Races of Paramecium. 555 



larger nuclei. The culture of large individuals was kept fifty- 

 three days. 



The vulnerable point in this case is the fact that the original 

 "room culture" is not stated to have come from a single individ- 

 ual. It may therefore have contained various races; and it is 

 possible that what the author did was merely to isolate an indi- 

 vidual of one of the large races — the suppressed fission having 

 nothing to do with the greater size. This would fully explain 

 the great size in the isolated culture, as compared with the usual 

 size in the room culture. According to Popoff, there were a 

 number of unequal fissions in this large race, producing small 

 individuals. Here was a chance for a crucial test of the question 

 whether from a race of a given size another of different size is 

 produced. If these small specimens had been cultivated sep- 

 arately, and had given a small race, while the original race con- 

 tinued large, all conditions for a demonstration would have been 

 fulfilled. But unfortunately, the author merely removed these 

 small specimens, in order that they might not bring down the 

 average size of the parent culture; we do not know what they 

 would have produced if farther cultivated. 



In making this analysis there is no desire to detract from the 

 value and interest of Popolf's beautiful work. He has put for- 

 ward almost the only plausible suggestion thus far as to the origin 

 of the races of different size, and the cases numbered here 5 and 6 

 may be said to furnish a certain amount of evidence for the cor- 

 rectness of this view. If the suggestion does turn out the correct 

 one, it will be of extraordinary interest. But we are here con- 

 fronted with a precise and definite question : Has it been dem- 

 onstrated correct or has it not? I am sure that Popoff will 

 agree with us that a cardinal principle for the advance of experi- 

 mental science is that nothing shall be held demonstrated that 

 has not been demonstrated. If it is true that the heritable cell 

 size is "not something definite, but is extremely variable," then 

 there should be no difficulty in getting cases fulfilling the simple 

 and definite conditions that will furnish a demonstration. In my 

 work with Paramecium I have found it easy to get races of di- 

 verse sizes so long as these precise conditions are not fulfilled; 



