33 



will represent the effect it will produce upon A. The four eggs 

 of the parasite will be deposited in four of the 32 hosts so that 

 only 28 of them will come to maturity; these will give birth to 

 56 young, eight of which will be killed by the parasites and 48 

 come to maturity; these will give birth to 96 young, of which 

 16 will be killed by the parasites. This process will continue to 

 the eleventh generation of the host, when it will only equal the 

 parasite in number and so be totally destroyed. Right up to the 

 last generation the host appears to be predominant and the final 

 reduction is sudden. This is a feature that one often observes 

 in nature. 



Ratio Betw^eex Host axd Parasite. 



That the utter extinction of the host does not take plac^ is 

 a very difficult problem to explain. Hyperparasitism only 

 pushes the question back one degree, and accidental death acts 

 upon host and parasite alike. 



Observations on several species of insects, extending over 

 wide areas, indicate that there is a certain ratio between the 

 numbers of the host and parasite. One would expect some such 

 ratio from inductive reasoning : parasitism could not exist with- 

 out it. 



How this ratio is maintained it is difficult to tell. It is not 

 through the birth rate, for many parasites are very much more 

 prolific than their hosts, and the length of time occupied in their 

 life cycle is often much shorter. It appears likely that the ratio 

 is due to the capacity of the parasite to discover its host. In 

 some species this capacity appears to be low, and the maximum 

 rate of parasitism is therefore low ; in other cases this capacity 

 is high, and the maximum rate of parasitism is consequently 

 high. 



When living in Africa, I often accompanied a friend shoot- 

 ing; he was by far the better shot, but whether game was 

 scarce or plentiful my bag generally stood in the same propor- 

 tion to his. This would indicate that we each had a certain 

 capacity for finding and bagging our game, and, within certain 

 limits, acted up to it. 



Uncivilized men, hunting with bow and spear, seldom, if 

 ever, exterminated game. Where game was plentiful the fam- 

 ily or tribe could increase till the district could not longer sup- 

 port it; then it would decrease or wholly or partly migrate. 



