on into the blood! The mischief is spread all over the
body at once.

It is not surprising, then, that the long course of natural selection
and survival of the fittest has resulted in the fixing in the blood
and the living cells immediately connected with it of extraordinary
protective powers. The floating scavenger cells (eater-cells or
phagocytes, first recognised as such and so named by Metchnikoff) are
already found in the blood of quite simple animals in worms,
shell-fish and insects. I have watched them with the microscope at
work in transparent minute living water-fleas eating up, and digesting
microbes which had got into the water-flea's blood. In higher animals
what we call "inflammation" is a condition--the result of a new and
advantageous mechanism--which consists in a local retarding of the
blood-current, effected by the action of the nerves on the muscular
walls of the blood-vessels, and the consequent escape of the
eater-cells into the injured or infected tissue, there to eat up and
destroy the injurious microbes or other particles. Special and
remarkable properties--chemical activities of an extraordinary
character--have been gradually developed in the floating phagocytes
and in similar non-floating fixed cells over which the blood flows.

These special chemical activities are of several distinct kinds. The
first is the power to convert the poison of a microbe into a destroyer
of that poison--toxin into antitoxin. The atoms of these poisons are
elaborately composed combinations of the organic elements. By a
"shake" or a "twist" (so to speak) administered by the living cells of
the blood the combination is altered, and the toxin becomes an
antitoxin, destroying by chemically combining with it the very toxin
from which it was formed. This is a far more efficacious method than
the supposed mithridatic "habituation" or "toleration" of a poison,
with small doses of which you have to be gradually prepared. The
healthy blood converts any one of a large series of microbe poisons
into antitoxins. It is true that apparent "opposites" are often
closely allied in Nature. Evil smells and tastes are closely allied to
sweet perfumes and flavours, and what is healthy and agreeable to some
men acts as virulent poison to others (_e.g._ shell-fish, egg,
quinine, opium). The smallest change in the substance administered or
the smallest difference in the living substance of an individual (what
is called "idiosyncrasy") make