6 Gary N. Calkins 



two forms which Maupas, and with him more recently Simpson, 

 held to be a distinctive feature, loses all value when considered 

 critically, while size relations in general are absolutely untrust- 

 worthy in settling questions of species. The variations which a 

 species of Paramecium passes through under different conditions 

 of vitality are so great, and last for such long periods, that no infer- 

 ence can be drawn from cell dimensions at any given time. Size 

 depends, apparently, upon two factors — the relative vitality, and 

 the rate of cell division — and these two may probably be merged 

 into one, which may be called the potential of vitality. Parame- 

 cium caudatum under different conditions shows wide variations 

 in size. When taken directly from the natural habitat, where food 

 is not overabundant, they are large, measuring on the average 

 315/14. The same forms cultivated on hay infusion, with its rich 

 food content, multiply rapidly, and do not grow individually to 

 the same size as the "wild" form. These measure on the average 

 (18 individuals killed at different periods of the cycle) only 206 /*, 

 with variations from 180 to 224 /x when the division rate was rela- 

 tively high — two divisions per day. When the potential of vitality 

 is nearly exhausted and the division rate is low, a similar small 

 size is noticed; but at such a time it is obviously due to a different 

 cause, probably a loss of metabolic energy. The same differences 

 are noticed in P. aurelia at different periods of vitality, and the 

 impossibility of considering size relations as of specific value is 

 clearly estabhshed. During the first 45 generations of P. aurelia 

 the division rate averaged only eight-tenths of a division per day, 

 which is lower than that for the G and X lines, which averaged 

 one and one-tenth divisions. Eighteen individuals killed at differ- 

 ent periods of the culture were measured during these 45 

 generations, and the average length was 224 /a, with variations 

 from 168 to 256 /A. After the loss of one of the micronuclei the 

 ♦> division rate increased to the remarkable rate of 2 . 2 per day on 

 the average for a period of four months, when vitality waned. 

 During this period of rapid multiplication the size averaged only 

 178 ft with variations from 148 to 212 /i. At this period the organ- 

 isms in culture would have been identified by any microscopist as 

 P. caudatum, although more than 40 /i, on the average, smaller 



