THE 



AMERICAN NATURALIST 



Vol. XLVI August, 1912 No. 548 



A CASE OF POLYMORPHISM IN ASPLANCHNA 

 SIMULATING A MUTATION 



PROFESSOR J. H. POWERS, Ph.D. 

 University of Nebraska 



During the fall of 1909 the writer accidentally dis 

 covered a case of heterogenesis in the genus Asplm/clma 

 which bore all the earmarks of a bona fide mutation. The 

 two forms concerned were quite sufficiently different to 

 be classed as distinct species, and even as strongly 

 marked species. The transition was sudden and com- 

 plete, without apparent intergradation. The transition 

 was also in one direction only, and it could not be con- 

 sidered as in any sense due to an immediate cross, be- 

 cause the reproduction of the Asplanchna, aside from 

 resting eggs, is wholly parthenogenetic. 



Subsequent study of the same species, during the fol- 

 lowing spring, as it appeared in several different loca- 

 tions, supplemented by extensive experiments, showed 

 that the phenomena in question were not those of typical 

 mutation, but are rather to be classed as striking in- 

 stances of polymorphism. As, however, their interest 

 depends in large measure, for the writer at least, upon 

 the ease with which they may be mistaken for mutation, I 

 will first describe the facts as they appeared in the 

 original collection of material. 



This collection was made about October 1, in the 

 remnant of a vile pool on the alkali flat west of the city 

 441 



