178 LORANDE LOSS WOODRUFF. 



The general behavior as well as the 'avoiding reaction,' etc., 

 of P. calkinsi is very similar to P. caudatum and P. aurelia, 

 though the broad and slightly curved anterior end of P. calkinsi 

 renders the revolutions on its long axis and its spiral path some- 

 what more conspicuous than in the more common species of 

 Paramecium. 



Paramecium calkinsi appears to be adapted to essentially the 

 same environmental conditions as P. aurelia and P. caudatum. 

 A series of experiments in which flasks of hay infusion were 

 seeded with both P. calkinsi and P. aurelia showed that, as a 

 rule, one and not both species attained a maximum development 

 in the same flask. Usually within a few days there was a heavy 

 growth of one and only a few struggling survivors, or none at 

 all, of the other species. There was no indication that one was 

 more hardy than the other under the conditions of the experiment ; 

 success or failure to dominate the culture apparently depended 

 on which form 'got the start,' so to speak. It seems probable 

 that the excretion products of the two forms are mutually 

 detrimental and that this is the determining factor in the com- 

 parative incompatibility of the two species. 23 This point is being 

 investigated. 



The rate of reproduction of the new species, as exhibited in pedi- 

 gree cultures throughout nearly a year, shows no factors which 

 distinguish it from P. aurelia, P. caudatum or P. bursaria, which 

 I have studied under similar conditions, since it is well known 

 that characteristic division rates obtain in different races of 

 the same species. The main pedigree culture of P. calkinsi was 

 carried from January 30 to November 20, 1920, when it died out 

 at the 32 ist generation (Fig. 7). During all this period a 

 constant culture medium of standard beef extract was employed, 

 and during certain experiments the temperature was maintained, 

 by a thermostat, practically constant at 24 C. The race is 

 now thriving in mass cultures which were seeded from the 

 pedigree lines. The animals are apparently in as healthy condi- 

 tion as ever. 



23 L. L. Woodruff, "The Effect of Excretion Products of Infusoria on the Same 

 and on Different Species, with Special Reference to the Protozoan Sequence in 

 Infusions," Jour. Exper. Zool., 14, 1913. 



