.33 



will represent the effect it will prodtice upon A. The four ei^gs 

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

 only 28 of tlieni w'ill couie to maturity; these will give birth to 

 5G young, eight of which will he killed by the parasites and 48 

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

 1 6 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 i)redominant and the final 

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

 in nature. 



Ratio Betweex Host axd Parasite. 



That the utter extinction of the host does not take place 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 witb- 

 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. 



