ZOOLOGY AND MEDICINE. 441 



hend in an equally clear manner the laws of vaccination and of 

 immunit}', for which medicine has never been able to advance an 

 acceptable hypothesis. 



Such are the results of the johagocytic theory. We may search in 

 vain in any other branch of biological science for an example of so 

 profound a revolution in doctrine, based upon so slight a phenomenon. 



In 1901 our learned colleague. Prof. B. Grassi, showed to the Con- 

 gress of Berlin the part which mosquitoes play in the propagation of 

 malarial fever. I do not wish to take up again this subject, which 

 he has treated with so much authority, but it may be useful to dwell 

 for a moment upon these insects, which are even more dangerous than 

 his brilliant paper indicated. In fact, they not only propagate 

 malaria] fever over almost the entire surface of the globe, but they 

 are, in countries less vast but yet of great extent, the agents for the 

 dissemination of maladies that rank among the greatest scourges of 

 humanity. Throughout the whole of this Torrid Zone they inoculate 

 with the Fihiria sanguinis, a nematode that lives in the connective 

 ti&sue or in the circulatory apparatus. Its embryos are carried along 

 by the torrent of the blood. They have a relation to various patho- 

 logical conditions, such as the hsematuria of hot countries and the 

 elephantiasis of the Arabs. 



In a more restricted region mosquitoes inoculate with yellow fever, 

 whose domain, formerlj' limited to tropical America, now extends to 

 the western coast of Africa, sometimes touches Europe, and is perhaps 

 on the eve of reaching the extreme East as soon as the Panama Canal 

 is finished. Mosquitoes are not, as might be supposed, mere inert 

 transmitters of the known or unknown parasites that cause these dis- 

 orders; on the contrary, in their organism such parasites undergo 

 more or less complicated metamorphoses. 



One of the most urgent problems in the hygiene of hot countries is, 

 then, since these facts are known, the study of the mosquitoes found 

 in different parts of the globe. The exact knowledge of the fauna 

 of a country, from this s^jecial point of view, is, as may be seen, of 

 the greatest interest for public health, since the country that is the 

 object of such an investigation may be declared dangerous or healthy 

 according as the species recognized as pathogenic are found to be 

 present or absent. 



To tell the truth, it is not necessary that every physician should 

 I)e able to determine with complete scientific accuracy the different 

 species of mosquitoes that may be presented to him ; still less is it 

 necessary for him to recognize them in the different stages of Qgg. 

 larvae, and nymph, as well as in the adult form. Such determina- 

 tions can only be made by naturalists who have made a specialty of 

 this work ; and here we have the unusual phenomenon of the cabinet 

 entomologist, to whom we must have recourse for the determination 



