clinical
experience speaks in general against Loewit's theory. For in infectious
diseases hyperleucocytosis is very common; and a transient leukopenia is
equally rare.

This contradiction to the experimental results obtained by Loewit is
easily explained when one reflects how different from the natural
processes of disease are the circumstances of experiment. In this case
the animal is by intravenous injection flooded at once with the morbid
substance, and a violent acute reaction of the vascular and blood
systems is the natural consequence. In natural infection, insidious and
increasing amounts of poison come quite gradually into play, and for
this reason, perhaps, hypoleucocytosis in the normal course of
infectious diseases is much rarer than in the brusque conditions of
experiment.

Upon the clinical importance of leucocytosis, particularly for the
infectious diseases and their various stages, an enormous mass of
observations has accumulated. Selecting pneumonia as the best studied
example, in the typical course of this disease the constant occurrence
of leucocytosis is undisputed; the increase usually continues up to the
crisis, and then gives place to a diminution of the leucocytes until a
subnormal number is reached.

Of special importance are the observations on an absence of leucocytosis
in particularly severe or lethally ending cases (Kikodse, Sadler, v.
Jaksch, Tschistowitch, Tuerk and others).

In many other diseases as well, the observation has been made that
hyperleucocytosis as a rule is only absent in specially severe, or in
some way atypical cases. Several observers (Loewy and Richter, M. Hahn,
Jacob), have been able to demonstrate experimentally for various
infections, that artificial hyperleucocytosis influences the course of
an artificial infection most favourably. The question, in what way does
this process contribute to the protection of the body, is at the present
time under discussion, and introduces the most difficult problems of
biology.

* * * * *

The ~morphological character of leucocytosis~ is certainly not simple, and
we must sharply separate various groups, according to the kind of
leucocyte increased.

The most important consideration is, whether cells capable of
spontaneous movement, and of active emigration into the blood, are
increased ("~active leucocytosis~"); or whether the number of those cells
is raised, to which an independent mobility can