Races of Paramecium. 507 



other. On the contrary, there was a reverse change, to a some- 

 what smaller size (p. 6), though the decrease in size is plainly such 

 as might be expected from the increased rate of fission at that time. 



Thus there appears to be no evidence that Calkins has ever had 

 under examination representatives of the real Paramecium aurelia, 

 as understood by Maupas, Hertwig, and the present paper. It 

 must be concluded that Calkins expresses the real state of the case 

 when he says '*It may be that my observation was made on a 

 chance abnormality,- that paralleled P. aurelia and that the real 

 P. aurelia retains its integrity as a species" ('06, p. 8). Further 

 evidence of this will be given in Part 2 of this paper, where it will 

 be shown that the two species are constant (at least for long peri- 

 ods), and that they differ in other respects besides themicronuclei. 



Thus in reading Calkins' recent textbook ('09) and his later 

 papers, the reader who desires to understand the relation of the 

 facts brought out to these two existing types (or species) will do 

 well to substitute throughout the specific name caudatum Ehr. 

 for that of aurelia Miill. 



Naturally the question comes up: How does such a condition 

 as the presence of an extra micro-nucleus or the absence of one 

 of the typical ones arise? In a single individual of the largest 

 race Lz there was found a condition which may possibly throw 

 some light on the point. Fig. 11 shows this individual as it ap- 

 pears upon the slide; fission was nearly completed when the ani- 

 mal was killed. The micro-nucleus in the posterior half (left in 

 the figure) appears typical in every respect, but that in the anterior 

 half is very unusual in appearance. At this stage of division the 

 daughter micro-nuclei are usually condensing or contracting into 

 the rather small resting condition. But this one is extended into 

 a spindle shape as though in preparation for another division. If 

 such a division took place we should, of course, have one of the 

 daughter cells with two micro-nuclei instead of with the normal 

 number, one. But it is possible that there is here represented only 

 a very great retardation in the assumption of the typical condi- 

 tion. 



* Of P. caudatum. 



