Variability of Body Size 



51 



and 22) for each of three species studied, Scenedesmus, 

 Gonostomum, and Colpoda, that the smaller individ- 

 uals had the higher specific gravities. If anyone had 

 set out to predict such a difference he would probably 

 have anticipated that smaller individuals had higher 

 specific gravities, because then the tendency to sink in 

 water would be approximately alike in all individuals. 

 For, the rate of sinking depends on the ratio of the 



1.12 



30 34 38 42 46 



Diameter in micra 



Figure 22. Frequency curve of diameters (solid lines), and correlation 

 of diameters with specific gravities (broken line), in the cysts of Gonosto- 

 mum sp. at 12.4° C. (Redrawn from Allison, '24, with the permission of the 

 editors of the Annals of Applied Biology). 



specific gravity difference to the body surface. But the 

 change of specific gravity with size is in no case suffi- 

 ciently large to compensate mechanically for the increase 

 of surface. 



Probably the same factor of density has been demon- 

 strated in another form by Fortner ('25) in Parame- 

 cium. He measured how much the bodies lost in vol- 

 ume upon being put into a sucrose solution, and found 

 (figure 23) that less shrinkage occurred in the small 



