THE president's ADDEESS. 



that we find the greatest complication in the structure and 

 mechanisms of the generative organs, the most extraordinary 

 and delicately-adjusted adaptations of the propagative phases, 

 and the most astonishing fertility. 



So long as the parasite has not made the necessary provision 

 for propagating its kind and disseminating its progeny, it is 

 against its best interests to kill or greatly weaken its host. The 

 interests of the two are bound up together, and the death of the 

 host, before the parasite has completed its reproductive arrange- 

 ments, necessarily entails the extinction of the parasite. The 

 ideal host, from the point of view of the parasite, is, one that is 

 " tolerant," that is to say, one that can support the presence of 

 the parasite and keep it supplied with the nutriment it requires^ 

 like a good mother or nnrse, without suffering in health and 

 vigour to any marked extent. When once, however, the parasite 

 has matured and produced its reproductive phases, the life or 

 death of the host may be a matter of indifference to it. In some 

 cases the death of the host may even be necessary for the dis- 

 semination of the parasite. 



From these general considerations it is seen that reproduction, 

 and above all the dissemination of the progeny in a manner 

 adapted to the special circumstances of its mode of life, are by 

 far the most important functions of a parasite. Turning now to 

 the special consideration of Protozoan parasites, we find that 

 diverse conditions of their mode of life and habitat impose a 

 corresponding diversity in their reproductive phases and their 

 modes of dissemination. 



The passage of a parasite from one host to another includes 

 two manoeuvres, so to speak : the passing out from the first 

 host, and the passing into the second. Primitively it may be 

 supposed that this migration was effected simply by the unaided 

 efforts of the parasite itself, that is to say, that the active motile 

 parasite would force its way out of one host, move freely in the 

 surrounding medium, and sooner or later attack and penetrate 

 a second host. This primitive method of transference doubtless 

 occurs in many cases, especially amongst epizoic forms. In the 

 case of entozoic forms its occurrence is more doubtful. It is- 



