ANOTHER CASE OF MULTIPLE ALLELOMORPHS. 237 



it might appear in the next generation if it went into a male, 

 just as yellow itself might have appeared in the same way. If 

 the chromosome in question had passed into a female no evidence 

 of the mutation would be seen, but it would appear in the second 

 generation males. The two cases are alike. Since, however, 

 the mutation has never appeared in the gray stock, although a 

 great many more gray flies have been seen than yellow flies, 

 it might be argued that a mutation in a locus is more likely to 

 be followed by another in the same locus than if no mutation 

 had taken place there. But mutations are such rare phenomena 

 that this argument does not carry much weight, and judgment 

 must be suspended until further mutations have shown them- 

 selves. 



OTHER CHARACTERISTICS OF SPOT. 



In general we are apt to seize upon the most evident char- 

 acteristic of a mutation, give it a name, and neglect to mention, 

 or overlook, other effects that may be associated with it. In the 

 case of the white eye mutation for instance the eye attracts our 

 attention, yet if red-eyed and white-eyed flies be put into alco- 

 hol the yellow, especially the yellow bands, become markedly 

 different in the two flies. The white-eyed flies show much whiter 

 bands. Evidently the mutation has affected other pigments 

 than those contained in the eye. 



In another mutation called club wing (as yet undescribed) 

 the wing pads of many flies fail to expand, although others do 

 expand even in pure stock. It was later found that a small bristle 

 on each side of the thorax is absent in the club stock irrespective 

 of whether the wing pads expand or not. This microscopic 

 change is a constant feature of the mutation and a better char- 

 acter for that reason than the more obvious although inconstant 

 one of the club-shaped wing. 



Likewise in "spot." In the black spot fly there is a distinct 

 light dot on the post thorax, and often a light band down the 

 middle of the thorax (Fig. 4a). These were called dot and dash, 

 but it was later noticed by Bridges that the dot and dash are 

 present in flies only for a short time and disappear as the flies get 

 older. All attempts to get stocks with permanent dot and dash 



