SPECIES HYBRIDIZATION 225 



tary material might be the same for the two species, the actual factors them- 

 selves might differ in certain respects, for example in the exact type of 

 character expression and in their power to react with a given set of 

 factors. If the differences be relatively slight, the factors might still be 

 able to interact with each other approximately in the normal fashion, 

 and to display allelomorphic relations dependent upon their position in 

 the hereditary material. On this point Detlefsen contributes very 

 important data which we shall consider somewhat in detail. 



The first set of observations relates to the differences between the 

 agouti factors of C. porcellus and C. rufescens. It is a common obser- 

 vation that the agouti pattern in rodents in general is a variable one. 

 Some of this variability is unquestionably due to the presence of modi- 

 fying factors, but not all such variations can be interpreted in this fashion. 

 Elsewhere we have pointed out that in mice a system of quadruple 

 allelomorphs includes the factors for yellow, black, gray, and gray with 

 white belly. In the rabbit, Punnett's results may be interpreted as 

 establishing the existence of a triple system of multiple allelomorphs 

 consisting of the factors for yellow, agouti, and black. Similarly in 

 the tame guinea-pig there are apparently allelomorphic variations which 

 affect the agouti pattern, but Detlefsen finds, nevertheless, that these 

 never condition the type of agouti presented in C. rufescens. Detlefsen 

 points out that agoutis in common restrict black or brown in the sub-apical 

 band of individual hairs so that the dorsal hairs present a barred appear- 

 ance. More powerful restriction is shown in the hairs of the belly, but 

 there is always a close correlation between the amount of restriction in 

 dorsal and ventral regions, for the darker the dorsal region, the darker 

 is the pigmentation of the ventral surface. The wild agouti factor was 

 distinguished by its weak restricting power, so that ordinarily the yellow 

 sub-apical band in the hairs of these animals was distinctly narrower than 

 in some agouti guinea-pigs. In some cases the lack of restriction was so 

 marked that only a slight sprinkling of agouti hairs in the adult gave 

 evidence of the existence of the agouti factor. Moreover, in some cases 

 the wild agouti pattern carried with it a ticked belly, a condition appar- 

 ently unknown in the tame guinea-pig. Some variation was observed 

 in the agouti patterns of the original individuals of C. rufescens and this 

 must not be forgotten in interpreting Detlef sen's results. Dark agoutis 

 produced by constantly mating wild agouti hybrids to tame non-agouti 

 guinea-pigs, were mated to tame agouti animals. We may represent 

 the factor for wild agouti by A', that for tame agouti by A, and that for 

 the allelomorphic condition in the tame guinea-pig by a. Following this 

 formula, then those dark agoutis produced by mating wild hybrid 

 agoutis to tame non-agoutis must have been of the genetic constitution 

 A' a. When such animals are mated to tame agoutis two types of animals 



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