190 Regulation of Size in Unicellular Organisms 



variability and mean are exactly those of the original 

 population. With respect to conditions there will al- 

 ways remain room for doubt, since no two individuals 

 can live in exactly the same place at the same time. 

 In truth it is no less remarkable that a number of popu- 

 lations belonging to one clone should show the same 

 degree of variability, than that they should all exhibit 

 the same mean of size or other character. 



Interdependence. It must be obvious that size can- 

 not vary independently of all other characters. Size 

 has been found to be correlated with fission rate, with 

 growth or assimilation rate, with ancestral reorgani- 

 zations of various sorts. Under a standard set of con- 

 ditions it was indicated, in the genus Colpoda, that 

 fission rate was the primary variable, so that size vari- 

 ability might result from it. Under conditions of par- 

 tial starvation, members of some species kept on re- 

 producing almost as rapidly as in the presence of 

 plentiful food, in spite of considerable decrease in as- 

 similation; body size decreased accordingly. Under 

 apparently uniform conditions, large sizes and high 

 rates of fission followed soon after the act of conjuga- 

 tion in some races. Obviously body size is not invari- 

 ably connected with any one internal factor, any more 

 than it is with any single external factor. 



Body size, then, is a characteristic which varies in 

 its own right, at the same time that it, like all other 

 characteristics, does not vary wholly independently of 

 the other characteristics. The intimate control of size 

 is, in a sense, incompletely understood until all the 

 functions and dynamic equilibria of the particular or- 

 ganism have been described. But one can, a priori, get 

 just as far by beginning the description with size, as 

 by beginning with reproductive rate, or assimilation 

 rate, or structural differentiation rate. Size can be 

 measured in most kinds of unicellular organisms; it is 



