402 BIOLOGICAL CONTROL 



or that, if the sequence be incomplete, the pest would not be held 

 in check. According to Thompson's view, control depends 

 primarily upon the ratio between the respective rates of repro- 

 duction of host and parasite. If the rate of reproduction of 

 one species of parasite is equal to, or greater than, the rate of 

 reproduction of the host, this parasite could, in many cases, 

 increase to the point when complete control of a host is achieved 

 by its unaided activities. The time taken for the attainments of 

 effective control of the host by the parasite will depend, not only 

 upon the reproductive ratios just mentioned, but also upon the 

 ratio between the number of individuals of a parasite introduced 

 and the number of individuals of the host at the time the intro- 

 duction is made. Thompson concludes that, although there are 

 cases where the percentage of parasitism of a given species may 

 never rise above a certain maximum, instances must be rare in 

 Nature where all the parasites of a host are thus limited in their 

 activities, and it will usually not be found true of most members 

 of a parasite sequence. According to him the theory is invalid as 

 a general theory of parasitic action, since it can only apply to a 

 limited number of special cases. On the other hand, Thompson 

 supports the general idea of the introduction of a sequence of 

 parasites for a reason entirely different from the one implied 

 by the theory. It is simply because by introducing a number 

 of species of parasites the probability is greatly increased of 

 selecting one whose reproductive potentialities are equal to, or 

 greater than, those of the host concerned and which, for this 

 reason, offers possibilities of exercising the desired degree of 

 control. 



Other Factors. Among other factors which may have a direct 

 bearing upon parasite introductions, the following will be briefly 

 discussed : — 



1. The subject of host-parasite relationship is discussed in 

 Chapter XIII, and little can be usefully added here. The bulk 

 of the information relative to this factor is of an empirical nature. 

 No means exist of predicting the results of parasite introductions 

 for the reason that it is not possible to forecast with any certainty 

 how a particular parasite will behave towards its possible hosts 



