120 INHERITANCE IN GUINEA-PIGS. 



guinea-pigs in appearance, and we find an allelomorph intermediate in 

 dominance between the intensity and albino factor to be responsible 

 for their condition. Sepias are similarly intermediate between blacks 

 and albinos and are due to the same allelomorph of intensity and albin- 

 ism. The series, light agouti of Cavia cutleri, dark agouti of C. rufes- 

 cens, and black, furnishes another example due to triple allelomorphs. 



In other cases, the intermediate type is an unfixable one, due to 

 imperfect dominance. Thus cream is the heterozygote between yellow 

 and albino. A "razor back" rough (rough C or D) is the heterozygote 

 between a type smooth except for the hind toes (rough E) and a full- 

 rough (rough A) . 



A series of deviations from the original type may depend on the 

 presence of a certain factor necessary for any deviation whose effect is 

 modified to different extents by independently inherited factors. 

 Rough A contains the same rough factor (R) as does rough E, but differs 

 in possessing an independent factor variation (s) favorable for rough- 

 ness. Most of the variation which we have ascribed to residual 

 heredity probably comes under this head. 



Deviations from type, which apparently form a natural series, may 

 be due to wholly independent factors whose effects are merely super- 

 ficially similar. A pink-eyed pale sepia superficially seems as good an 

 intermediate between an intense black and an albino as does a black- 

 eyed sepia, yet the former is due to a variation which is wholly inde- 

 pendent of albinism; the latter is due to an allelomorph of albinism. 

 White-spotted animals are sometimes called partial albinos and con- 

 sidered as natural intermediates between the self-colored type and 

 albinos, but genetically they are wholly distinct. Black, agouti, and 

 self yellow form a series which is due to three allelomorphs in mice, but 

 in guinea-pigs two wholly independent sets of factors are involved. 



Finally, we must recognize series of variations in which no Mendelian 

 factors have yet been isolated. The series of white-spotted and yellow- 

 spotted types and the series of polydactylous types are examples in 

 guinea-pigs. Further, in all series of variations, to whatever extent 

 analysis has been carried, there always remains some unanalyzed varia- 

 tion. In many cases such variations are known to be hereditary and 

 can be assigned to the residual heredity of particular stocks. Such 

 unanalyzed variations, however, are probably in general complicated 

 by variation which is not hereditary, due apparently to irregularities 

 in development. If we can measure the importance of such non- 

 hereditary variation by the extent of irregular asymmetry met with, it 

 is very important in white and yellow spotting, in the variations in the 

 development of extra toes on the hind feet, and is noticeable in varia- 

 tions in roughness. 



In the continuous series of variations several of these phenomena 

 have generally been found together. In the series from smooth to full- 



