120 



THE STRUGGLE FOR EXISTENCE 



diminishing prey should perish not in consequence of their having 

 been devoured by the predators but from other causes, we could not 

 be entitled to draw any conclusions in respect to the properties of the 

 predator-prey relations in the given chain. 



(5) We may be told that after we have "snatched" two components 

 out of a complex natural community and placed them under "arti- 

 ficial" conditions, we shall certainly not obtain anything valuable 

 and shall come to absurd conclusions. We will therefore point out 

 beforehand that under such conditions it is nevertheless possible to 

 obtain periodic oscillations in the numbers of the predators and prey, 

 if we but introduce some complications into the arrangement of the 



J 



Fig. 30. The elementary interaction between Didinium nasutum and Para- 

 mecium caudatum (medium of Osterhout). The environment is not com- 

 pletely favorable for Didinium, and it begins to die out too early. Numbers 

 of individuals pro 5 c.c. From Gause ('35a). 



experiments. As yet we have only separated the elementary inter- 

 action between two species, and noted some of its fundamental 

 properties. 



However, why is the theoretical equation of the mathematicians 

 not realized in our case? The cause of this is apparently that a purely 

 biological property of our predator has not been taken into account 

 in the equation (21a). According to this equation a decrease in the 

 concentration of the prey diminishes the probability of their en- 

 counters with the predators, and causes a sharp decrease in the multi- 

 plication of the latter, and afterwards this even leads to their partly 

 dying out. However, in the actual case Didinium in spite of the in- 

 sufficiency of food continues to multiply intensely at the expense of 



