ng foes.

The three divisions of the struggle for existence are so important not
only in purely scientific respects, but also in connection with the
analysis of human biology, that we may look a little further into their
details, taking them up in the reverse order. Regarding the environmental
influences, the way that unfavorable surroundings decimate the numbers of
the plants of any one generation has already been noted, and it is typical
of the vital situation everywhere. English sparrows are killed by
prolonged cold and snow as surely as by the hawk. The pond in which
bacteria and protozoa are living may dry up, and these organisms may be
killed by the billion. Even the human species cannot be regarded as exempt
from the necessity of carrying on this kind of natural strife, for scores
and hundreds die every year from freezing and sunstroke and the thirsts of
the desert. Unknown thousands perish at sea from storm and shipwreck,
while the recorded casualties from earthquakes and volcanic eruptions and
tidal waves have numbered nearly one hundred and fifty thousand in the
past twenty-eight years. The effects of inorganic influences upon all
forms of organic life must not be underestimated in view of such facts as
these.

In the second place, the vital struggle includes the battles of every
species with other kinds of living things whose interests are in
opposition. The relations of protozoa and bacteria, conger-eels and other
fish, English sparrows and hawks, plants and herbivorous animals, are
typical examples of the universal conflict in which all organisms are
involved in some way. Again it is only too evident that human beings must
participate every day in some form of warfare with other species. In order
that food may be provided for mankind the lives of countless wild
organisms must be sacrificed in addition to the great numbers of
domesticated animals reared by man only that they may be destroyed. The
wolf and the wildcat and the panther have disappeared from many of our
Eastern states where they formerly lived, while no longer do vast herds of
bison and wild horses roam the Western prairies. Because one or another
human interest was incompatible with the welfare of these animals they
have been driven out by the stronger invaders.

That the victory does not always fall to the human contestant is
tragically demonstrated by the effects of the incessant assaults upon man
made by just one kind of living enemy,--the