36 



THE STRUGGLE FOR EXISTENCE 



of the logistic curve in a numerical example. Let us turn our atten- 

 tion to the growth of a number of individuals of an infusorian, Para- 

 mecium caudatum, in a small test tube containing 0.5 cm 3 of nutritive 

 medium (with the sediment; see Chapter V). The technique of 

 experimentation will be described in detail further on. Five individ- 

 uals of Paramecium (from a pure culture) were placed in such a 

 microcosm, and for six days the number of individuals in every tube 

 was counted daily. The average data of 63 separate counts are 

 given in Figure 4. This figure shows that the number of individuals 

 in the tube increases, rapidly at first and then more slowly, until 

 towards the fourth day it attains a certain maximal level saturating 



2 3 V S 6 



Days 



Fig. 4. The growth of population of Paramecium caudatum 



the given microcosm. The character of the curve should be the same 

 if we took only one mother cell at the start. Indeed, if one Para- 

 mecium is isolated and its products segregated as a pure culture, the 

 generation time of each cell is not identically the same as that of its 

 neighbors, and consequently at any given moment some cells are 

 dividing, whereas the others are at various intermediate stages of the 

 reproductive cycle; it is, however, no longer possible to divide the 

 population up into permanent categories, since a Paramecium which 

 divides rapidly tends to give rise to daughter cells which divide slowly, 

 and vice versa. The rate of increase of such a population will be 

 determined by the percentage of cells actually dividing at any instant, 

 and the actual growth of the population can be plotted as a smooth 



