PROTOZOA 315 



spread; surra and mal de caderas certainly do not, as these are diseases 

 of Asia and South America respectively, and tsetse flies are not found in 

 either of these countries. There is, in fact, no reason to doubt that any 

 blood-sucking fly can transmit trypanosomes from the blood of one 

 host to that of another. In view of this the horse and stable flies, so 

 common in North America, would, in the presence of trypanosomiasis, 

 amply supply the means for its spread. 



In recent years important advances have been made in the study 

 of the r61e of arthropods in the spread of disease. Common knowledge 

 as to its powers for carrying bacterial infection has condemned the 

 fly to the swat, but it is as essential hosts, and not as purely mechanical 

 carriers, that these invertebrates furnish the greatest field for research. 

 Much has already been accomphshed in working out the life histories 

 of the parasites of insects and ticks, including parasites which have no 

 apparent connection with diseases of higher animals, for these, po- 

 tentially at least, may not be so harmless to higher animals as may at 

 first appear. Change of habitat, as from one part of the body to another 

 in the same host, or from a host of one species to that of another, fre- 

 quently leads to great alteration in the mode of life of an organism 

 which, relatively harmless in the insect, may in the vertebrate evolute 

 into more harmful parasitism with the development of pathogenicity. 

 The newer a parasite is to the animal harboring it, the less it is in har- 

 mony with its environment. Protozoa which produce acute forms of 

 disease have less adaptation to their environment than those producing 

 a chronic type of malady. This discord between parasite and host is 

 manifested by acute disturbances which may result in the death of the 

 infected animal. Such parasitic diseases of a chronic course are usually 

 correlated with a greater degree of adaptation of the parasite to its 

 host and also with acquired resisting powers of the host to the specific 

 action of the parasite. 



The scale of evolution through the saprophytic, parasitic, and patho- 

 genic is thus exhibited by certain groups. The Spirochetida, long, 

 deUcate Protozoa with a corkscrew-formed body, may be found as in- 

 habitants of the body-cavities, of normal mucous surfaces, of inflamed 

 mucous surfaces, as parasites which have penetrated the tissue, and as 

 blood parasites. This series is sufficient to show how parasitism may 

 evolute by various gradations from harmless commensalism to distinct 

 parasitism and pathogenicity. When the habit of living in inflamed or 

 ulcerated tissues is reached the power of penetrating healthy tissues 

 soon follows which, with the multiplication of the spirochetes in such 

 situations, causes destruction of invaded tissue and local disturbances. 

 The products of this tissue destruction, together with those coming from 

 the dead bodies of the parasites, form toxins which, getting into the 

 blood, produce the general toxemic symptoms. The final stage of 



