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THE AMERICAN NATURALIST [Vol. XLIX 



no evidence of coupled modifiers has yet been discovered, 

 that the existence of such assumed modifiers seems at 

 present doubtful. 



Besides the triple or quadruple series of agouti allelo- 

 morphs now known for mice, guinea-pigs and rabbits, at 

 least three other Mendelian factors concerned in the pig- 

 mentation of rodents vary discontinuously in this way. 



1. Castle (1905) and Punnett (1912) have shown that 

 the Himalayan rabbit possesses a form of albinism allelo- 

 morphic with that of ordinary albino rabbits, and that 

 both are allelomorphic to ordinary pigmentation. Guinea- 

 pigs show an even more extended series of albino allelo- 

 morphs (Castle, 1914, Wright, unpublished data). 



2. Punnett (1912) has discovered in rabbits an alterna- 

 tive form of the " extension" factor, one in the presence 

 of which the agouti factor produces a less amount of tick- 

 ing than normally. He describes it as a darkened exten- 

 sion, i. e., as ordinary extension modified by a coupled 

 darkening factor. This is of course only an alternative 

 form of statement to saying that extension occurs in two 

 forms, for he discovered no cases in which the hypothet- 

 ical coupling was broken. The three allelomorphs in the 

 case of Punnett 's rabbits were accordingly: 1, ordinary 

 extension; 2, darkened extension, and 3, restriction. 



3. In still another Mendelian factor affecting the pig- 

 mentation of rodents discontinuous variation occurs at- 

 tended almost certainly by the formation of a series of 

 allelomorphs. Cuenot (1904) stated that white-spotting 

 in mice occurs in a graded series of conditions as regards 

 the amount or extent of the white areas. He found that 

 widely separated stages in the series Mendelize on cross- 

 ing, i. e., that the segregates fluctuate about modal con- 

 ditions corresponding roughly with the conditions of 

 spotting found in the respective parents crossed, and he 

 concluded the number of allelomorphs which it would be 

 possible to find in the series to be indefinitely great. Sub- 

 sequent studies of the subject made by Little (1914) in 

 mice, and by Castle and Phillips (1914) in rats, have not 

 served to simplify the matter, and yet they confirm Cue- 



