THE PRESIDENT'S ADDRESS. 213 



whole existence in one host. Kossmann attempted what may be 

 called a physiological classification, viz. : (1) Dios?notici, or vege- 

 tative, without independent digestive organs ; (2) Digestorii, with 

 independent digestive organs, and these latter are divided into 

 Sedentarii and Vagantes. Mr. Geddes classifies according to the 

 nature and degree of the parasitism into commensualists ; parasites 

 within the same species, and hyperparasites. Megnin divides para- 

 sites into two great classes, viz., harmless and hurtful ; and, indeed, 

 no better summary of the different views can be given than the 

 well-known quotation from Terence, Quot homines tot sententice. 



I will now refer to a few of the varied and curious forms which 

 parasitism assumes. 



Amongst the order of beings to which I have paid the most 

 special attention, namely the Acarina, there are numerous examples 

 of almost every kind. Unquestionably the best known is the itch- 

 mite (Sarcoptes Scabiei), which has brought itself very un- 

 pleasantly under the notice of the human race. It is a true 

 parasite, living its whole life in or upon one individual, and feeding 

 upon the substance of its host. The literature relative to this tiny 

 creature is so voluminous that Fiirstenberg's summary of it occupies 

 172 closely-printed folio pages of his book, and has rightly been 

 called a marvel of human labour ; and yet Fiirstenberg wrote in 

 1861 and the literature has been rapidly increasing ever since. It 

 is curious that in his magnificent work Fiirstenberg carefully draws 

 every species of itch-mite as having two pairs of chelate mandibles, 

 a thing not only incorrect in itself, but also entirely unknown in the 

 whole order of Acarina. I do not propose to add to the mass of 

 printed matter on this not very attractive subject, and will simply 

 remind you that it is an error to suppose that cleanliness is an 

 entire protection against this unwelcome visitor. It doubtless 

 assists considerably ; but although the adult, egg-bearing female 

 lives in burrows in the skin and is probably rarely transferred, yet 

 the immature forms and the young adults live on the surface, and 

 being minute, hyaline creatures, not visible to the naked eye, may 

 be passed from individual to individual by the morning newspaper, 

 the handle of a door, the clothes from the wash, or in a thousand 

 other fashions. Luckily, now that the disease is understood, a cure 

 can be effected with absolute certainty ; but amongst animals, 

 which cannot rub each other with sulphur and the like, the result 

 is different, and the largest and fiercest carnivora are sometimes 



