iv PREFACE 



because of the characteristic black spots), led him to the belief that 

 many human ills are similarly due to minute and microscopic forms of 

 life, and so paved the way for the later generalization which now domi- 

 nates medicine — the germ theory of disease. Ahead of his times in 

 recognizing the present-day axiom that epidemics are ended by pre- 

 vention rather than by individual treatment, Pasteur patiently advised 

 and demonstrated, in connection with the silk industry, that perfect 

 silkworms and moths would not develop from eggs having pfebrine 

 corpuscles on them. It is of no importance that these corpuscles were 

 not recognized by him as the spores of a protozoon, but the important 

 results which followed their discovery, and which led to increased 

 length of human life, and to the mitigation of human and of animal 

 suffering throughout the civilized world, make an increasingly sub- 

 stantial monument to the patience, courage, and virility of this man 

 of pure science, who, by the apotheosis of scientific method, proved 

 these unknown corpuscles to be the cause of this silkworm disease. 



The recently opened chapter of the protozoan diseases of man 

 might have been earlier studied had these observations of Pasteur 

 upon the spores of Nosema bombycis been followed up. The parasitic 

 protozoa were known and the free-living forms had been brought into 

 prominence in scientific circles through the controversies over the cell 

 theory and the theory of spontaneous generation, but more than thirty 

 years were to elapse before general acceptance of the first human 

 disease attributable to protozoa. 



The other minute organisms, bacteria and yeasts, whose presence 

 Pasteur had demonstrated in his experiments on fermentation and 

 spontaneous generation, were not neglected. In the hands of R. 

 Koch the means of studying bacteria were perfected, and "culture" 

 methods were introduced which soon raised bacterial research to the 

 dignity of an independent branch of biological science. The ease with 

 which bacteria could be studied, thanks to these methods, and the 

 rapidly increasing list of bacterial diseases, seemed to divert the atten- 

 tion of specialists from the pursuit of protozoan diseases and to confine 

 it to research on those of bacterial origin. Attempts were repeatedly 

 made, however, to cultivate protozoa as the bacteria are cultivated, on 

 artificial media, but until the present decade such efforts, for the most 

 part, were fruitless. The difficulties in applying the artificial culture 

 method to the protozoa are due, essentially, to the differences in their 

 mode of nutrition. Some of them, indeed, are similar to the bacteria in 

 being saprophytic or saprozoic (to use Blanchard's expressive term), 

 absorbing liquid or dissolved proteid matter through the body wall. 

 Such forms lend themselves to the culture method, and the success of 

 Novy andMacNeal and others with trypanosomes,herpetomonads, etc., 

 in artificial liquid media follows from this nutritional characteristic. 

 Other forms of protozoa, as, for example, the parasitic amebse, may or 



