2 PROTOZOAN PARASITISM 



Scattered far and wide over the earth, they live 

 wherever they find sufficient moisture even for a short 

 period of activity, lying dormant when their medium 

 is no longer liquid. In their dormant phase, which 

 we call cysts, they protect themselves by a capsule 

 formed from their own secretion, in which they may 

 survive drought conditions for variable lengths of 

 time. 



Warm wet regions are thus most favorable to them 

 and the waters of such climates teem with their num- 

 bers. The sands of the desert, the dust of the earth 

 and air, the ice of frozen regions, however, harbor 

 them, waiting for favorable conditions for continued 

 activity. 



It is probably not to be supposed that the forms 

 which we see today are the same as their ancestors, 

 nor is it to be thought that they are the forbears of 

 higher animals. Such is sometimes the conception 

 of the ignorant concerning the meaning of evolu- 

 tion. 



Undoubtedly the species which we know today as 

 free living forms and as parasites upon higher animals 

 and man have evolved from common ancestors. 

 Adaptation to changed or new environment has pro- 

 duced division into species, races and strains, specifi- 

 cally suited to the environment in which they live. 

 Thus the different species of one kind of protozoa 

 living in freedom or as parasites in man and other 

 animal hosts are close akin, at times difficult to dis- 

 tinguish from each other, and yet usually distinct 



