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THE AMERICAN NATURALIST [Vol. XLVII 



condition appears, its extent being determined by other 

 factors. 



The extension of spotting would be considered as due 

 to successive mutations which could only be realized after 

 the first stage ss had occurred. Such stages would be 

 represented by sss^s^ sss 2 s 2 , sss 3 s 3 or complexes of these, 

 namely, sss 1 s 1 s 3 s 3 or ss# 1 s 1 ]S' 3 s 3 , etc. Selection would 

 then consist in eliminating from such combinations dif- 

 ferent factors. The hypothesis is in a sense complex, but 

 so are the facts. We shall consider this hypothesis later 

 after our facts have been presented. 



Castle has recently pointed out that there are cases of 

 yellow-and-white-spotted guinea-pigs that breed true. 

 In these he assumes that a chromogen factor (the one 

 that makes any color possible) is irregularly distributed. 

 Hence, wherever color is produced that color is yellow. 

 Where no color is produced, because of the absence of the 

 color producer, white results. Black-and-white races, if 

 such exist (Castle does not specifically mention such 

 races except black-and-white from tricolors of the tri- 

 color series), would fall under a similar scheme. Yellow- 

 and-black animals also exist with no white (Castle). In 

 this case the color factor for black is assumed to be dis- 

 tributed irregularly. 



Castle's explanation for the tricolors is as follows: 



Now the tricolor race is a yellow one spotted both with white and with 

 black, i. e., it results from irregularity in distribution through the coat 

 of two different chemical substances, the color factor and the black 

 factor. These two factors are known to be independent of each other 

 in heredity. See Castle (1909). It is therefore not to be supposed 

 that they will commonly coincide in distribution. If the black factor 

 extends over all the colored areas, the animal will be black-and-white. 

 If the black factor falls only on areas which lack the color factor, it 

 will produce no visible effect, and the animal will be yellow-and-white. 

 If, finally, the black factor falls on some of the colored areas but not 

 on all of them, those in which it falls will be black, the others yellow, 

 and the uncolored areas of course white. Hence a tricolor will result. 

 But the gametic composition of these tricolors will not be different 

 from that of the black-and-whites or red-and-whites produced by the 



