SPECIES HYBRIDIZATION 223 
ences exist between the two species under consideration. When we 
observe the number of differences in habit, form, size, etc., which are 
known to obtain between the two species, this assumption does not appear 
to do violence to actual facts in the case. Baur has sought by systematic 
hybridization investigations to determine which of the known factors of 
the hereditary material of A. majus are also contained in that of A. molle. 
From these investigations he concludes that A. molle certainly possesses 
the factors indicated by the incomplete formula BBDDEEFFIU, in which 
B represents a factor for yellow flower color; D, a factor for extension of 
pigment to the tube of the corolla; H, the factor for zygomorphic flowers; 
F, a base factor for red flower coloration which is epistatic to B; and l, 
a recessive factor which determines a low intensity of flower coloration. 
His success in determining the presence of these factors in the hereditary 
material of A. molle has led Baur to conclude that it is entirely within the 
range of possibility to analyze completely the differences which exist 
between these two undoubted species. All the unusual flower forms, 
therefore, which are obtained by crossing them are to be regarded as the 
results of peculiar factor interactions. We have pointed out in previous 
chapters that it is not always possible to predict the character expression 
of a given set of factors from a knowledge of their known expression in 
certain combinations. That this condition is here operative is borne out 
in part by the fact that certain flower types which appeared in F» did 
not reappear even among fairly large numbers in the F'; generation from 
such fF, plants. We consequently can state with assurance in spite of 
unsatisfactory ratios and peculiar character expressions that the results 
obtained in this species cross may reasonably be interpreted in harmony 
with Mendelian doctrine. 
Detlefsen’s Cavy Hybrids.—A similar line of investigation in animals 
has led Detlefsen to similar conclusions. He crossed the tame guinea- 
pig, Cavia porcellus, of which many different races have been produced 
under domestication, with the wild C. rufescens. The latter differs from 
the tame guinea-pig in a number of respects. It is very much smaller, 
weighing about half as much as the tame guinea-pig, and in skeletal 
measurements and other characters it is definitely set off as a distinct 
species from C. porcellus. In color it is of the agouti type common to 
all wild rodents, but the agouti differed from that of the tame guinea- 
pig in having less power to exclude black and brown from the hair than 
has the agouti of the tame animals, consequently individuals of the wild 
C. rufescens have darker coats than those of the tame porcellus. 
By crossing C. rufescens and its hybrids with porcellus with various 
races of porcellus, Detlefsen was able to study the inheritance of the 
following factors in this species cross: 
A—the agouti factor, which operates by restricting the black or brown 
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