202 



ZOOLOGY 



Economic 

 results from 

 study of 

 Protozoa 



was necessarily in the bodies of warm-blooded animals 

 at all. They are parasites of Arthropods, which are con- 

 veyed to mammals and other animals when the insects 

 or arachnids suck their blood. Thus the parasites have 

 alternate hosts, belonging to very different classes of 

 animals, both of which must be present for the comple- 

 tion of the entire cycle of normal activities. It is note- 

 worthy in this connection that the alimentary canals of 

 many Arthropods, such as insects and centipedes, are 

 inhabited by Gregarines, a group of Sporozoa which 

 .have nothing to do with disease in higher animals. We 

 may infer, though we can never prove, that millions of 

 years ago, in Carboniferous times, the great cockroaches 

 then so abundant were infested by these parasites, at a 

 time when no warm-blooded animals had evolved. We 

 do not know how it first came about that alternation be- 

 tween a vertebrate and an invertebrate host was estab- 

 lished, or by what means a parasite was able to accom- 

 modate itself to the strange environment of warm 

 blood. We do know, however, that this happened more 

 than once ; for trypanosomes and malaria parasites are 

 little related, and certainly evolved from quite different 

 branches of the great protozoan stem. 



5. With the establishment of the theory of alternate 

 hosts, a great new field of preventive medicine was 

 opened up. It would be difficult to exaggerate the value 

 of the various discoveries which have given us knowl- 

 edge of the course and mode of transmission of yellow 

 fever, malaria, sleeping sickness, and other diseases. 

 Through them the most fertile regions of the world are 

 opened up to the white man ; and while innumerable 

 deaths are prevented, our food supply is increased enor- 

 mously. We have only begun, as yet, to take advantage 

 of the offerings of science in this direction ; but it is 



