The Keys of Death. 
215 
1879.] 
beings indifferent to their adtion may gradually be formed 
by a process of natural sele(5tion. Scarcely any other hy- 
pothesis will explain the fadts that certain races of men 
resist disease-poisons to which others succumb, and that an 
epidemic on its first visit to any country commits such 
frightful ravages. A striking instance of this was shown by 
the late spread of the measles in the Fiji Islands. It is 
exceedingly probable that the resistance of the slug to 
digitalin and atropin may have been produced in the same 
manner as the negro’s non-susceptibility to yellow fever. 
Indeed one of the most curious fadts connected with poisons 
is that individual men, and doubtless animals, by beginning 
with small doses, can gradually habituate themselves even 
to the most deadly compounds. The legend of Mithridates 
contains a basis of solid truth. Without depending upon 
disputed cases, such as that of the arsenic-eaters of Styria, 
the history of opium-eating gives abundant evidence of this 
acquired immunity. We at one time knew a gentleman, 
connected with the Inland Revenue in a town of the West 
Riding, who was no more affedted by a wine-glassful of 
laudanum than is an ordinary man by the same quantity of 
sherry. Very similar is the case with tobacco, which, though 
it produces severe constitutional disturbance in persons un- 
accustomed to its use, is afterwards borne without the 
slightest inconvenience. The question may, indeed, be 
raised whether a similar immunity from snake-poison might 
not be acquired by habituating the system to doses at first 
infinitesimal, and gradually becoming larger and larger. 
Our knowledge concerning the physiological adtion of such 
poisons is as yet exceedingly rudimentary, and it is quite 
possible that there might be some serious objection to such 
inoculation. Still we should recommend this idea to the 
experimental notice of the medical profession in countries 
such as India and Australia, where venomous snakes abound, 
and where biological research has not yet been fettered at 
the bidding of a hysterical humanitarianism. 
We mentioned above that on the entrance into the system 
of a true poison morbid phenomena at once make their 
appearance. To this rule there is an exception, i.e ., when 
the poison is administered in successive doses too small to 
produce the usual alarming symptoms, but too large or too 
frequent to allow the system to become habituated to their 
effects. Certain poisons indeed, such as lead, accumulate 
in the system, whilst others — colchicin being perhaps the 
best example — are cumulative in their adtion. We have 
