THE president's ADDRESS. 13 



phases of the parasite should be sufficiently abundant in the blood 

 to make sure of infecting the invertebrate that sucks the blood 

 and transmits the parasite. To ensure this taking place it is 

 essential that the parasite should multiply and swarm in the 

 blood to an extent that may, in many cases, lead to pathogenic 

 and even fatal results. Hence one interest of the parasite may, 

 so to speak, clash with another, and the all-important object of 

 dissemination cannot be attained without bringing about a state 

 of affairs which puts an end to the host and to the parasite with 

 it ; a result which, though detrimental to the parasite, may not 

 be seriously so, if the death of its host do npt supervene until 

 there has been time for the dissemination of the parasite to have 

 taken place. No reasonable person, however, could expect any 

 adaptation to be absolutely perfect. 



The inevitable conclusion from a general consideration of the 

 facts of parasitism amongst Protozoa is, therefore, that it is 

 hardly ever to the advantage of the parasite to be pathogenic, 

 and still less so to be lethal to its host. This conclusion is borne 

 out by the facts, for when the known forms of Protozoan para- 

 sites are reviewed and considered in their entirety, it is found 

 that the vast majority of them are quite harmless to their hosts. 

 Pathogenic and lethal parasites are the exception amongst the 

 Protozoa, and are greatly in the minority when compared with 

 the harmless forms. The attention of investigators has been so 

 much focussed on the disease-producing forms, on account of their 

 practical importance, that they appear at first to overshadow 

 the harmless forms which obtrude themselves less on the public 

 attention, though far more numerous. Thus, if trypanosomes 

 are mentioned, most people think at once of the deadly parasites 

 of sleeping sickness or other diseases ; and it comes almost as a 

 shock to find that the fishes and frogs of our waters, and the 

 wild birds and animals of this and other countries, are commonly 

 infested with trypanosomes without being perceptibly the worse 

 for these parasites. In fact, the disease-producing Protozoa are 

 to be considered as exceptional and aberrant forms ; the effects 

 they produce on their hosts are detrimental to their own interests, 

 and they are instances of what has been termed by Metchnikoff 



