66 PKESIDKXTIAL ADDRESS— SECTION D. 



In connection with parasites, it may be noted that all forms 

 of parasitism are more or less degrading ; reproduction seems to 

 be the chief aim. On the other hand, parasitism implies plasticity 

 and often polymorphism, for the organism has to adapt itself to 

 many changes of environment. In some cases the parasite has 

 become so specialised in its physiological relations with the host 

 that one phase of its life can be passed only in one host, a second 

 phase, often one of rest or metamorphosis, in a second host, and 

 the final or adult condition is attained only in a third host. 

 Examples of such will be afforded later. 



Specificity of parasites to one vertebrate host is often well- 

 marked. In other cases, the parasites have greater powers of 

 adaptation and can live in several hosts. Thus, the human tape- 

 worm. Taenia solium, is specific to man, but Dipylidium. caninum 

 of the dog can not only infect dogs, but can live in wolves, jackals, 

 rodents, and occasionally man. Arthropod parasites, again, may 

 be specific to their hosts, and, in some cases, the association of 

 host and parasite occurred at a very early stage in the evolution 

 of both groups. An instance of this rigid specificity is afforded 

 by the Mallophaga parasitic on birds, as was shown by Harrison, 

 who indicated how the systematic relationships of different Mallo- 

 phaga are indicative of relationships of the genera of birds on 

 which thej^ occur. Incidental parasitism, the result of temporary 

 or accidental association, is also known. 



In connection with certain of the intestinal Protozoa, parti- 

 cularly Flagellata, numerous species have been created often 

 merely on the criteria of size and of occurrence in another host. 

 Morphologically, many of these organisms, such as species of 

 Herpetomonas and Giardia, are almost identical; biologically, 

 their life-histoi'ies are the same. Pathologically, they may have 

 widely different effects on the hosts they infest. Potentially, all 

 are capable of producing fatal effects on any new, susceptible host 

 into which they may be introduced. As has been stated repeat- 

 edly and shown experimentally, the newer the parasite is to the 

 host the more lethal are its effects. With age-long habituation to 

 one host generations of parasites are produced that have inherited 

 the mutual toleration established through such long association, 

 and their pathogenic properties are either sublimated or reduced. 



At the same time, it seems to me highly probable that many 

 species of parasites will idtimately prove to be varieties of the 

 same organism, the primitive strain having been acted upon by 

 different environments, charged with slightly different potentiali- 

 ties as a result of their reactions with these environments, and 

 finally modified slightly morphologically in accordance therewith. 

 Just as the chemist and the physicist have worked together to find 

 the universal ion. witli numerous potentialities, each combination 

 resulting in differences of form and constitution, so it seems 

 ])ossible that the parasitologist, after prolonged study of the 

 numerous environmental factors that may be at work, will arrive 

 at the condition of affairs where there is a universal trypanosome, 

 berpetomonad or other genus of parasites, modified according to 

 its environment and potentialities. 



