57
Para. 31
and by æstivo-autumnal parasites, I never once detected a single quartan para-
site in a case of fever contracted in that spot. Nevertheless, it was easy to
obtain examples of these parasites simply by going to the next regiments
situated a mile or two away in the cantonment—at the same season and in the
same climate, be it noted. Again, at Bangalore, from October 1895 to March
1897, I observed only the tertian and æstivo-parasites again. Not a single case
of quartan was found in the station itself ; and, moreover, all the cases of tertian
came from one limited part of the town. It is impossible to explain these
facts—of which I am quite sure—on any theory attributing the morphological
differences to climatic ones. In Madras, according to Sturmer [35], quartan
parasites abound. In the Sigur Ghât I found only quartan and æstivo-
autumnal ; and the same precisely at Kherwara, Rajputana. Hence the preval-
ence of the different varieties must vary according to locality—even when the
localities are very contiguous.
In my experience, moreover, two forms are so rarely found together in the
same patient, especially in recent infections, and so seldom succeed one another
in him, that when such things occur they are much more reasonably explained
by the hypothesis of mixed infections than by that of climatic influence. It is
difficult to see how external climatic changes can alter the morphological charac-
ters of the parasites more than those of the red and white cells of the blood ;
while it is a pretty general biological law that morphological differences imply
specific ones. It is frequently argued that the types of fever said by Golgi and
Marchiafava to be associated with the types of the parasites are not rigidly
observed in connection with them. All I can say is that I have generally
found this association in recent infections ; but that the study requires great
thermometric and microscopical care—which is not always given—in order to
avoid errors due to the presence, appearance or disappearance of parallel genera-
tions, to the use of quinine, and, possibly, to the existence of secondary
fevers.
On the whole, I have personally felt it necessary to abandon all doubt
regarding the existence of plurality of species. Further than this, as I stated
long ago [23], I find in India exactly the same specific characteristics in the
parasites as the Italians and others have done elsewhere. Murray, in his
admirable little monograph [34], records the same experience.
It has often been thought that the parasites of birds are all forms of the
same polymorphic organism. My cultivations of proteosoma in grey mosquitoes
appear to set the question at rest, because I have always failed hitherto in cul-
tivating the other forms known as halteridium. Proteosoma produced by me
in sparrows, weaver-birds and a crow, by the bites of these mosquitoes, had always
precisely the morphological characters of the original proteosoma from which
the virus was obtained.
I must now go farther, however, and note that while morphological
difference is almost enough of itself to imply specific difference, we have no
justification for assuming that morphological similarity, or even identity, implies
specific identity. Two organisms may be closely alike yet may be really quite
differant. Thus, there may be many quartan parasites morphologically identical
yet specifically distinct ; producing perhaps distinct secondary effects on the
human host ; differing in virulence and, may be, in details of their life-history
outside the human body. This possibility appears to have been generally lost
sight of. We know, for instance, that many local fevers appear to differ largely
in their secondary effects on the patients, some producing hæmoglobinuria,
others diarrhœa, others having a peculiar incidence on the liver or spleen, and so
on ; although the microscope seems invariably to disclose only the three known
morphological types. Again, according to many writers, there are two if not
three varieties of the æstivo-autumnal parasites, though all are very similar in
form. Such considerations suggest that each morphological type covers many
really distinct species ; that, in short, these types imply not only specific but
generic difference ; that each genus contains many distinct species, indistinguish-
able from each other under the microscope, but really possessing subtle
differences in other respects. At all events, we cannot definitely say that such
is not the case, or even that such is improbable.
Considering, however, the three readily distinguishable varieties—the quar-
tan, the tertian, and the æstivo-autumnal parasites—let us see how the presence of
I