DESTRUCTION OF ONE SPECIES BY ANOTHER 129 



out does not usually go on to the end owing to the heterogeneity of 

 population and presence of immune individuals, which are in a cer- 

 tain degree equivalent to the individuals protected in the refuge: 

 they are not carried away by the process of destruction which goes on 

 to the end among the non-immune ones. The nature of this "refuge" 

 is very complicated, and it is interesting to quote here the following 

 words of Topley ('26, pp. 531-532) : "Most of us who have been con- 

 cerned at all with the problem of immunity have been accustomed 

 to take the individual as our unit. When we take as our unit not the 

 individual but the herd, entirely new factors are introduced. Herd- 

 resistance must be studied as a problem sui generis. One factor 

 peculiar to the development of communal as opposed to individual 

 immunity may be referred to here. A herd may clearly increase its 

 average resistance by a process of simple selection, by the elimination 

 through death of its more susceptible members. . . . That some proc- 

 ess of active immunization will be associated with the occurrence of 

 non-fatal infection may safely be assumed, though its degree and 

 importance may be very difficult to assess, so that we must allow for 

 the possibility that the spread of infection which is killing some of 

 our hosts is immunizing others. 



"A very imperfect analogy may help to depict the position. Sup- 

 pose we take a number of stakes of different thickness, plant them 

 in the ground and expose them to bombardment with stones of vary- 

 ing size from catapults of varying strengths. After a certain time 

 we shall find that a number of the stakes have been broken. This 

 will not have happened to many of the thicker stakes, but other sur- 

 vivors will consist of thinner stakes, around which ineffective missiles 

 have formed a protective armour. Survivors of the latter class are 

 in a precarious state; subsequent bombardment may displace the 

 protective heap, and perhaps add its impetus to that of the new mis- 

 sile. Survivors of the former class may eventually be destroyed by 

 a missile of sufficient momentum." 



(3) The experiments of epidemiologists dealing with the influence 

 of immigration on the course of an epidemic among mice in a limited 

 microcosm are also very interesting. One can distinctly see here 

 that a continuous interference from without acting upon a definite 

 population causes periodic oscillations in the epidemic which disap- 

 pear immediately as the interference ceases. Let us quote Topley 

 again: "When susceptible mice gain access to the cage at a steady 



