No. 549] A CASE OF POLYMORPHISM 



545 



size of a few individuals, with perhaps a somewhat dis- 

 proportionate expansion in breadth of corona. Yet the 

 results of a preliminary six weeks' culture of this type of 

 .1. brif/litirclli are essentially similar to those which fol- 

 lowed my attempts to modify the first type: they are 

 negative. 



Much more extended and varied experiments must he 

 made before reaching final conclusions upon the con- 

 stancy of these two races of A. brightwelli. Yet they 

 certainly promise to hear out in the main Jennings's pre- 

 diction of relatively fixed races within the species. 



Yet the foregoing does not entirelv complete the pic- 

 ture of variation as I have found it in A. bright wrlli. 

 While the rearing of thousands of individuals and the 

 examination of a very large amount of material in nature 

 give the appearance of two stable genetic types, yet in the 

 rarest instances mutation-like changes of the most 

 marked character probably do occur, just as they do in 



While at Sylvan Lake it occurred to me that the very 

 favorable conditions under which A. bright ivelli was there 

 developing, including the preying upon a number of dif- 

 ferent organisms, were as well adapted as possible to 

 bring about mutational changes such as I had found to 

 occur in X amphora. Day after day I examin ^ la W 



a favorable morning at the very close of my stay at the 

 lake enabled me to collect several liters of plankton as 

 thick as cream in consistency, with perhaps four fifths of 

 its bulk living Asplauchna. Pouring this in the thinnest 

 possible lavers, into broad dishes, and placing these above 

 a black surface, I proceeded, by means of a powerful 

 reading lens, to search for any individual Asplnudm,, 



could not. of ,-ourse. I... thn^ dot-tod. To my great sur- 

 prise, my search was finally rewarded by the finding of 

 three individual rotifers, and three only, of quite aston- 

 ishing proportions. They were certainly Asplanchna; 



