Protozoa are a group of animals organized on a different prin-
ciple from the rest. They are, as we have just seen, non-cellular
animals with peculiar lives and habits. Structurally and func-
tionally they differ, in many ways, from all other animals.
Now all the chief biological generalizations almost all general
propositions relating to such phenomena as birth, growth,
development, sex, reproduction, heredity, variation, and death
have been derived from observations made upon the larger
multicellular animals. When general ideas were formulated on
such subjects the Protozoa were practically left out of account.
When the more important facts about the Protozoa are firmly
established, we shall be able to recast many of our biological
theorems in a more satisfactory form. The Protozoa offer
us, in other words, a new world of animals for generalization,
and a new standpoint from which to survey our old-world
zoological knowledge. The discovery of the Protozoa was to
zoology what the discovery of America was to geography. But
we are still, in protozoology, in the i6th century. For our
knowledge of the new world we must still depend upon travellers'
tales, upon reports of things ill-observed and misunderstood,
marvels and myths and mysteries. But some day we shall have
accurate and faithful records, and then protozoology will come
190
PROTOZOOLOGY
into its own. As yet we are hardly on the threshold of the new
biology, but for those who delight in the destruction of error and
the advancement of true learning, the protozoological prospect
is already full of hope.
The Applied Science. The chief practical appli cations of
protozoology are to medicine. Certain of the Protozoa live as
parasites in the bodies of men and animals, and thereby cause
diseases. Some of these are so important that they are widely
known for example, malaria and sleeping sickness and the
elucidation of such diseases is one of the most interesting and
recent chapters in biology. Protozoology also has certain applica-
tions to agricultural science, because many Protozoa inhabit the
soil, but their value is still doubtful.
The founder of protozoology was the first to find Protozoa
inhabiting the living bodies of other and larger animals. In
1681 he described one such " animalcule " which was living in
his own intestine. In 1683 he described and depicted others
from the intestine of the frog. All these are recognizable, with
fair certainty, at the present day. Leeuwenhoek did not suggest
that these " parasites " were in any way concerned in the
causation of disease, and it is probable, indeed, that the forms
which he observed are not. But already at that date the " mi-
crobe " theory of disease-production was in existence, for it was
guessed at long before any " microbes " were discovered; and
consequently we find that, even in Leeuwenhoek's lifetime, the
suggestion was put forward that his " little animals " might be
the " causes " of certain disorders. We find, for example, an
early fellow of the Royal Society remarking, in 1683, of a
" murren " which had raged among the cattle in central Europe,
and of which the cause was undiscovered: " I wish Mr. Leewen-
hoeck had been present at some of the dissections of these in-
fected Animals, I am perswaded He would have discovered some
strange Insect or other in them." Mr. Leeuwenhoek's successors
have, on many a like occasion, fulfilled the expectations of
" the ingenious Fred. Slare, M.D., and F.R.S.," but his " s.trange
insects " they now call " Protozoa " or " Bacteria."
From the time of Leeuwenhoek to the present day the parasitic
Protozoa have been studied with increasing attention. Their
relation to diseases has been gradually elucidated, though we
are still very far from finality in our knowledge of this ab-
sorbingly interesting subject. The history of our knowledge is
long, and the discoveries have followed devious ways too
devious and intricate to be more than touched upon here.
Our knowledge of protozoal diseases diseases colloquially
said to be " caused " by protozoal parasites really begins as
recently as the middle of the ipth century, when Louis Pasteur
(1822-1805) began his researches on a disease of silkworms
called pebrine. Applying to the investigation of this disease
the genius which stamps his work on " microbes " generally,
Pasteur first discovered its causes, and then deduced methods for
its prevention. The " cause " he found to be a microscopic
parasite, now called Nosema bombycis and classified among the
Protozoa. Although Pasteur did not know that the parasite
was a protozoon, his work on pebrine and other microbic diseases
was of fundamental importance for protozoology, because it
demonstrated the methods by which such diseases can be studied
and elucidated. Pasteur's scientific principles were impeccable,
and equalled only by his own practical applications of them.
It is common knowledge that he founded modern bacteriology;
but in so doing he also laid the foundations of medical proto-
zoology. To the casual reader it may seem strange that the study
of silkworms can have any bearing upon medicine, or could in
any way contribute towards the alleviation of human suffering.
But there was another practical result of Pasteur's work which
everyone will immediately appreciate, since it can be expressed in
pounds, shillings and pence. Before pebrine attacked the silk-
worms of France the silk industry yielded an annual revenue of
130,000,000 francs to the State. After the disease had raged for
a dozen years the revenue had fallen to 8,000,000, and the
industry was on the brink of ruin. To have discovered the causes
of the disease, and to have devised, as a direct consequence,
means for its control, and, as a further consequence of this, to
have rehabilitated the whole silk industry these are practical
results which everyone can understand. And one has but to
remember that protozoal diseases may affect man himself and
his larger domesticated animals not merely silkworms to
realize the practical possibilities of protozoology.
Towards the close of the ipth century medical protozoology
became linked up with another branch of zoology entomology,
the science which deals with insects. This connexion has nothing
to do with the silkworms just mentioned, but arose through the
discovery of the part played by certain other insects in the
causation of protozoal diseases. The discoveries in this field
began, once more, with the investigation of a disease of domes-
ticated animals; but the pioneer was not, in this case, the
Frenchman Pasteur, but the Scotsman David Bruce. His work
is of such importance that we must notice it at this point.
The Work of Bruce. Some parts of Africa are the home of
certain large blood-sucking flies called " tsetse." The " Fly
Country " is uninhabitable except for wild animals; and long
before its full significance was understood, the fly itself was
recognized as a serious obstacle to the opening-up of Central
Africa. Livingstone, the greatest of all African explorers, was so
impressed with the fly's importance in this connexion that he
put a vignette of a tsetse on the title-page of his Missionary
Travels (ist ed., 1857). Live stock taken into the " Fly Country"
rapidly succumbs to a disease which is called " nagana " in
Zululand, where Bruce's original investigations were made.
The disease was also called " tsetse-fly disease," since it was
believed by the European settlers to be, caused by the bite of the
fly. The natives believed, however, that it was " caused by
the presence of large game, the wild animals in some way con-
taminating the grass or drinking-water."
Bruce began his work in Zululand after an abortive attempt
in 1894 in Sept. 1895 (the month of Pasteur's death). His full
report on his researches is dated May 1896. In this almost in-
credibly short space of time he demonstrated that nagana is
caused by a protozoal blood-parasite since named Trypanosoma
brucei, after its discoverer; that the parasite lives normally in
the blood of big game, without harming them; and that it is
conveyed from animal to animal by the tsetse. When the fly
sucks the blood of an infected animal it becomes itself infected
with the trypanosomes, which are subsequently re-inoculated
into other animals by the fly when it sucks their blood. If these
other animals are domestic stock, such as oxen or horses, they
become infected with trypanosomes, contract nagana, and die.
If they are wild game, such as antelopes, they also become in-
fected, but develop no disease. In nature the trypanosome
lives in the game and the flies alternately, the fly acting as an
intermediary in the spread of infection from antelope to antelope.
The big game indigenous in the country are habituated to
and proof against the infection; domestic animals foreigners,
introduced by man are not, and when infected usually die.
Bruce thus succeeded in extracting elements of truth from
both the European and the native beliefs, and was able to com-
bine them into a true theory of the causation of nagana. At the
same time he threw a flood of light on many other protozoal
diseases, and suggested all sorts of possibilities concerning their
causation and prevention. He forged new links between proto-
zoology and medicine and between entomology and protozoology.
It is true there were other lights and other links before. Try-
panosomes were known, and known to cause diseases, before
Bruce went to Zululand. Timothy Lewis and Griffith Evans
had observed similar parasites in India more than a decade
earlier; and Theobald Smith and Kilborne, in America, had
demonstrated in 1893 that the disease of cattle known as " Texas
fever " a disease also caused by a blood-inhabiting protozoal
parasite is transmitted from beast to beast by the agency of
ticks. But Bruce's work was solid, complete, and demonstrative.
By clean experiments and right reasoning he contributed more
to science in a few months than hundreds who have followed
up his work have since been able to contribute in many years.
In work of this sort it is the quality, not the quantity, that
counts. Later researches have but served to enhance the
PROTOZOOLOGY
191
magnitude and difficulty of the problem which confronted Bruce
in 1895; and to find a just parallel to the masterly manner in
which he solved it, we must go back to Pasteur. There is, indeed,
the same simplicity, the same directness, the same insight in
the work of both these men. Their works are enduring demon-
strations of the method of science: they are a delight to read,
and illustrate on every page the favourite maxim of Boerhaave:
Simplex sigillum veri.
The following-up of Bruce's discoveries and the working-out
of details and consequences have led to the accumulation of
an immense amount of new knowledge protozoological, ento-
mological, and medical. We can do no more than mention it
here. We must, however, notice one of the first-fruits of his
labours the application of his results to the study of human
dissasss. This application was made mainly by Bruce himself.
A few y^ars after he had done his great work on nagana he
attacked the problem of sleeping sickness, a human disease
which has depopulated large areas of Central Africa. Bruce
and his collaborators were able to show that this disease is
similar to nagana. It is likewise caused by a trypanosome,
which is conveyed to man by the bite of a tsetse-fly, and which is
capable of living in other animals. In this case the parasite had
been previously seen by Forde and Button, and by Castellani.
But its relation to human disease and the part played by the
tsetse in its transmission were first clearly demonstrated through
the work of Bruce.
Malaria and Other Diseases. We must now notice another
disease, which is known by name to all malaria, " the scourge
of the tropics." This disease, as we now know, is also carried
from man to man by the agency of a blood-sucking fly in this
case a mosquito, and it is also caused by a blood-inhabiting
protozoal parasite, though it is one very different from that
which causes nagana. Moreover, this parasite lives in men and
mosquitoes only. After undergoing a peculiar development in
the blood of a human being, it is sucked up with his blood by a
mosquito when it feeds upon him. Provided that the mosquito
is of the right sort, the parasites in the blood if they are in
the proper stage of development undergo further remarkable
changes in the mosquito's body. Thereafter the mosquito is
able to infect other men with the parasites, which it injects into
their blood in the process of sucking. And so the life of the para-
site continues.
The foregoing is the briefest synopsis of a very complicated
story, in which almost every event has been worked out in great
detail. Hundreds have contributed to this work, though some
of them can hardly be said to have cooperated in it. Indeed,
such bitter fights have taken place among them that it has now
become almost impossible to mention the names of some workers
without offending others. The history of these discoveries would
give an unpleasant shock to anybody simple enough to believe
that men of science always labour for truth and the advancement
of knowledge rather than for fame and personal gain. Fortunate-
ly the names of the leading discoverers are now known to almost
everybody, and their individual achievements are no longer in
dispute. Even the " general reader " is familiar with the name
of Laveran, the great Frenchman who, in 1880, discovered the
malarial parasites in human blood; of Patrick Manson, the
founder of modern tropical medicine, who divined, in 1894,
the part played by the mosquito; of Ronald Ross, who, inspired
by Manson, first worked out in 1898 the complete development
of the malarial parasite of birds, and thus solved the general
problem; and of Grassi and his fellow- workers in Italy, who
immediately confirmed Ross's work and extended and success-
fully applied his results to the study of malaria in man. When
the igth century ended the story was almost complete.
It will be evident that malaria, nagana, and similar diseases
are not purely protozoological problems. It will also be obvious
that such diseases might be stamped out and prevented by
attacking either the protozoal parasites which " cause " them,
or the insects which transmit them, though there could have
een but little hope of success in coping with such diseases be-
fore the life-histories of the parasites were discovered. When
protozoology, entomology and medicine have solved their re-
spective parts of such problems, then many tropical regions
which are now forbidden ground will become habitable for man
and beast. The practical importance of protozoology in cases
such as these is self-evident. The facts speak for themselves.
Malaria is a far commoner disease than nagana, and the dis-
coveries relating to it have therefore made a far wider appeal
to the public. It intrigues the public to hear that there would
still be no Panama Canal but for the great discoveries in con-
nexion with malaria. It would excite them but little to hear that
some obscure tribe of Zulus could now keep cattle in places
where it was previously impossible. But the advancement of
science is not measured in such terms, and science values most
highly those who discover and enunciate new principles. Already
we can observe that the problems presented by nagana and
malaria are similar, and that most of the generalizations which
their solution can give us are, indeed, the same. We can see,
too, that history, in the end, is generally just. Consequently,
we may hazard a guess that in years to come the historian of
science, in his impartial search for beginnings and great names,
will not fail to note the sequence of the discoveries which we
have just considered, and will apportion his praise accordingly.
The World War Period. Medical protozoology, like many
another branch of science, received a powerful stimulus from the
World War of 1914-8. Not only was much of the previously
acquired knowledge put into practice, but this practical applica-
tion in turn revealed or emphasized the gaps, defects, and errors
in many current conceptions, and so led ultimately to the
prosecution of new researches and the acquisition of much new
knowledge. Surveyed from the most general standpoint, the war
appears to have taught us little that was new regarding malaria
and the other protozoal diseases already mentioned. Its chief '
protozoological contribution has been to our knowledge of those
Protozoa which live in the human intestine, and more especially
to the elucidation of the disease called amoebic dysentery. We
may therefore say a few words on this subject at this point.
The Protozoa known as " amoebae " form a large and inter-
esting group. Most of the species live independently in such
places as ponds, ditches, or the soil; but some of them live in
the bodies of other animals, and one of them called Entamoeba
hislolytica was already known before the war to live in the
human bowel and " cause " amoebic dysentery. The parasite
was discovered by Losch in Russia as long ago as 1875. Its
real relation to dysentery, however, was not made clear, though
much debated, until just before the war, when the admirable
researches of two American workers in the Philippine Islands
E. L. Walker and A. W. Sellards were published. During the
war their results have been confirmed and greatly extended,
chiefly by the investigations of British workers. As a con-
sequence, we now know as much about amoebic dysentery as we
do about malaria or the diseases due to trypanosomes. There
are several points here which are worthy of mention.
We now know that no less than five different species of
amoebae may live in the intestine of man, though only one of
these the " dysentery amoeba " already mentioned ever does
him any harm. Moreover, we now know also that amoebic
dysentery is a comparatively rare disease. There are many
different kinds of dysentery, and the kind due to amoebae is
far from being the commonest. Before the war amoebic dys-
entery was generally recognized as a disease more or less re-
stricted to the tropics, though certain other kinds of dysentery
occur all over the world. "The curious fact brought into prom-
inence by the war is that the dysentery amoeba itself is very
common almost everywhere. This parasite, which can cause, by
its presence in the bowel, a violent and sometimes fatal form of
dysentery, usually does no such thing. Very many people, in
all parts of the world, are infected with it, but very few ever
suffer any appreciable harm from its presence. The parasite
and the person who harbours it are usually suited to one another
in such a way that they can live together comfortably, oblivious
of the existence of one another. There are, for instance, in the
British Isles at this moment many thousands of people who are
PROTOZOOLOGY
heavily infected with these disease-producing parasites, and
yet enjoying perfect health.
Another curious feature of amoebic dysentery is the circum-
stance that it cannot be contracted from a person suffering from
the disease. The people responsible for the spread of infection
are those who harbour the parasite but themselves suffer no
ill consequences from its presence. The explanation of these
seemingly contradictory facts is really quite simple, now that
we know the life-history of the amoeba and its relation to disease.
It is a popular fallacy to suppose that any parasite is the sole
" cause " of any disease. A disease is a joint result of many
antecedent factors, and in the present case it would probably be
nearer the truth to say that the person who harbours the amoeba,
rather than the amoeba itself, is the " cause " of amoebic dys-
entery. For dysentery results only when the infected person
happens to be abnormally sensitive to infection with the amoeba,
and the condition is as harmful to the parasite as it is to the
patient. Normally man and amoeba fit one another, and there
is no trouble. Abnormally there is a misfit, and amoebic dys-
entery is the consequence. The disease is really an unimportant
side-show in the life-history of the parasite, the result of its
being planted in unsuitable soil.
The foregoing considerations will serve to show once more the
value of protozoology in the study of human diseases. What
hope could there ever be of eradicating a disease such as amoebic
dysentery if we remained in ignorance of the life-history of the
parasites connected with it? We might cure every case of the
disease we might conceivably prevent the death of every
patient who contracted it; but even if we did, it is now clear
that this would have no effect whatever upon the continuance
and prevalence of the disease itself. Such procedure could not
possibly stamp out amoebic dysentery, or even reduce by one
the annual number of cases of this disorder. This is not to
say that protozoology has yet enabled us to do either of these
things; but it has enabled us to formulate the problem cor-
rectly, and has shown the uselessness of expending our energies
in wrong directions. Greater results will follow when our
knowledge is greater and more properly and consistently applied.
It has been supposed for so long that the parasites which
produce protozoal diseases are peculiar to tropical or sub-
tropical countries that the discovery of the dysentery amoeba
in Britain may seem surprising. It is really not so surprising as
the circumstance that nobody, until quite recently, had thought
of looking for it here. And there are many equally remarkable
parallels. To mention only those diseases and parasites which
we have already noted, we can now say that malaria occurs
indigenously in Britain though this was hardly suspected
until recently; and that parasites closely similar to those which
cause nagana and Texas fever have now been discovered in
the sheep and cattle of the United Kingdom. How far these
observations are of practical importance the future will show,
but already they clearly indicate that protozoology may be
studied with profit at home no less than abroad.
Organization and Training of Workers. In conclusion, we
shall now note very briefly what has already been done for the
promotion of protozoology as a branch of science.
As a profession it still hardly exists. Most of those who have
enlarged the science have been zoologists or medical men en-
gaged in teaching other subjects and in practising their pro-
fessions. Many great discoveries have been made by men who
cannot be described as protozoologists^ But the science has now
become so vast, from the amassing of myriads of complicated
details, that it can no longer be regarded as an occupation for
anyone but a highly trained specialist. The amateur toying
with his microscope, the ordinary zoologist or physician working
in occasional vacations or leisure hours snatched from practice,
can no longer expect to make any solid contributions to proto-
zoology. In future all great advances in knowledge must come
from those who are bred up as protozoologists who not only
have the necessary physical and mental gifts for this most
difficult study, but who also are prepared to devote their lives
and energies to it, and to it alone.
Modern science has already developed to such unwieldy pro-
portions that it has ceased to be coherent and has burst asunder
into separate segments. The day of the " scientist," with all
science for his province, is gone for ever. If men of science are
to escape the fate of the builders of the Tower of Babel, it can
only be by conscious cooperation. Each worker must do his
own special work, but must do it with due regard for his fellow-
labourers in adjoining sections, and with the plan of the whole
building constantly before his eyes. Protozoology must, accord-
ingly, develop along its own lines and by the labour of proto-
zoologists, but it must remain in touch with the rest of zoology
and with medicine and with all other sciences whose collaboration
is likely to be mutually beneficial. We can already observe the
bad effects of non-collaboration in the modern school of proto-
zoology which originated with Fritz Schaudinn in Berlin.
Over-specialization has there led after beginning on an ad-
mirable foundation of fact to fantastic speculation and the
promulgation of doctrines which are biologically unsound.
One of the good results of the World War was to encourage
the collaboration of workers in different branches of science,