180 ANIMAL LIFE AND HUMAN PEOGEESS 



a hypothesis a great deal of laborioiis experimental work will 

 have to be carried through. 



Nor must we forget that there are distinguished students of 

 genetics who look at the matter from another point of view. 

 Chief among them is Professor Castle, who bases his opinion- 

 mainly upon a very extensive and valuable series of experi- 

 ments dealing with the inheritance of pigmentation in the coat 

 of rats. In this species there is a pattern form known as 

 " hooding." In a typical hooded rat the head and fore part 

 of the body are pigmented, while a pigmented stripe runs 

 along the back and includes the tail. The rest of the coat 

 is white. The extent of the white area varies considerably. 

 Starting with more or less typical hooded rats Castle made 

 two main series of experiments. In the one he selected out 

 and bred from those animals which possessed the greatest 

 amoimt of pigmentation ; in the other he used animals which 

 showed most white. In each case the process of selection 

 was carried through sixteen to seventeen generations, the end 

 result being that two strains were established, of which one 

 showed a very much greater pigmented area than any of the 

 original rats, while in the other strain the amount of white 

 was greatly increased. 



Castle further claims that when the selective process is 

 reversed either strain can be brought back to the original 

 state of pigmentation at about the same rate as that at which 

 it departed from it. Now the hooded character behaves as a 

 simple recessive to self-colour when crosses are made between 

 these two forms. Consequently Castle considers that a single 

 genetic factor can undergo quantitative change under the 

 influence of selection. The point here at issue is one of 

 fundamental importance to all work where quantitative 

 characters are involved. 



Castle's views have not escaped challenge from other genetic 

 workers. In America, where people are much more alive to 

 the importance of these things, they have become the subject 

 of keen controversy. In their recent book on The Mechanism 

 of Mendelian Heredity Morgan and his colleagues have 

 examined Castle's results in some detail and have pointed 



