CHAPTER VI. 



THE PATHOGENIC FLAGELLATES. 



It is a well-recognized zoological principle that some groups of 

 animals, families, orders, classes, or even phyla, may be stationary, 

 so far as evolution is concerned, and not easily adapted to new environ- 

 mental conditions. Other groups, on the other hand, are remarkable 

 for the variety of structures, for ready adaptability to new conditions, 

 and, in general, for their high "potential of evolution." 



Similarly with the protozoa we meet with the same variations; the 

 infusoria, for example, both ciliates and suctoria, are highly differ- 

 entiated, and, as shown by the well-defined orders and families, are 

 fairly stable in evolution, while the mastigophora, on the contrary, 

 possess a remarkable power of variation and a high potential of evolu- 

 tion. It is among these latter forms that we meet with all methods 

 of nutrition and with all grades of organization connecting animals 

 with plants, while it is here, also, that we look, especially among the 

 colony forms, for cellular division of labor or developmental processes, 

 that may throw light on the origin of multicellular from unicellular 

 animals. 



With their great power of adaptation combined with the variety of 

 available modes of life, it is to be expected that many types of flagel- 

 lated unicellular parasites should be known, and among them, that 

 we should find numerous cases of incomplete adaptation. This is 

 particularly probable in organisms like the hematozoic flagellates, 

 where the uncertain conditions of the definitive invertebrate and 

 secondary vertebrate hosts make stability of form and life cycle 

 difficult to work out. There is reason to believe, with R. Koch, that 

 certain types of trypanosomes are established, or are "good" species 

 (e. g., Trypanosoma leivlsi, T. ihelleri), while others are undoubtedly 

 in that phase of adaptability which De Vries calls the period of muta- 

 tion. While such an hypothesis probably contains an element of truth, 

 it is just as well to keep it for the present as a generality, and not to 

 apply it as the famous bacteriologist does, to specific cases until after 

 the life histories of such cases are known. "Good" or "bad" species of 

 protozoa, especially in this group, have no scientific standing until the 

 life cycle is accurately establislicd, and "degrees of virulence" or 

 '"promiscuity of secondary (vertebrate) hosts" have no more to do with 

 establishing a protozoan species than the salt- or fresh-water habitat 

 has to do with actinophrys, chilodon, or colpoda, and whether there is 



