External Factors and Size 125 



Such might well be the case when the shift of tempera- 

 ture was one of only five degrees, as happened in their 

 experiments; but if the organisms were transferred 

 from 25° to 10°C. the amount of growth required to 

 attain the new size would be so great that at least a 

 much prolonged duration of one generation would be 

 required to carry it out. 



The observations of Middleton ('18) indicate that 

 not only size but also fission rate may be persistently 

 modified by temporary subjection of Stylonychia to 

 temperature differences. What is needed is a simul- 

 taneous study of size and of fission rate in the same 

 series of individuals, in order to find which of the two 

 characteristics is most susceptible to the influence. 



Jollos ('13) found that the size responses of Para- 

 mecium to temperature were of several varieties, de- 

 pending, he believed, upon the particular clone studied. 

 When put into a higher temperature some maintained 

 their characteristic adult size, or became slightly 

 larger; others became rapidly and permanently smaller 

 (just as in Rautmann's experiments) ; still others be- 

 came smaller at first and after many generations regu- 

 lated back to about the original size. Jollos seemed 

 to find that only in the last case, which is illustrated in 

 figure 38, did the organisms remain healthy at the 

 higher temperature; others died within eight weeks. 

 This is evidence that body size is of vital importance, 

 and closely correlated with ability to survive. 



When put into lower temperatures Paramecium 

 either became permanently larger (just as in Raut- 

 mann's experiments), or else became larger at first and 

 later returned to the original size (illustrated in figure 

 38). The regulation back toward the original size 

 occupied two to four months in the lower temperatures 

 as compared with one to eight weeks in the higher 

 temperatures. So far as Jollos observed, the fission 



