194 Regulation of Size in Unicellular Organisms 



time, and fission begin later when a normal age has 

 been attained. 



The data also furnished no decision upon the various 

 theories of size. Is the theory of Spencer realized in 

 any specific instance ? Does it matter that expenditure 

 is proportional to volume and supply is proportional 

 to surface? Is it minimum work for a race to increase 

 the sizes of its individuals instead of regulating their 

 diffusing capacities more precisely? None of these 

 questions can be answered now; they await investi- 

 gation. 



2. Individuality 



One of the classical problems of biology with which 

 the regulation of body size is intimately connected is 

 the question : what constitutes an individual ? Among 

 unicellular organisms, less than among any other 

 kinds, does an individual always correspond to a dis- 

 crete portion of living material. A series of generations 

 shares characteristics; an entire population is intra- 

 dependent. With respect to size, how does a double 

 monster compare with the normal individual, or how 

 does the double-sized individual which arose by living 

 at a low temperature or by receiving ultraviolet radia- 

 tion compare with one which is normal at a higher 

 temperature or protected from radiation? 



Limits of divisibility. A partial answer seemed to 

 fit these questions at the time when it was believed 

 that the divisibility of a protozoan individual was 

 limited by some invisible organization in the body 

 mass. To the present-day investigator, however, it 

 seems more likely that divisibility is limited only by 

 the conditions under which living material is kept from 

 dissipating into its fluid environment. This is a physi- 

 cal limitation rather than one which is significantly 

 physiological or morphological. Considerable experi- 



