xceedingly severe. In like manner, influenza
will slay its hundreds in a tribe of less than a thousand members.
Chicken-pox will become so virulent as to be mistaken for smallpox.
Several of the epidemics of alleged smallpox that have occurred among
Indians and other savage tribes are now known to have been only measles.
At first, pathologists were inclined to receive these reports with some
degree of skepticism, and to regard them either as travelers' tales, or
as instances of exceptional and accidental virulence in that particular
tribe, the high death-rate due to bad nursing or horrible methods of
voodoo treatment.

But from all over the world came ringing in the same story, not merely
from scores of travelers, but also from army surgeons, medical
missionaries, and medical explorers, until it has now become a
definitely established fact that the mild, trifling diseases of infancy,
"colds" and influenzas of civilized races, leap to the proportions of a
deadly pestilence when communicated to a savage tribe. Whether that
tribe be the Eskimo of the Northern ice-sheet or the Terra del Fuegian
of the Southern, the Hawaiian of the islands of the Pacific or the
Aymaras of the Amazon, all fall like grain before the scythe under the
attack of a malady which is little more than the proverbial "little
'oliday" of three days in bed to civilized man. Evidently civilized man
has acquired a degree and kind of immunity that uncivilized man has not.
Either the disease has grown milder or civilized man tougher with the
ages.

The probability is that both of these explanations are true. These
diseases may originally have been comparatively severe and serious; but
as generation after generation has been submitted to their attack, those
who were most susceptible died or were so crippled as to be seriously
handicapped in the race of life and have left fewer and less vigorous
offspring. So that, by a gradual process of weeding out the more
susceptible, the more resisting survived and became the resistant
civilized races of to-day.

On the other hand, any disease which kills its victim so quickly that it
has not time to make sure of its transmission to another one before his
death, will not have so many chances of survival as will a milder and
more chronic disorder. Hence, the milder and less fatal strains of germs
would stand the better chance of survival. This, of course, is a very
crude outline, but it probably represents something of