INFLUENCE OF CLniATE ON ANIMAL IHSKASi:. 489 



and from the traveller Burchell. who visited this country abov.t 

 that year. Writing at that time from the Roi^seveld Mountains. 

 he says : — 



Tlie Hantclicrg district. Ix-ini; in tlie dircctinn of iKirth iKirth west 

 from tlie Ro,t>Keveld mountains, is said to be very higli land and remark- 

 able for bein.ti' 1 nc of a few situations in this part of the countr\- where 

 horses arc not liable to tb.e horse-sickness which rases during the sunnner 

 season, and annnall.\' carries off oreat numliers. 



Sjieakinjj' of Klaarwater. lie stated: 



-\ftcr the months of Octolier no frost is expected for seven months, 

 but in tlic mornins of .May it is always found to return, and is tlie signal 

 for the return of the I'oer's horses from the Ro.nye\eId. whither they are 

 sent in January to avoid the Paardczicktc. a fatal distemper to which they 

 are liable in the hotter months. Those who object to sending their 

 horses such a long distance from tiie settlement are content to run the 

 risk of keeping them in the Landsberg. an elevated mountainous country 

 lying in north north west direction. This, liowever. not being so cold 

 as the Ro.ggeveld, is less safely to lie depended upon. It does not seem 

 that the distemjier acquires its fidl force until the beginning of Feliruary. 

 but after then the lower districts of the whole of the extra tropical parts 

 of South Africa are as far as my experience enables me to speak. 

 subject to its baneful eft'ects. Experience has shown that the hrst frost. 

 whene\er it happens, fortunately puts a stop to its further ravages. 



Tints wrote Bitrche-11 over a hntidred )ears ago, ])aying a 

 high tri])nte to the observation of the Dutch farmer even at that 

 time. 



It has, moreover, been observed that a certain amount of 

 safety is conferred on horses in bad districts by stabling them 

 during the night, and from shortly before stinset until some time 

 after sttm^ise. 



If we V. ere now to consider in detail the di.sease known as 

 l)lue tongue, we could bring out exacth' the same jioints as we 

 ha\e noted in regard to horse-sickness. It also is a disease of 

 the wet sttnuner season and of low-h'ing places near the vicinity 

 of water. It is rarely met Avith after the first frost has a])iieared 

 until the next summer sets in again, is rare in shee]) stabled in 

 closed sheds between siuiset and sunrise, and it is a disease 

 especially |)revalent in exceptionally wet seasons. The similarity 

 between the conditions under which the diseases occin-red is. 

 indeed, so striking that the Boers observing it, and also noting 

 that in certain cases of horse-sickness the tongue assumed a 

 slightl}- bhtish colour, came to the conclusion that both diseases 

 were really due to one catisal agent acting on two different 

 species of animals. The latter conclusion has, however, since 

 been shown to be wrong b\- ])roving that each of the two diseases 

 is due to a distinct and separate filterable virus. 



Here, then, we have two diseases, influencing the distribu- 

 tion and prevalence of which the climatic and tellurical factors 

 are seen to be of extreme imj^ortance ; hence it is of especial 

 interest to see if we can explain the reasons for this influence. 

 Such an explanation, however, cannot be found in exactly the 

 same way as we have seen in the cases of the other diseases 

 hitherto considered. In those cases we were dealing with condi- 



