R C. PUNNETT 317 



is a far less satisfactory explanation of the facts than the hypothesis of 

 independent factors such as I have suggested. Moreover on this latter 

 hypothesis there is no need to drag in any conception of " mutual 

 modification." Such a conception is in my opinion entirely opposed to 

 the general trend of modern genetic work. If, after adequate analysis, 

 cases are found which afford indubitable evidence for such a process, 

 then it must be carefully taken into account. But after many years 

 spent in breeding Dutch rabbits I am strongly inclined to believe that 

 the facts will eventually receive an explanation on a strictly factorial 

 basis; nor at present do I see good reason for supposing that the factors 

 concerned shew any more tendency to undergo modification through 

 crossing than in the countless instances in both plants and animals 

 where apparent clean segregation is the rule. 



[The experiments on Dutch rabbits alluded to in this paper have 

 been carried on with assistance from the Development Commission 

 Fund administered by the Board of Agriculture For those dealing 

 with heterochromia iridis grants have been received from the Govern- 

 ment Grant Commission.] 



EXPLANATION OF PLATE XI. 



Fig. 1. Typical Dutch rabbit. The right eye shews some heterochromia. Note the small 



white patches on the back. 

 Fig. 2. White Dutch. In this animal both eyes were blue. 

 Fig. 3. Spotted Dutch. The pigmentation in most animals of this strain is rather more 



pronounced. 

 Fig. 4. Fi rabbit ex Typical Dutch x White Dutch. 



