ADAPTATION OF ORGANISMS 337 



and taking with the blood certain sexual stages of the para- 

 site which can develop in the cold-blooded insect. And now 

 the Mosquito is the host. In its stomach the sexual phase of 

 the life history of the malarial parasite takes place, fertiliza- 

 tion occurs, and finally the numerous products of the zygote 

 work their way to the mouth parts of the Mosquito, where 

 they await an opportunity to enter the human blood. 



The life history of malarial parasites exhibits a continuous 

 series of adaptations to parasitic life: the nicety of the ad- 



FIG. 182. A trypanosome (Trypanosoma theileri) from the blood of cattle. Magni- 

 fied about 3000 times. (After Liihe.) 



justment being especially well illustrated at the transfer 

 from Man to Mosquito, since all the parasites which enter 

 the stomach of the latter are digested except those sexual 

 forms which are ready to initiate the sexual part of the cycle 

 in the new host. 



But the acme of parasitic associations is only attained 

 when the adaptations of parasite and host have become so 

 complete that the latter 'pays the price' without any un- 

 toward results. Thus the Antelopes and similar Mammals 

 of certain regions of Africa harbor in their blood various 

 species of Protozoan parasites, known as TRYPANOSOMES, 



