8 president's address. 



reflected on to malarial fever of man, and this example should 

 serve to illustrate in a most striking way how pure scientific 

 investigations have led to far-reaching practical results. 



It having been proved that the flies and insects act as hosts 

 of the disease-causing germs, it was a natural sequence to draw 

 the conclusion that the destruction of the carriers would be a 

 means of clearing a country from disease. This has frequently 

 been called an impractical application of a theoretical deduction. 

 It has, however, been undertaken most seriously in many parts 

 of the world, and the Americans have given us the example 

 in Panama. You will remember that when the first attempt 

 was made to build that canal, the mortality from fever, particu- 

 larly yellow fever, another mosquito-transmitted disease, was 

 enormous. The destruction of all mosquito life has now 

 reduced the mortality to that of a European town. I regret 

 to say that in South Africa we have been somewhat slack in 

 making use of all this knowledge. Private enterprise in the 

 form of an Anti-malarial Association, with its headquarters in 

 Johannesburg, is now trying to disseminate modern knowledge 

 about the cause of malaria and its prevention, more particu- 

 larly for the benefit of those whom it affects most, and who. 

 in the ordinary course of things, would hardly hear about it. 

 and are unfortunately the people who least believe it. 



It remains now to show you that all this modern knowledge 

 and the discussions it led to can be usefully applied to some 

 of the problems which are in my particular line of research, 

 viz., to diseases of our domesticated animals. I shall mention 

 but two, known probably to you all, and winch are of great 

 economical importance — horse-sickness in equines. and blue- 

 tongue in sheep. Long before any expert came in contact with 

 him, the observant farmer quite rightly classed these two 

 diseases in one group. He even went so far as to say they were 

 identical, but here is an opinion which we are not able to 

 support. There are nevertheless more similarities than 

 differences in the two ; they resemble each other' in nature of the 

 cause, both being due to micro-organisms of infinitesimal 

 minuteness, so small that none of our modern microscopes can 

 detect them. The theory of our modern microscope teaches us 

 that there is a limit to visibility beyond which objects can no 

 longer be recognised. The so-called ultramicroscope. which 

 makes use of a different principle of illumination, and allows the 

 detection of objects varying in the magnitude of a molecule, 

 has in these two diseases failed to enable us to demonstrate 

 an organism so far. It must be there, nevertheless, and we 

 conclude this from the experiment that we are able to transmit 

 the disease by inoculation with blood from a sick to a healthy 

 animal, in which latter, after a definite incubation time, it 

 appears, thus shewing that a development must have followed. 

 Tt having been demonstrated that the malady was inoculable, it 

 formed the subject of much speculation to explain the observa- 



