48 
sponding “ tonicity ” in tlieir organisation, so with man ; 
the denizen of equatorial regions is by a law of nature 
restricted to his geographical limits.* Are we, then, to view 
the prospect as a law of nature that tropical regions are 
destined to be for ever peopled by a human race whose 
physical and mental characteristics are still to be such as have 
been described ? That in those regions, amidst dense forest, 
dank, luxuriant, but unwholesome vegetation, amidst swamps, 
marshes, and lagoons, tenanted by fierce animals, hideous and 
fierce creeping things, tropical man must continue as he has 
heretofore been. According to my own view, the laws of 
climate determine that such must happen. 
29. The geographical distribution of disease realms is no 
less defined than that of other phenomena in Nature. 
Meteorological conditions are among the most important of 
the factors to be taken into account in determining growth, 
development, and health of man, as of other organised beings. 
But other concurrent circumstances also exert their influ- 
ence, favourably or unfavourably as the case may be. Among 
them latitude, local situation, nature and elevation of the soil, 
the presence or otherwise of rivers, lakes, swamps, forest or 
other vegetation, desert tracts, and so on — in fact, physical 
climate generally, together with habits of a people, their food 
in relation to produce of the land, their habits, and so on — all 
concur to stamp diseases among communities with a special 
character.-}* 
30. The tropical zone is bounded north and south by the 
mean annual isothermal line of 80 deg. F. The diseases 
which prevail in greatest constancy and frequency within 
* The following particulars are from the Army Medical Blue Book for 
1S79. They refer to sickness, mortality, and invaliding among white 
and black troops respectively in the West Indies, viz. The ratios are per 
1 , 000 . 
Admitted, white troops . . . 
... 641* 1 
black troops 
.1152- 8 
Died „ „ ... 
... 10-27 
» v 
. 17*57 
Invalided „ „ 
... 27T0 
?? 
. 27-23 
Constantly sick „ 
... 32*63 
JJ JJ 
. 54-90 
following are the averages for ten years prior to 1879 : — 
Admitted, white troops 91 1'0 black troops 
.1047-03 
Died „ „ ... 
... 11-47 
JJ J> 
, 19-42 
Invalided „ „ 
.. 19-32 
j5 „ say' 
27-23 
Constantly sick „ 
... 41-93 
» jj 
. 54-90 
t In the tables quoted the ratio invalided is not given ; that here entered 
is in accordance with what took place in 1879. The same reports show that 
1 These and some of the succeeding remarks are based upon the chapter, 
“Medical Geography,’ 7 in Dr. Aitken’s Science and Practice of Medicine. 
