Races of Paramecium. 551 



ough to indicate that such changes do not occur readily nor often 

 in that animal, and to make desirable a very careful examination 

 of the evidence submitted by Popoff, in order to see how far it 

 is demonstrative. 



In Paramecium it is very easy to find races of differing size 

 when one is working with a culture not known to be derived 

 from a single individual, but this changes completely as soon as 

 our culture is known thus to have origin from a single specimen. 

 If one assumed, to begin with, that the Paramecia in a ' Vild" cul- 

 ture were all alike, it would be easy to collect evidence seeming 

 to indicate that differentiation in size had occurred ; but the con- 

 clusion so drawn would be an entirely mistaken one. Demonstra- 

 tive evidence of the origin of heritable differences in size can then 

 come only when it is known positively that the differing indi- 

 viduals or lines were derived originally' from a single specimen. 

 This requirement can be met in two ways: (1) by working with 

 a culture that was begun with a single individual and has been 

 kept uncontaminated; (2) by actual observation of the fission that 

 produced the two lines of different sizes — with of course immediate 

 isolation of the two products of the fission. 



How far do the cases set forth by Popoff meet these require- 

 ments? In his summing up Popoff says, "In the discussion up to 

 this point I have set forth nine cases in Stentor coeruleus (includ- 

 ing the specimens of normal size) and two in Frontonia leucas, 

 in which it was possible by inequalities of division or by experi- 

 mental operation to change and to fix the cell size, and to culti- 

 vate the size-varieties thus produced for a long time side by side" 

 ('09, p. 163). It will be worth while for us to look into each of 

 these cases, to see how far they fulfil the conditions required for 

 demonstration. These conditions, as we have seen, are: 



1. Either the entire culture from which the new race comes 

 shall be known to have been derived from a single specimen; 



2. Or, the author shall have observed the fission that produced 

 the unequal races, isolating the two individuals produced at 

 once, so as to know that both came from the same individual. 



Now, it should first be noted that since Popoff (as we have 

 seen) counts in his eleven cases the unchanged normal specimens 



