remarks[671] that white men "born in the torrid zone walk barefoot
with impunity in the same apartment where a European, recently landed, is
exposed to the attacks of the _Pulex penetrans_." This insect, the too
well-known chigoe, must therefore be able to distinguish what the most
delicate chemical analysis fails to distinguish, namely, a difference
between the blood or tissues of a European and those of a white man born in
the country. But the discernment of the chigoe is not so surprising as it
at first appears; for {276} according to Liebig[672] the blood of men with
different complexions, though inhabiting the same country, emits a
different odour.

Diseases peculiar to certain localities, heights, or climates, may be
here briefly noticed, as showing the influence of external
circumstances on the human body. Diseases confined to certain races of
man do not concern us, for the constitution of the race may play the
more important part, and this may have been determined by unknown
causes. The Plica Polonica stands, in this respect, in a nearly
intermediate position; for it rarely affects Germans, who inhabit the
neighbourhood of the Vistula, where so many Poles are grievously
affected; and on the other hand, it does not affect Russians, who are
said to belong to the same original stock with the Poles.[673] The
elevation of a district often governs the appearance of diseases; in
Mexico the yellow fever does not extend above 924 metres; and in Peru,
people are affected with the _verugas_ only between 600 and 1600 metres
above the sea; many other such cases could be given. A peculiar
cutaneous complaint, called the _Bouton d'Alep_, affects in Aleppo and
some neighbouring districts almost every native infant, and some few
strangers; and it seems fairly well established that this singular
complaint depends on drinking certain waters. In the healthy little
island of St. Helena the scarlet-fever is dreaded like the Plague;
analogous facts have been observed in Chili and Mexico.[674] Even in
the different departments of France it is found that the various
infirmities which render the conscript unfit for serving in the army,
prevail with remarkable inequality, revealing, as Boudin observes, that
many of them are endemic, which otherwise would never have been
suspected.[675] Any one who will study the distribution of disease will