The Rate of Multiplication 75 



centration of various solutes 9 , likewise are almost uni- 

 versally potent. 



Methods have been devised of controlling and hold- 

 ing almost constant every one of these factors; and it 

 is when these conditions are controlled most accur- 

 ately that the greatest constancy of reproductive rates 

 has been observed. 



The best control has been attained in the case of 

 organisms such as green flagellates, which can syn- 

 thesize their own organic compounds from a medium 

 of inorganic salts. For biophytic species it is best to 

 feed a pure culture of other organisms washed free of 

 the medium in which they in turn were grown (Belar, 

 '28). 



Absolute invariability of cultural conditions can 

 theoretically never be attained; and it will always re- 

 main a bare possibility that the seemingly inherent 

 residuum of variability in rate of reproduction and in 

 other functions is environmental in origin. 



2. Internal Factors 



To study exactly how constant the rate of repro- 

 duction is, it is necessary not merely to count the num- 

 bers of individuals at intervals, but to time accurately 

 the very minute at which each individual is produced 

 by fission. This has been done only for the infusorian 

 Colpoda (Adolph, '29) ; the variability found is shown 

 in figure 31. 10 The statistical consideration of fission 

 rate leads to a useful conception of the processes of 

 multiplication, and from it can be found, as will be 



"Nowikoff ('08),Ugata ('26), Hammett ('29), Peters ('04), Spek ('20), 

 Hartmann ('24a), Packard ('26), Middleton ('28). Further references to 

 many potent solutes and some physical influences are given by Ball ('25a). 



10 Similar observations of the time required for one complete generation 

 have been made on bacteria and yeasts, as in the work of Barber ('08) and 

 Peskett ('25), but no frequency curves nor individual determinations were 

 reported. 



