PRESENT CRISIS IN EVOLUTIONARY THOUGHT 19 



subtractive nature of the factorial mutation is that of the 

 Primula "Coral King," a salmon-colored mutant, which was 

 suddenly given off by a red variety of Primula called "Crim- 

 son King." Such a mutation is obviously based on the loss of 

 a germinal factor for color. The loss, however, is sometimes 

 partial rather than total, as instanced in the case of the purple- 

 edged Picotee sweet pea, which arose from the wholly purple 

 wild variety by fractionation of the genetic factor for purple 

 pigment. Even where the mutational variation appears to be 

 one of gain, as happens when a positive character appears 

 de novo in the phenotype, or when a dilute parental character 

 is intensified in the offspring, it is, nevertheless, interpretable 

 as a result of germinal loss, the loss, namely, total or partial, 

 of a genetic inhibitor. Such inhibitive genes or factors are 

 known to exist. Bateson has shown, for example, that the 

 whiteness of White Leghorn chickens is due, not to the absence 

 of color-factors, but to the presence of a genetic inhibitor — 

 "The white of White Leghorns," he says, "is not, as white in. 

 nature often is, due to the loss of the color elements, but to 

 the action of something which inhibits their expression." (Ad- 

 dress to the Brit. Ass'n, Smithson. Inst. Rpt. for 1915, p. 368.) 

 Thus the sudden appearance in the offspring of a character not 

 visibly represented in the parents may be due, not to germinal 

 acquisition, but the loss of an inhibitory gene, whose elimina- 

 tion allows the somatic character previously suppressed by it 

 to appear. Hence Bateson concludes: "In spite of seeming 

 perversity, therefore, we have to admit that there is no evo- 

 lutionary change which in the pres^ent state of our knowledge 

 we can positively declare to be not due to loss." {Loc. cit., 

 p. 375.) 



Another consideration, which disqualifies the factorial mu- 

 tant for the role of a new species, is its failure to pass the test 

 of interspecific sterility. When individuals from two distinct 

 species are crossed, the offspring of the cross is either completely 

 sterile, as instanced in the mule, or at least partially so. But 

 when, for example, the sepia-eyed mutant of the vinegar fly is 



