Variability of Body Size 53 



no evidence that speed of locomotion has any survival 

 or other value within the range of sizes found in a 

 single clonal culture. This and similar consequences 

 of surface will be reconsidered in Chapter X. 



Toxicity. Various quantities, such as toxicity, 

 have been shown to be correlated with age of individ- 

 uals. Whether or not age, then, is important because 

 of its size correlative, is by no means clear. Estabrook 

 ('10) found that certain alkaloids killed Paramecium 

 most readily, soon after fission; but one alkaloid, at 

 least, killed old individuals more readily. It is obvious 

 (from figure 23) that any agent whose action depen- 

 ded upon the concentration of water or of some solute 

 within the body would have a different effect upon 

 large and small individuals. It was shown in the pre- 

 vious chapter that the concentration of water probably 

 changed during the growth of individuals. 



Summary. It is clear that definite chemical and 

 physical differences exist among the individuals of a 

 clonal culture which depend solely upon the fact that 

 the individuals are diverse in size. These differences 

 have not helped to distinguish what produced the di- 

 versity in size; yet it is fairly probable that a change 

 in specific gravity or in the ability to buffer toxic re- 

 agents would of itself induce modification of the size, 

 or select among sizes, or both. It has not been demon- 

 strated directly that the small diversities of size which 

 prevail among individuals of one age, one clone, and 

 one culture, are accompanied by physical and chemi- 

 cal differences that matter, but it is likely to be so. 

 This may help to understand the variability which 

 exists among such individuals, but it does not help to 

 describe how the variability arose. This residuum of 

 variation can at present only be recognized and meas- 

 ured. 



