Size as Structure and Function 17 



The diversity of sizes within the group of unicellular 

 species is not generally appreciated, for the simple rea- 

 son that in practice the magnification of the micro- 

 scope or the scale of the illustration is suited to con- 

 venience. To the investigator, as well as to the text- 

 book reader, there appears to be little difference of 

 size between an average flagellate and a bacterium. 

 Even upon a single page one usually views a hetero- 

 geneous selection of species without reference to their 

 relative sizes. To bring to mind just what it means to 

 compare the bulks of various species, representatives 

 of a number are rendered to scale in figures 4 and 5. 

 To get a small bacterium delineated as the shortest 

 possible line it was necessary to increase the linear 

 scale ten times, therefore the volume scale a thousand 

 times, between figure 4 and figure 5. 



Size factors. The number of factors which may 

 possibly limit the size attained by a particular organ- 

 ism is very large. The number of combinations of 

 these factors is still larger. While some of the factors 

 may be recognized as physical forces or chemical con- 

 centrations, others can at present be described only as 

 physiological complexes. While equations could be 

 fitted to many correlations between body size and 

 quantity of environmental influence, the meanings of 

 the equations would at present remain unidentified. 

 The working out of these correlations and the quanti- 

 tative comparison of the various influences is the task 

 of the following chapters. 



