EVOLUTION IN PARASITISM 63 



free-living form, quite independent of any animal. 

 Certain of its members, having been ingested by 

 some insect, say, become habituated to living in the 

 alimentary tract of the new host. At first the host, 

 perhaps, was caused inconvenience by its invader, but 

 later became accustomed to its presence and immune 

 to any evil effect. From the main alimentary canal 

 the flagellate reached other organs of the body of 

 the host at any rate, in some cases. 



A further stage in the scale may have been the 

 result of the acquiring of the sanguivorous habit by 

 the insect. The latter, having in turn become para- 

 sitic externally on higher animals, pierced the flesh 

 of a vertebrate, and in so doing injected a small 

 number of its contained parasites into the wound. 

 Many probably died quickly in the blood-stream, 

 but some, being of a hardier nature than their fellows, 

 began to divide, to develop along somewhat different 

 lines from their ancestors, and finally to become the 

 pathogenic agents of such maladies as the tsetse-fly 

 disease, nagana, or even sleeping sickness. 



Such is one speculative view, interesting enough 

 in itself, and being an hypothesis for which more 

 can be said than for most. For in this case there 

 are a certain number of facts on which the above 

 speculation is based. Recently it has been shown 

 that H. ctenocephali, from the gut of the dog-flea, and 

 Crithidia fasciculata, from a mosquito, can be success- 

 fully inoculated into mice. Such experiments furnish 

 examples of leishmaniases (see Chapter X.) in the 

 making. Further researches on these lines will be 

 awaited with interest. 



