8a ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



much poon : <>t" persisting as a parasite of that particular host 



anin 



re thus comes about in Nature a kind of equilibrium between 



t he ho- .mil the parasites of a particular region that equilibrium 



:o to be upset by (a) the introduction of new host animals into that 



or (b) the introduction of new parasites. There are then brought 



my with one another hosts and parasites between which 



has had no time to bring about the elimination of susceptibility 



.ml tlu-rc are now liable to occur violent outbreaks of disease, lasting 



until tin- particular species of host animal is completely exterminated 



or on the other hand, a condition of tolerance or repellence has been 



gradually brought about by the weeding out of the more susceptible 



and the survival of the less susceptible strains. 



Such disturbances of equilibrium occur doubtless frequently in 

 Nature but particularly striking examples have come about through 

 the action of man. 



white man colonizes parts of the world infected by the parasite 



iria and he suffers greatly from the attacks of the malarial parasite 



towards which the aboriginal negro inhabitants have become tolerant, 



while continuing to act as carriers or reservoirs of the disease. Again 



:<>duces domesticated animals into regions in which they fall 



! to epidemics of Trypanosomiasis or Piroplasmosis caused by 



bich spread to them from the native animals in which they 



thout causing any obvious disease. 



it may be the parasite rather than the host which is trans- 



mtu unaccustomed regions. Such seems to have been the case 



with .slrrpini: >ickness, which has long existed on the West Coast of 



Africa but which, conveyed up the course of the Congo by carriers, 



l>ed among the natives of Uganda into an epidemic of the greatest 



compared with the comparatively feeble outbursts on the 



West Coast where susceptibility had been in the course of time gradually 



dimini 



I'.OOKS R)k I'VKTHER STUDY 



I. GENERAL TEXT-BOOKS 



Minchin. Introduction to the Study of the Protozoa. 

 Doflein. Lchrbuch der Protozoenkunde. 



