140 



THE STRUGGLE FOR EXISTENCE 



can be seen in the following manner: by plotting as ordinates the 



values log ( 6 2 — j^- — j^ ) corresponding to different values of abscissae 



Ni, we should obtain a straight line which, as Figure 40 shows, is actu- 

 ally observed. The slope of this straight line is characterized by the 

 coefficient X, which thus expresses the rate at which the relative in- 

 crease of the parasites approaches its maximal value with the increase 

 of the density of hosts. 



(2) To summarize: We expected at the beginning of this chapter 

 to find "classical" oscillations in numbers arising in consequence of the 

 continuous interaction between predators and prey as was assumed 





o 



I 



20 VO tO 



Concentration of- the host (N,) 



80 



Fig. 40. Connection between' the progeny of one pair of the parasite, Mor- 

 moniella vitripennis, and density of the host, Phormia groenlandica, according 

 to Smirnov and Wladimirow. From Gause, '34c. 



by Lotka and by Volterra. But it immediately became apparent 

 that such fluctuations are impossible in the population studied, and 

 that this holds true for more than our special case. The correspond- 

 ing analysis showed definitely to what biological adaptations this 

 impossibility is due. This has enabled us to find a particular system 

 possessing no such adaptations and in this way to observe "classical" 

 fluctuations under very specialized conditions. 



It is to be hoped that further experimental researches will enable 

 us to penetrate deeper into the nature of the processes of the struggle 

 for existence. But in this direction many and varied difficulties will 

 undoubtedly be encountered. 



