PREFACE 



ALL who appreciate the importance of knowledge of the causes of 

 disease, and who have watched the progress of pathology in recent 

 years, must realize that the subject dealt with in the following 

 pages is now of the highest importance in Medicine. It has been 

 my aim to enable those who have not already done so to form 

 an idea of this progress, by giving an outline of the work which 

 has proved that protozoa are the cause of certain diseases. 



In addition to this I have recorded the present position of 

 knowledge, including my own observations and views, in regard 

 to three of the capital diseases of man small-pox, syphilis, and 

 sarcoma. In tracing this record I have tried to restrict myself to 

 a purely objective view, and to make it quite clear where observa- 

 tion ends and interpretation begins. 



With regard to the descriptions of personal observations, I 

 have done my best to make them reflect truly the facts of Nature 

 to which they relate ; and as to my interpretations, I may be 

 permitted to say that during the past few years new facts have 

 been established which harmonize with and support them. 



J. JACKSON CLARKE. 



1 8, PORTLAND PLACE, 

 LONDON, W., 



December 15, 1907. 



