No. 540] A CASE OF POLYMORPHISM 



531 



saccate type is in its most typical condition, at least so 

 far as form is concerned. Of all the minor fluctuating 

 forms found among the saceates the one that most 

 suggests the humped type is that which I have character- 

 ized above as urn-shaped. The bulging sides of such a 

 form might readily be thought to be the forerunners of 

 at least the lateral humps; but during the entire study I 

 have been unable to observe the humped form originating 

 from this urn-like variety. 



Furthermore, the saltations from type to type do not 

 necessarily occur, and I think do not usually occur, when 

 the amount of fluctuating variability is greatest. This is 

 especially true of the formation of the campanulates from 

 the hump-bearers. This transition occurs when the 

 latter are at a culmination of development and vigor, and 

 in this condition the species is, until saltation occurs, 

 relatively uniform. 



Under the third question, as to causes of fluctuating 

 variability, I will record at present but two points, one 

 general and one special. 



The greatest amount of general fluctuation seems 

 always to occur under relatively unfavorable conditions. 

 Favorable conditions, on the other hand, tend to produce 

 full development with relative uniformity among the 

 individuals of any given type. 



One special instance of variation interested me so 

 much that I followed it whenever found, in the effort to 

 get at its exact cause. This is the variation in the length 

 of the three conspicuous humps which characterize the 

 commoner form. The amount of this variation is, 

 although I have not measured it, very great. The humps 

 may be but angular projections upon the body's outline, 

 or they may elongate until they might be appropriately 

 described as finger-shaped. The type represented in the 



repeated in countless numbers, with but moderate varia- 

 tion, so long as conditions are normal, whicli means 

 chiefly, so long as the food supply is uniform and ade- 



