COMPETITION FOR COMMON FOOD IN PROTOZOA 109 



a is equal to about +0.5; in other words, P. aurelia suffers from a 

 slight depressing influence of P. caudatum. Later the inhibitory- 

 action of one species upon the growth of another begins to manifest 

 itself more and more in proportion to the quantity of food consumed, 

 because the larger is the part of the food resources already consumed 

 the less is the unutilized opportunity for growth. In our calculations 

 for P. caudatum from the second and for P. aurelia from the fourth 

 days of growth we have identified the coefficients of competition with 

 the coefficients of the relative food consumption, i.e., a = 1.64, /3 = 

 0.61. It is obvious that this is but a first approximation to the actual 

 state of things where the coefficients gradually pass from one value 

 to another. The entire problem of the changes in the coefficients of 

 the struggle for existence in the course of the growth of a mixed popu- 

 lation (which apparently are in a great measure connected with the 

 fact that the Paramecia feed upon living bacteria) needs further 

 detailed investigations on more extensive experimental material than 

 we possess at present. 



(5) It remains to examine the second stage of the competition, 

 i.e., the direct displacement of one species by another. An analysis 

 of this phenomenon can no longer be reduced to the examination of 

 the coefficients of multiplication and of the coefficients of the struggle 

 for existence, and we have to do in the process of displacement with a 

 quite new qualitative factor: the rate of the stream which is repre- 

 sented by population having completely seized the food resources. 

 As we have already mentioned in Chapter III, after the cessation of 

 growth a population does not remain motionless and in every unit of 

 time a definite number of newly formed individuals fills the place of 

 those which have disappeared during the same time. Among differ- 

 ent animals this can take place in various ways, and a careful biologi- 

 cal analysis of every separate case is here absolutely necessary. In 

 our experiments the principal factor regulating the rapidity of this 

 movement of the population that had ceased growing was the follow- 

 ing technical measure: a sample equal to yu of the population was 

 taken every day and then destroyed. In this way a regular decrease 

 in the density of the population was produced and followed by the 

 subsequent growth up to the saturating level to fill in the loss. 



During these elementary movements of thinning the population 

 and filling the loss, the displacement of one species by another took 

 place. The biomass of every species was decreased by ro daily. 



