CHAPTER III 

 VARIABILITY OF BODY SIZE 



The dimensions of two individuals, in unicellular 

 species as in man, are almost never exactly alike. 

 The only way in which the role of various influences 

 can be studied, therefore, is by seeing whether groups 

 made up of considerable numbers of individuals differ 

 from one another. Evaluations of quantitative differ- 

 ences require not only the construction of significant 

 averages, but also the comparison of the degrees to 

 which the individuals composing each average are 

 themselves diverse. 



1. Variability and Its Measurement 



Modifications of size are induced by many sorts of 

 changes in environmental conditions. But before these 

 are compared, it is necessary to find how much vari- 

 ability occurs when conditions are so far as possible 

 kept constant. Those variations which occur under 

 what appear to be constant conditions will always 

 carry the possibility that they are none the less due to 

 environmental differences, either past or present. Un- 

 til they are shown to be such, they may legitimately 

 be considered as physiological variations; that is, vari- 

 ations due to internal factors. Ultimately, of course, 

 every variation is a resultant of many factors, both in- 

 ternal and external. Some of the variations can prob- 

 ably not be controlled or equated completely by any 

 possible technique. In the case of unicellular organ- 

 isms it must be realized that the environments of two 

 individuals that are in the same body of culture me- 

 dium may in reality differ markedly. 



42 



