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 BBDDEEFFU, in which 

 B represents a factor for yellow flower color; D, a factor for extension of 

 pigment to the tube of the corolla; E, the factor for zygomorphic flowers; 

 F, a base factor for red flower coloration which is epistatic to B\ and /, 

 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 2 did 

 not reappear even among fairly large numbers in the F 3 generation from 

 such F z 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 



