No. 548] A CASE OF POLYMORPHISM 



459 



The suggestion of cannibalism as the cause was thus 

 all but conclusive ; but I was able to press the proof one 

 step farther. My isolation cultures, started with one or 

 a few individuals, invariably failing, I decided to try 

 cultures started with a larger number of individuals, 

 each one of which was first subjected to examination un- 

 der the microscope. 240 individuals were thus isolated 

 and placed in a single stender dish, care being taken to 

 reject any that deviated never so little from the normal 

 humped type, especially as to extra size. In the rapidly 

 multiplying culture thus started there appeared within 

 a week several typical campanulates, and the whole sub- 

 sequent course of the culture duplicated the develop- 

 ments which had taken place in the mass cultures already 

 described. This is as near as I have come to actually 

 observing the production, by the humped Asplanchna, 

 of its larger humpless congener. I have not witnessed 

 the birth of the one from the other or seen it in uteri. 

 The demonstration of fact is, however, sufficiently com- 

 plete, and the farther conclusion is sufficiently plain, viz., 

 that the transition in this direction between the two types 

 is a relatively rare one. Not every humped Asplanchna 

 possesses the power, whether this depends upon size, 

 ingestive reaction or digestive capacity, to produce the 

 larger form. Indeed, this power would seem to reside in 

 but very few individuals. 



As soon as I had reached even the tentative conclu- 

 sion that cannibalism must be the cause of this marked 

 heterogenesis, I set about to determine, if possible, why 

 this should be the case. Was there some magic in the 

 cannibalistic diet as such? Was it merely a case of rich 

 nutrition? Or was the result perhaps due to the mere 

 size of the ingested food organisms? 



Excessive feeding with most of the usual food organ- 

 isms of the species was plainly without result. So long 

 as the Asplanchnas were relatively few or greatly out- 

 numbered by their food organisms they glutted them- 

 selves to repletion without unusual consequences in re- 



