102 THE SMALLEST LIVING THINGS 



as is heat-resistance. This has an importance in connection with 

 certain types of human disease. Malaria organisms, for ex- 

 ample, may become adapted to quinine poisoning and be able 

 to resist large doses of the drug. Many pathologists regard the 

 malarial relapse as due to such "quinine-fast" organisms, which 

 become virulent under certain conditions of the blood. 



The irritability of protoplasm, together with its power of 

 adjustment or adaptation, underlies all parasitism. Together 

 with water and food, protozoa are constantly being taken into 

 the digestive tracts of higher animals. In the stomach they enter 

 a denser and an acid medium in which the vast majority of such 

 water-dwelling forms are quickly killed. Some protozoa, par- 

 ticularly those already adjusted to a denser medium, are better 

 able to withstand the hardships of the new environment. Such 

 resistant forms may be the making of parasites. But they must 

 be adapted also to withstand the action of proteolytic ferments 

 which operate in the alkaline medium of the intestines. Such 

 resistant forms are said to be immune to digestive fluids, and 

 they may or may not become the causes of diseases, the result 

 of the association of the host and the parasite being dependent 

 upon the nature of their adaptations. In general, we may say 

 that the more recent the association of host and parasite, the 

 more apt is the result to be pathogenic. For example : the blood 

 disease known as nagana, which attacks the cattle of southern 

 Africa, and which is due to the flagellate, Trypanosoma brucei, 

 is only mildly harmful to the native cattle but is quickly fatal 

 to European cattle. 



Associations for Mutual Good 



Great numbers of protozoa have become adapted to life in 

 the cavities, fluids, and tissues of higher animals. The digestive 

 tract in particular is a center of activity for such adapted forms, 

 and no animal is exempt. In some cases these protozoa are harm- 

 less guests, in which case they are called commensals, or they 

 may be even beneficial to the host by keeping down the growth 

 of bacteria. In some cases, rare indeed, they have made them- 

 selves indispensable to their host by preparing the food on which 

 the latter is dependent. Thus the highly complex flagellates, 

 known as Hypermastigidae, e.g., Trichonympha (Fig. 58), in- 



