100 



THE STRUGGLE FOR EXISTENCE 



Taking the volume of the larger P. caudatum equal to unity, the 

 volume of P. aurelia can be easily expressed in a relative form. Un- 

 der different conditions the relative volume of P. aurelia varies some- 

 what, but for Osterhout's medium it can be taken as equal on an 

 average to 0.39 of the volume of P. caudatum. In this way, in order 

 to pass from the growth of the number of individuals of the two spe- 

 cies of Paramecia to the growth of their volumes, we can leave without 

 alteration the number of individuals of P. caudatum, and only dimin- 

 ish the number of individuals of the small P. aurelia by multiplying 

 it in every case by 0.39. 



(5) Figure 21 represents graphically the growth in the number of 



TABLE X 



Measurements of Paramecium caudatum and Paramecium aurelia (after fixation) 

 Specification of measurements is taken from Figure 20 



ORIGIN OF PARAMECIA 



Growing culture with Osterhout's 

 medium 



Old culture with Osterhout's medium . 



Culture with the buffered medium. . . 



AVERAGE 

 VALUES FOR 

 P. CAUDATUM 

 IN DIVISIONS 

 OF OCULAR- 

 MICROMETER 



18.3 

 6.0 



17.1 

 5.1 



a = 18.0 

 b = 6.2 



AVERAGE 

 VALUES FOR 



P. AURELIA 

 IN DIVISIONS 



OF OCULAR- 

 MICROMETER 



13.8 

 4.2 



12.6 

 3.8 



a = 12.9 

 b = 4.8 



CALCULATED 

 VOLUME OF 

 P. AURELIA 

 (VOLUME OF 



P. CAUDATUM 

 = 1) 



0.39 



0.37 

 0.41 



0.429 



individuals and in the volumes of P. caudatum and P. aurelia culti- 

 vated separately in a medium changed daily for 25 days. The 

 general character of the curves shows that the growth of population 

 under these conditions has an S-shaped form. At a certain moment 

 the possibility of growth in a given microcosm is apparently ex- 

 hausted, and with a continuously maintained level of nutritive re- 

 sources a certain equilibrium of population is established. The oscil- 

 lations of population round this state of equilibrium are not governed 

 by any apparent law, and depend on various accidental causes (varia- 

 tion in temperature of the thermostat, a slight variability in the 

 composition of the synthetic medium, etc.). A comparison of the 



