366 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



we know much of its life-history, and certain investigators 

 do not hesitate to express their belief that it belongs to the 

 genus Spirochceta. Yellow fever is transmitted only by the 

 yellow fever mosquito. 



Infantile paralysis, or poliomyelitis, is also caused by one 

 of these 1 ultra-microscopic parasites, which can probably be 

 distributed, as recently indicated by experiments with mon- 

 keys, by the common stable-fly, Stomoxys calcitrans. The dis- 

 ease seems also to be directly contagious. 



Cattle plague, canine distemper, swamp fever of horses, hog 

 cholera and several other diseases also belong to the group pro- 

 duced by invisible parasites. Some of them will probably 

 be found to be caused by animal parasites, others by bacteria. 



1 It has recently been found to be visible in certain stages, although 

 invisible in others. 



