313 
1888 - 89 .] Dr R. W. Felkin on Tropical Diseases. 
of the various diseases dealt with. Although it must he allowed 
that such factors as heat, moisture, the character of the soil, and so 
forth, do exert a great effect on the production of disease, it is most 
difficult to estimate justly or with accuracy the parts they severally 
play in the etiology of disease. A wide field for further research 
presents itself in this direction, and one of my objects in compiling 
this paper has been to draw attention to a subject which deserves 
close and extended investigation. 
In looking at the maps which I have drawn, it will be seen, on 
comparing them with Plate XIV., that all the diseases depicted, with 
the exception of' scurvy, are endemic in various areas where the 
highest mean annual temperature of the globe is to be found, and 
it will also be noticed that they are most prevalent in regions 
having a rainfall of over 50 inches in the year. 
In investigating the etiology of tropical diseases, certain other 
factors have to be taken into account, factors which I have been 
obliged almost entirely to exclude from this paper, as, had I dealt 
with them, it would have involved considerable space and a much 
larger number of maps. The factors I refer to are the races 
inhabiting tropical regions, and their habits and customs, the 
geology and physical geography of the countries, and the character 
of the vegetation met with. The regions where the diseases I have 
included in my list are endemic are characterised by distinctive 
features with regard to food supply. They are chiefly, although not 
entirely, within the zone where a vegetable diet obtains, and where 
tropical grains and fruits are indigenous. 
In conclusion, I should like to mention Dr Lawson’s theory of 
pandemic w r aves of disease coinciding with the isoclinal lines which 
are found depicted upon Plate XIV. For instance, in regard to 
fever, he believes there is a factor which determines its appearance 
at points more and more northward in successive years, and he 
apparently proves that this factor, whatever it may be, revives peri- 
odically every second year, or at some multiple of two years, and then 
passes over a more or less extensive portion of the earth’s surface, 
giving epidemic impulse on its way to various diseases, and finally 
disappearing in the north. He found that the position of these 
pandemic waves, as he calls them, could be defined from year to 
year, approximately at least by referring them to lines of equal 
