1900.] on Malaria and Mosquitoes 3 



been generally endorsed by innumerable observations made since their 

 time by tbe general experience of mankind, by statistics, and by 

 the fact that malaria can often be actually banished by means of 

 drainage of the soil. But Laveran had uo\v shown the disease to be 

 due to a parasite of the blood. How reconcile these facts ? There 

 appeared to be but one way of doing so namely, by supposing that 

 the organism lives a free life in the water or soil of malarious places, 

 from which it enters man by the respiratory or digestive tracts. To 

 prove this it was necessary to discover it in the water or soil of 

 malarious places. But how make this discovery? The organism 

 is not a bacterium, but an animal parasite. It cannot be taken from 

 the living blood and sown on the surface of a gelatine film. Experi- 

 ments have proved that it can be inoculated from man to man by the 

 intravenous injection of fresh infected blood ; but this is a very 

 different thing to cultivating it in an artificial medium. At all 

 events, experiments in this line have always failed and are not in 

 the least likely to succeed. Tbe parasites simply perish when taken 

 from their natural habitation, the blood. It was therefore extremely 

 unlikely that we should ever be able to follow up their life-history 

 by this means which has proved so successful as regards the bacteria. 

 It remained only to find them in the soil or water by direct search. 

 But how identify them among the host of Protozoa which live in 

 these elements? Certainly not by their form or appearance. As 

 known to us at that time, they were simply minute amoebae ensconced 

 in the red corpuscles and accurately adapted for such a life. Now 

 red corpuscles do not exist in soil and water ; if the parasites live in 

 the latter, they must possess some other form to that which they 

 possess in the blood, and the clue afforded by identity of appearance 

 fails us. The only remaining method open to us would have been 

 to attempt to produce infection by each one in turn of the numerous 

 species of Protozoa found in the water and soil of malarious places 

 a task of great magnitude, and one which we now know would have 

 failed. Indeed, it was actually attempted by several observers, and 

 actually did fail. 



Such was the state of things up to the end of the year 1894. 

 Speaking for myself, I can well remember the hopeless feelings with 

 which 1 then regarded the problem. Fortune, however, was to be 

 kinder to us than I had dared believe. At this very moment the key 

 to the solution of the problem had already been indicated by 

 Dr. Patrick Hanson. 



I have said that since the original discovery of Ray Lankestcr 

 numerous haematozoa or rather hrcmocytozoa have been found in 

 man and various animals. All these are generally classed by zoolo- 

 gists in Leuckart's order of the Sporozoa, and are usually divided 

 into three groups groups which are not very closely related, except 

 for the fact that all the organisms concerned are parasites of the red 

 corpuscles of the blood. One group found in reptiles consists of 

 parasites closely allied to the G-regarinidae, another is found iu oxen, 



