4 i 6 POPULAR SCIENCE MONTHLY 



The organisms causing the several diseases that I have dealt with 

 in the preceding account fall in three of the four great groups of 

 protozoa, the I?ifusoria alone being unrepresented. The causes of 

 tropical dysenter}^ of rabies, and possibly of small-pox, are related to 

 the ordinary rhizopod types; the causes of sleeping sickness, trypano- 

 somiasis and syphilis belong to the animal flagellates, and here, pos- 

 sibly, belongs the organism of yellow fever; the causes of malaria, and, 

 probably, of splenomegaly, dum-dum fever and the like belong to the 

 sporozoa. 



The effects of these different parasites upon their hosts differ in 

 different cases. Sometimes they poison the host by the liberation of 

 toxins, as in the case of malaria or yellow fever ; sometimes by local 

 lesions or general tissue disorganization, as in tropical ulcer, sleeping 

 sickness, syphilis and rabies; or sometimes by the mere mechanical 

 obstruction to normal physiological processes by the accumulation of 

 parasites in capillaries and ducts, while in still other cases the para- 

 sites stimulate the latent dividing energy of tissue cells and lead to 

 abnormal tumor-like growths. In many cases knowledge of the organ- 

 ism and of its mode of life has led to preventive measures and to the 

 great saving of human life. In yellow fever, for example, the war- 

 fare on mosquitoes will stop entirely its epidemic nature and, thanks 

 to Carroll's brilliant experiments, yellow fever to-day, like malaria or 

 trichinosis, is an advertisement of ignorance or criminal negligence 

 in communities where it exists. Preventive measures hold down the 

 ravages of small-pox, anti-rabic serum lessens the malignancy of hydro- 

 phobia, while different specifics are fatal to other kinds of parasitic 

 protozoa, quinine to Plasmodium malaria, mercury to Treponema 

 pallidum, and recently Koch's specific to Trypanosoma. With the 

 knowledge of these other pathogenic protozoa and of their modes of 

 life investigation will bring out the means of combating them and 

 thus of reducing human suffering, and this prospect if for no other 

 reason, is a full justification of the many commissions that have 

 been appointed, and of the vast sums of money that have been spent, 

 to further protozoan research. 



