No. 566] NEW VARIETIES OF RATS AND GUINEA PIGS 73 



shown by Darbishire some ten years ago. This indicates 

 that the two variations are not only genetically distinct, 

 but are physiologically complementary. The albino lias 

 defective metabolism for producing yellow (and in conse- 

 quence brown and black also) ; the pink-eyed animal has 

 the full mechanism for forming yellow, but its brown and 

 black producing mechanism is defective. Together they 

 possess the full mechanism of normal color production. 

 Hence the reversion on crossing. 



White spotting is clearly due to neither of the above 

 modifications, but to a different change in the metabolism 

 so that no pigment at all is produced. For an albino rab- 

 bit or guinea-pig may, as already observed, bear consider 

 able black or brown pigment, but a white spot either on 

 an albino, on a pink-eyed animal, or on a fully colored 

 animal is entirely devoid of pigment. The paradox of a 

 white spot on an albino is obtainable by crossing a white- 

 spotted colored race with an albino race, which develops 

 some pigment in the fur, as for example the Himalayan 

 race of rabbits and guinea-pigs. In this way English- 

 marked Himalayan rabbits and spotted albino guinea- 

 pigs have been produced in my laboratory. 



Postscript: While this paper was in press, Mr. Tilling, 



eyed yellow rat was caught on a ship at Liverpool. The 

 fact that the pink-eyed variety was found in the same gen- 

 eral region leads him to believe that both variations were 

 introduced on ships from some foreign country. It would 

 be of much interest to know from what country or conn- 

 tries. Any information on this point obtainable from 

 rat-catchers or others would be welcome. 



