SELECTED PAPERS 



provoking agents that do not show an unmistakable relationship with 

 forms that occur in a free-living state in nature, the so-called sapro- 

 phytic types. In view of this fact we are led to ascribe the appearance 

 of parasites to the occurrence at some time of an event whereby some 

 of these saprophytes, exposed as are all living organisms to gradual 

 modifications, have succeeded in overcoming the natural resistance 

 that every living cell complex offers against invasion by species- 

 foreign life. There is every reason to assume that this process has started 

 in the dim past of the developmental history of life on earth. In the 

 case of very ancient fossils, dating back to the Permian (200 million 

 years ago) the palaeontologists have found clear-cut indications of 

 infectious diseases, and some scientists are even inclined to ascribe the 

 extinction of numerous animal species during prehistoric times to 

 such diseases. 



Now it seems far from improbable that a similar transformation of 

 a saprophytic into a parasitic mode of life may occasionally occur 

 even to-day. This possibility is increased by the fact that in recent 

 years the microbiologist has managed to induce mutations in numer- 

 ous microbes with the aid of irradiation, and it is readily conceivable 

 that similar effects are produced under natural conditions by cosmic 

 rays. 



Furthermore, we have to count with the possibility that during the 

 repeated transmission from host to host the uniformity of the milieu 

 may cause modifications of the parasite that may have either good 

 or bad consequences. 



Whereas the first invasion generally leads to a violent reaction, 

 causing either host or invader to succumb acutely, a modicum of 

 adaptation is prone to induce in the host a chronically proceeding 

 alteration ; and eventually a symbiosis, or coexistence with mutual ben- 

 efit, may be established in which the host no longer shows any signs 

 of damage. I may here refer, for example, to the well known spiro- 

 chaetosis of mice ; and our fellow member, Swellengrebel, has discov- 

 ered a sensational example in human pathology when he established 

 that particularly the younger generation of aborigines in Surinam was 

 often strongly infected with virulent malaria parasites without appar- 

 ently suffering any ill effects. 



Such an adaptation may, however, equally well lead to other con- 

 sequences. In numerous cases it has been found that passage through 



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