild stock, shows a different variability, the pigmentation of the 

 extracted recessives being increased in extent." Castle and 

 Phillips discuss the theory of modifiers but discard it in favor 

 of the effect of selection upon unit characters. 



In the main the work of the last decade has supported the 

 pure line hypothesis. As noted above this hypothesis is in full 

 accord with the modern conception of inheritance. For this 

 reason no doubt it has been much more readily accepted than if 

 the reverse were the case. However, the possibility must not be 

 lost sight of that, as has happened in the past, our present con- 

 ception of the hereditary process may be materially altered in 

 the future. In view of such a possibility it would seem well to 

 make certain of the facts and to study these from various points 

 of view. 



The present paper is, in a way, a preHminary report. The 

 selections have been carried on for only three generations. This 



^"MacCurdy, H. and Castle, W. E. Selection and Cross-breeding in 

 Relation to the Inheritance of Coat-prgments and Coat-patterns in Rats 

 and Guinea-pigs. Carnegie Institution of Washington, Publication No. 

 70, 1907. ' 



