86 RESEARCH IN PROTOZOOLOGY 



reservoir hosts for human intestinal protozoan infections. The sec- 

 ond hypothesis concludes that the intestinal protozoa of man and 

 the lower mammals represent different species, that the host-para- 

 site relations are rigid and that the lower mammals may not serve 

 as reservoir hosts for human intestinal protozoan infections. 



Obviously the earlier view was formulated as the result merely 

 of observing the presence of intestinal protozoa of the lower 

 animals similar in appearance to those seen in man. Because they 

 presented related morphological characteristics they were held to 

 be the same species. As a rule little or no experimental work was 

 carried out before conclusions were drawn. Later, certain investi- 

 gators resorted to the experimental method to support their con- 

 tentions but the experiments were usually carelessly conducted, 

 without adequate controls and with insufficient care in the selec- 

 tion of parasite-free animals before the transmission experiments 

 were carried on. 



With evident differences in morphology, which became appar- 

 ent as the result of investigations by the cytologist, certain species 

 came to be clearly differentiated on the basis of morphology. This 

 gave the necessary stimulus to unlimited species differentiation 

 which, coupled with a few negative results obtained from inade- 

 quate transmission experiments, led to the formation of the second 

 or negative view which assumes, purely on the basis of host en- 

 vironment, that parasites within different species of mammals are 

 different. 



To hold rigidly to this second and restricted contention is just 

 as unfortunate as to accept the first extreme view, for it seems 

 probable that each species of host and of parasite must be con- 

 sicjered on the basis of its own individual relationships. Hence a 

 third or interm,ediate view, which is the one held by the writer, is 

 apparent. This may be stated as maintaining that certain parasites 

 are non-rigid and can be transferred with facility from host to 

 host while others may be found to be restricted to individual hosts. 



The factors involved in determining these relationships are as 

 yet quite obscure but it seems probable they are mostly biochemical 

 in scope. Variation in hydrogen-ion concentration in the intestine ; 

 variation in type of diet, i.e., whether carnivorous, vegetarian or 

 omnivorous ; or whether of high or low protein content ; and the 

 type of vitamines present, all appear to be of importance in such 

 determinations. The type and strength of digestive secretions pres- 



