12 THE president's ADDRESS. 



alternation of hosts and a complicated life-cycle, which has been 

 most completely studied in the case of the malarial parasites of 

 man, transmitted by the agency of mosquitoes. 



6. In some cases the parasite may penetrate the ovary of its 

 host, pass into the ova, and thus infect the embryo and the next 

 veneration of the host. Transmission of this kind is known in a 

 certain number of cases : for instance, in the "pebrine" disease of 

 silkworms caused by Xosema bombi/cis. It is never the sole method 

 of transmission, but is always supplementary to other methods. 



To turn now to the methods by which Protozoan parasites 

 penetrate into new hosts. We find four known methods, which 

 can be summarised very briefly after what has been stated 

 already. The commonest is the method of casual or contamina- 

 tive infection, where the host infects itself by taking vip the 

 propagative phases of the parasite accidentally from its surround- 

 ings. Most usually infection takes place by way of the mouth, 

 with the food ; but it may be effected by the respiratory organs. 

 Other modes of infection are the contagious, as in dourine already 

 mentioned ; the inoculative, as in malaria and other diseases 

 caused by blood-parasites ; and the so-called hereditary method, as 

 in Xosema homhycis and other cases. 



From the foregoing summary of the methods by which Protozoan 

 parasites are propagated and spread from one host to another, it 

 is clearly seen that there are only very few cases in which it is 

 of direct advantage to the parasite to cause the death of the host, 

 and even then only when the propagative phases of the parasit 

 are fully matured. In cases where the parasite is disseminated 

 by its host being devoured by another animal, it is necessary for 

 the propagation of the parasite that the host should die a violent 

 death, but not through the agency of the parasite; on the con- 

 trary, the interests of the parasite are best served by the host 

 remaining in good health until the fatal moment arrives, unless 

 it be supposed that some weakening of the host renders it more 

 liable to become the prey of its enemies. Blood-parasites, trans- 

 mitted by the inoculative method, are not perceptibly benefited by 

 the host's illness, and certainly not by its death. It is, however, 

 very necessary for this method of dissemination that the required 



