e facilities
of transportation offered by modern inventions. The subject has
therefore received the careful study of medical geographers and
statisticians.

I can give but a brief statement of their conclusions. They are to the
effect, first, that when the migration takes place along approximately
the same isothermal lines, the changes in the system are slight; but as
the mean annual temperature rises, the body becomes increasingly unable
to resist its deleterious action until a difference of 18 deg. F. is
reached, at which continued existence of the more northern races
becomes impossible. They suffer from a chemical change in the condition
of the blood cells, leading to anemia in the individual and to
extinction of the lineage in the third generation.

This is the general law of the relation to race and climate. Like most
laws it has its exceptions, depending on special conditions. A stock
which has long been accustomed to change of climate adapts itself to any
with greater facility. This explains the singular readiness of the Jews
to settle and flourish in all zones. For a similar reason a people who
at home are accustomed to a climate of wide and sudden changes, like
that of the eastern United States, supports others with less loss of
power than the average.

A locality may be extremely hot but unusually free from other malefic
influences, being dry with regular and moderate winds, and well drained,
such as certain areas between the Red Sea and the Nile, which are also
quite salubrious.

Finally, certain individuals and certain families, owing to some
fortunate power of resistance which we cannot explain, acclimate
successfully where their companions perish. Most of the instances of
alleged successful acclimatization of Europeans in the tropics are due
to such exceptions, the far greater number of the victims being left out
of the count.

If these alleged successful cases, or that of the Jews or Arabs, be
closely examined, it will almost surely be discovered that another
physiological element has been active in bringing about acclimatization,
and that is the mingling of blood with the native race. In the American
tropics the Spaniards have survived for four centuries; but how many of
the _Ladinos_ can truthfully claim an unmixed descent? In Guatemala, for
example, says a close observer, _not any_. The Jews of the Malabar coast
have actually become black, and so has also in Africa many an Arab
claiming direct d