Thus far we have considered the behavior of the individual as a whole
in his response to a certain type of noci-influences. We have been voicing
our argument in terms of physical escape from GROSS physical dangers,
or of grappling with GROSS NERVE-MUSCULAR enemies of the same or of
other species. To explain these phenomena we have invoked the aid
of the laws of natural selection and phylogenetic association.
If our conclusions be correct, then it should follow that in the same
laws we may find the explanation of immunity, which, of course,
means a defensive response to our MICROSCOPIC enemies. There should
be no more difficulty in evolving an efficient army of phagocytes
by natural selection, or in developing specific chemical reactions
against _*microscopic enemies_, than there was in evolving the various
nociceptors for our nerve-muscular defense against our _*gross enemies_.
That immunity is a chemical reaction is no argument against
the application of the law of natural selection or of association.
What essential difference is there between the chemical defense of
the skunk against its NERVE-MUSCULAR enemies and its chemical defense
(immunity) against its MICROSCOPIC ENEMIES?
The administration of vaccines becomes the adequate stimulus which
awakens phylogenetic association of a chemical nature as a result
of which immune bodies are produced.
In discussing this subject I will raise only the question whether
or not the specific character of the inaugural symptoms of some
infectious diseases may be due to phylogenetic association.
These inaugural symptoms are measurably a recapitulation of the leading
phenomena of the disease in its completed clinical picture.
Thus, the furious initiative symptoms of pneumonia, of peritonitis,
or erysipelas, of the exanthemata, are exaggerations of phenomena
which are analogous to the phenomena accompanying physical injury
and fear of physical violence. Just as the acute phenomena of fear,
or those which accompany the adequate stimulation of nociceptors,
are recapitulations of phylogenetic struggles, so may the inaugural
symptoms of an infection be a similar phylogenetic recapitulation
of the course of the disease. A certain amount of negative
evidence is supplied by a comparison of the response to a dose
of toxins with the response to a dose of a standard drug.
No drug in therapeutic dosage except the iodin compounds causes
a febrile response; no drug causes a chill; on the other hand,
all specific toxins cause febrile responses and many cause chills.
If a species of animal had been poisoned by a drug during vast periods
of time, and if natural selection had successfully established
a self-defensive response, then the administration of that drug would
cause a noci-association (chemical), and a specific reaction analogous
to that following the administration of Coley's toxins might be expected.
Bacterial noci-association probably operates through the same
law as that through which physical noci-association operates.
Natural selection is impartial, however. It must be supposed that it
acts impartially upon the microscopic invader and upon the host.
On this ground one must infer that, in accordance with the same law
of natural selection, the bacteria of acute infections have met
by natural selection each advance in the struggle of the host
for immunity. Hence the fast and furious struggle between man
and his microscopic enemies merely indicates to what extent natural
selection has developed the ATTACK and the DEFENSE respectively.
This struggle is analogous to the quick and decisive battles
of the carnivora when fighting among themselves or when contending
against their ancient enemies. But when phylogenetically strange
animals meet each other, they do not understand how to conduct a fight:
natural selection has not had the opportunity of teaching them.
The acute infections have the characteristics of being ancient enemies.
On this hypothesis one can understand the high mortality of measles
when it is introduced into a new country. By natural selection,
measles has become a powerful enemy of the human race, and a race
to which this infection is newly introduced has not had the advantage
of building up a defense against it by the law of natural selection.
May not the phenomena of anaphylaxis be studied on associational lines?
Then, too, there may be chemical noci-associations with enemies
now extinct, which, like the ticklish points, may still be active
on adequate stimulation. This brief reference to the possible
relation of the phenomena of the acute infections to the laws
of natural selection and of specific chemical noci-association has
been made as a suggestion. Since the doctrine of evolution explains
all or nothing, I have included many phenomena to see how reasonable
or unreasonable such an explanation might be.