No. 548] 



A CASE OF POLYMORPHISM 



4 49 



tionship between the two types seemed to leave the evi- 

 dence for mutation sharp and clear : the transition from 

 the giant humpless Asplanchna to the smaller humped 

 type was sudden and complete; it involved many char- 

 acters ; it was being effected in many individuals simul- 

 taneously; it was a transition in one direction only; in- 

 termediate types and fluctuating varieties seemed, in all 

 the material studied, no more in evidence than was to be 

 expected in any two species of one genus. At the close 

 of this part of my study it seemed that I had found an 

 ideal case of mutation, agreeing in every particular with 

 the definitions of the concept put forward by its origi- 

 nator, DeVries. 



However, just as I was finishing the study of the pre- 

 served material at hand, and deeming that I had reached 

 the above conclusions with full certainty, I was suddenly 

 halted by coming into possession of more living material, 

 and material so surprising that my well-worked-out re- 

 sults again assumed the character of problems, and I 

 resolved, if possible, to subject the whole to the test of 

 experimental method and further observation. 



It was about the middle of March, 1910, that the 

 Asplanchna began hatching in a large covered aquarium 

 jar in which I had placed some of the resting eggs the fall 

 before. Between the last of March and the 10th of 

 April the species also began to appear gradually in two 

 ponds, in which, fortunately, the available food organisms 

 for many weeks were wholly different. Observations 

 were thus begun upon the species under diverse condi- 

 tions, even from the start. And, as they were continued 

 with but few and short interruptions through the spring, 

 summer, and well into the succeeding fall, 2 the variety of 

 conditions under which the development of the species 

 was followed included very wide extremes. Thus, during 

 a cold period in early April the species multiplied in 

 shallow ponds upon which ice was forming nearly every 



'Since the above was written nearly all of the phenomena recorded have 

 again been observed during a second season. 



