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SCIENCE. 



[N. S. Vol. XXII. No. 555. 



organisms to disease. Even now the new 

 field has been recognized and the Landon 

 School of Tropical Medicine has appointed 

 this year an investigator in protozoology— 

 however unfortunate the form of the term 

 may be. There are already listed more 

 than thirty of the Protozoa which para- 

 sitize the human body. Regarding many 

 of them our knowledge is exceedingly 

 scanty, but of others it may be affirmed 

 definitely that they are the cause of dis- 

 eases which rank among the most dangerous 

 to which man is subject. Among these 

 forms I have included only those that are 

 distinctly recognizable in structure as Pro- 

 tozoa, though their life histories and exact 

 relationships are yet unexplained; but be- 

 yond these limits lie a vast horde of un- 

 identified structures, interpreted by some 

 observers as parasitic protozoa, but re- 

 garded by others as parasitic fungi and by 

 still others as products of cellular degen- 

 eration or other pathological changes. Such 

 are the cancer parasites of several investi- 

 gators, the organisms of leucsemia, scarlet 

 fever and other diseases. No doubt some 

 of these will be shown by further research 

 to be in fact independent organisms of 

 parasitic habit and the cause of disease, 

 and it seems probable that many of these 

 will fall within the group of Protozoa, the 

 unicellular animals. Thus has been opened 

 up a new field in which the microscope is 

 the essential instrument of investigation. 

 All the work to be done here depends upon 

 this instrument, without which the very 

 existence of these organisms would have 

 remained unsuspected. Following close 

 upon the wonderful discoveries of the his- 

 tologist, the pathologist, the bacteriologist 

 and the clinician, these studies furnish new 

 evidence of the supreme importance of the 

 microscope in the development of scientific 

 medicine, in the attainment and preserva- 

 tion of the health of mankind. 



There is left but a paragraph in which 

 to mention another aspect of the subject 

 of this address. Even under, the narrow 

 limits of the topic — animals in relation to 

 disease— there is one phase which in justice 

 to them should not be entirely omitted. 

 Animals stand also distinctly as preventers 

 of disease ; and this in the first place as 

 destroyers of disease germs. Among the 

 Protozoa which have already been exploited 

 as the producers of disease, are found also 

 the organisms which play the most impor- 

 tant part in the purification of sewage- 

 contaminated streams by consuming the 

 bacteria. These forms are specifically cili- 

 ates, of which the common slipper animal- 

 cule (Paramecium) may serve as a typical 

 form ; they abound in all waters, especially 

 in those containing decaying matter, and 

 devour countless numbers of bacteria. 

 Through their activity it becomes possible 

 for one city to drink the diluted sewage of 

 another city higher up on the watershed 

 without losing all its citizens from intes- 

 tinal diseases. 



Modern science has also made use of ani- 

 mals in combating disease; as producers 

 of antidotes, either in the form of cow-pox 

 or vaccine, or in the role of test animals 

 and of serum producers, manufacturing 

 antitoxines of various sorts, many animals 

 discharge in this way a most essential func- 

 tion in modern life. But the discussion of 

 this phase lies beyond the demands of the 

 present occasion. 



In closing let me call your attention to 

 the bearing these studies on the relations 

 of animals to disease have on the science 

 of medicine. Any rational method of cure 

 depends upon the distinct recognition of 

 the cause of the malady. Any other basis 

 gives unlimited opportunity for chicanery 

 and fraud and for the despoliation of the 

 people in the name of medicine, so general 

 to-day. But more than that, preventive 



