THE president's ADDRESS. 



difficult to detect.* These facts strongly suggest, as has been 

 pointed out by medical authorities, that the parasites produce 

 a specific toxin. Only in a single case, however, has it been 

 claimed that a toxin has been isolated from a Protozoan parasite, 

 namely the " sarcocystine " produced by the parasites of the 

 genus Sarcocystis (Sarcosporidia).t The subject is one which 

 remains to be investigated, and in the present state of knowledge 

 it would be unprofitable to discuss it further here. 



The interaction of host and parasite is a question of great 

 practical importance, which in consequence has been studied 

 chiefly from the medical standpoint, that is to say, from the point 

 of view of the host. I propose here, as I have already said, 

 to deal with the question as a naturalist, and to discuss the 

 problem rather from the point of view of the parasite ; and wntli 

 this object I shall begin by a brief consideration of the facts of 

 parasitism in general. 



A parasite is, like any other living being, an organism strug- 

 gling for existence in a hard and cruel world. The difficulties, 

 however, against which a parasite has to contend, in the struggle 

 for existence, are different in many respects from those \vhich 

 beset a free-living organism. When once a parasite has obtained 

 a footing in its proper host, the question of food-supply is solved 

 for it, since it finds itself lodged in the midst of abundant nutri- 

 tion so long as its host lives. On the other hand, if the species 

 is to be maintained, it is essential that the offspring of the 

 parasite should be able to infect new hosts a matter usually 

 of great difficulty, and one in which the chances are all against 

 the parasite in most cases. To ensure dissemination of the 

 species a large number of offspring must be produced, and special 

 adaptations and mechanisms may be necessary. Hence in para- 

 sites, the more they become specialised and adapted to parasitic 

 Hfe, the more the organs and functions of nutrition tend to 

 become simplified, and the greater the tendency to elaboration 

 of the reproductive system. It is esjDecially amongst parasites 



* Compare Laveran and Mesnil, " Trypanosomes and Trypanosomiases," 

 translated by Nabarro (Bailliere, Tindal & Cox, 1907), pp. 146-150. 

 t Comjjtes Rendus Soc. Biol. Paris LI. (1899), pp. 311-14. 



