N0.54S] A CASE OF POLYMORPHISM 



455 



as soon as it had assumed complete expansion, appeared 

 fully a third larger than its saccate parent. The act of 

 giving birth to such heterogenetic issue is a decidedly 

 prolonged and painful process for a rotifer, very differ- 

 ent from the ordinary, sudden expulsion, well known in 

 Asplanchna. 



Turning to the consideration of the actual cause of the 

 transition from the saccate to the larger humped form, 

 I have not discovered it definitely. The cessation of 

 the rapid rate of reproduction, already spoken of, seems 

 a partial proximate cause, though it itself I can not ex- 

 plain. It seems in part a mere matter of the number 

 of generations following hatching from the resting egg. 

 But a copious food supply and generally favorable con- 

 ditions probably favor the change. In early spring the 

 saccate form maintained itself for several weeks in a 

 pond where, in July, after drying up and being refilled 

 by rain, the saccate form began to produce the humped 

 form sparingly within four days, and was very soon sup- 

 planted by it. In another pond of purer water and seem- 

 ingly less well adapted to the species, the saccate form 

 appeared scantily in early May, and struggled on spar- 

 ingly for three weeks, feeding mainly upon the flocculent 

 masses of blue-green alga, but somewhat upon Brach- 

 ionus; it then disappeared entirely, having produced no 

 form but its own so far as I could discover. Yet in isola- 

 tion cultures from this same stock, when fed on Para- 

 mecium, I raised, after several generations, a number of 

 rather small humped Asplanchna. 



While I have seen little evidence, thus far, that either 

 of the larger types give rise to the saccate form in nature, 

 yet in two small cultures I have produced a population of 

 from one hundred to several hundred small saccates by 

 the degeneration of the humped stock. In one case the 

 colony was a very old one and the degeneration possibly 

 due to this fact ; in the other it was plainly the result of 

 diluting the culture medium with tap water. The small 

 saccates, even when they had dwindled to a half dozen, 



