president's address. 9 



tions which the framers had been collecting ever since they knew 

 it, ami which principally apply to the climatical and tellnrical 

 conditions under which it appears. You have probably all heard 

 that the tanner interpreted his observations to the effect that 

 the dew is the cause. There is nothing ridiculous in this 

 theory. Remember that our knowledge of micro-organisms as 

 causes of disease is practically only a science of yesterday, 

 remember that the English translation of the name " malaria " 

 for the disease of that name means " bad air," and it is only a 

 few years back that science admitted of such a theory a> the 

 probable cause; that is just as our farmers have done and are 

 still doing for horse-sickness. The observations of the farmer 

 are correct in details. We give them the right interpretation 

 when we substitute for the name " dew " the name " blood- 

 sucking night insect." Under the conditions under which dew- 

 is formed horse-sickness and blue-tongue appear most frequently, 

 and these conditions are most favourable for the breeding of 

 mosquitoes and other blood-sucking insects. This being so, 

 the question might be put to us, " But are there any direct 

 proofs to this effect?'' If we had all the proofs, we would no 

 longer speak of a theory, and we must speak of a theory until 

 the actual blood-sucking insect has been demonstrated and 

 until the experiments have been made under such con- 

 ditions that no doubts are left any longer. Indirectly, the 

 theory has been so well founded that the only missing link is 

 the insect itself. The reason why this link has not been demon- 

 strated yet is the fact that we do not know sufficient of all the 

 nocturnal blood-sucking insects of South Africa, of which 

 various genera and many species exist; we do not yet know how- 

 to breed and handle them for such delicate experiments as are 

 required to bring the proofs with horse-sickness and blue- 

 tongue. Notwithstanding this, the theory has its practical 

 value, inasmuch as it shows in which way protective measures 

 can be adopted, and what has been said about the destruction 

 of mosquitoes in connection with human malaria applies equally 

 well to the diseases under discussion. The theory goes still 

 further. Seeing that flying insects must be accepted as being 

 the transmitting agencies, we conclude that there also must be 

 a reservoir somewhere from which these insects obtain the virus 

 This is perhaps the most interesting point. The horse alone 

 in the case of horse-sickness, and the sheep in the case of blue- 

 tongue, is not sufficient to represent that reservoir. When 

 recovered, the blood of these animals no longer contains any 

 virus. Furthermore, horeses, when introduced into a wild 

 country where before there had never been any equines. are 

 liable to contract the disease. Again, the almost " explosion- 

 like " expansion when climatical conditions are suitable do not 

 allow us to conclude that the sick animal alone is responsible, 

 and we naturally ask, " Where does the virus come from ?" 

 By analogy with tsetse and human malaria we accept the exis- 



