CHAPTER TWENTY-SIX 



PROTOZOA AND DISEASE 



I. PARASITISM has arisen independently in various Origin of 

 groups of Protozoa. It represents an effort on the part Jmong 181 

 of these animals to extend their range, to find new oppor- Pr <>tozoa 

 tunities for existence. The fluids within the bodies 

 of animals appear to be especially suitable for proto- 

 zoan life, but the species which are found as parasites 

 are not identical with those living free. They have 

 special characters which fit them not merely for para- 

 sitic life in general, but for life in a particular kind of 

 animal, the involuntary host. 



We may imagine the evolution of a parasitic proto- 

 zoan type to have been somewhat as follows. Origi- 

 nally an inhabitant of the waters surrounding or im- 

 bibed by the prospective host, it finds its way into the 

 alimentary canal, where it becomes established and at 

 the same time modified for the new mode of life. Then, 

 after a time, it penetrates the walls of the gut, and occu- 

 pies the blood or some other body fluid, and is now an 

 obligatory parasite. All this will doubtless take a very 

 long time, and requires perhaps millions of generations 

 of the evolving organism. That it should happen at 

 all is rather surprising, when we consider the extraor- 

 dinary stability of the free-living species. These latter 

 have remained true to type in the presence of tropical 

 heat and arctic snows, and through immense periods of 

 time. 



The parasites have certainly undergone more rapid 

 change, fitting themselves for life in various hosts, some 

 of which are themselves of comparatively recent evolu- 

 tion. Herein they met the necessities of the situation, 

 showing a power of adaptation where nothing else would 



199 



