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 Detlefsen’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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