280 Proceedings of Royal Society of Edinburgh. [sess. 
is now known to have visited Egypt, Senegambia, and Tripoli, and 
the valley of the Nile as far as Khartum ; also Arabia along the 
coasts, and Mecca. It is epidemic all over India and Further India, 
and in Batavia; and has appeared in Shanghai, Amoy, and the 
island of Formosa. It has also visited Keunion, Tahiti, Zanzibar, 
and the Canary Islands ; and epidemics have overspread the West 
Indies and a great part of the Southern States of the Union, as 
well as the northern shores of South America. Space w r ill not 
permit of indicating the various areas of diffusion during the 
several great epidemics of Dengue, but it must suffice to state that its 
greatest area of diffusion lies between 33° N. lat. and 23° 30' S. lat. 
Remarks . — The period of incubation of Dengue is probably 
about six days. It is a highly infectious disorder, spreading with 
extreme rapidity. Summer and early autumn are undoubtedly the 
Dengue season, and the disease appears to depend on a high tempera- 
ture for its production. In the tropics as well as in the more 
tropical zones, nearly all the epidemics of Dengue have been in the 
hot weather, and as soon as a great fall of temperature takes place 
the disease declines rapidly. It is probable that the moisture of 
the atmosphere has little or nothing to do with the production 
of Dengue ; as a rule it is chiefly confined to coast districts, to the 
courses of great rivers, and to places having a low altitude. It has 
been noticed in various epidemics of Dengue that it spreads in a 
curious way amongst various classes of the community. Every 
race, nationality, age, and sex may be attacked by the disease, 
although in separate epidemics a remarkable immunity has been 
noticed on the part of certain classes. Sometimes Europeans will 
be attacked, and natives enjoy comparative freedom from the 
disease; again, in other cases, natives will be almost solely attacked; 
sometimes children suffer more than adults, or the reverse may 
obtain. For instance, Pasque says, speaking of the epidemic at 
Benghazi, that it was noticeable for the decided immunity 
experienced by the blacks, but they are attacked as much as anyone 
else in Egypt and Senegal. Christie remarks that in one or two of 
the epidemics which he witnessed in Zanzibar, the natives suffered 
less than the Europeans. In the epidemic in Mauritius in 1873, 
hardly any children were attacked by the disease. In the Deccan 
epidemic of 1872 there was noticed a peculiar predisposition to 
