mind which has not quite disappeared even in these days of perfect
instruments. Even the great Linnaeus (17071778), who attempted
to catalogue and classify all animals and plants, and thereby founded
modern systematic biology, never really overcame his suspicions
sufficiently to incorporate the Protozoa firmly in his system. His
mental attitude is shown in the name " Chaos infusorium," with
which, in 1767, he dubbed a mixed lot of questionable protozoal
organisms the term Chaos itself having been suggested, no doubt,
by Ovid's " rudis indigestaque moles."
But already at this period many workers were convinced that
the Protozoa or " Infusoria," as they were then called, from their
occurrence in infusions have a real existence. The once notorious
John Hill (1716-1775), in the course of his. journalistic, theatrical,
medical, and botanical adventures, turned his attention to micro-
scopes; and in 1752 he described and, for the first time, scientifically
named, a number of Protozoa which he had seen in infusions. Up
to this time writers had been content to call them by diminutives of
the names of larger and more familiar creatures, or occasionally by
names suggested by comparison with some common object. We
thus find the early protozoologists describing their observations upon
" little insects," " worms," " fishes," and even " reptiles," and upon
" the slipper," " the sun," " the trumpet," " the gimlet," or " the
bell animalcule." It was not until 1773 that a serious attempt was
made to reduce the chaos to order by careful observation and descrip-
tion and classification of the " Infusoria." This notable work was
done by the Danish naturalist, O. F. Miiller (1730-1784); and his
last book, published posthumously in 1786, is the first systematic
treatise on protozoology. It is a remarkable work, full of shrewd
observations, and showing astonishing insight, but containing , of
course, many mistakes which were inevitable at that period. Many
of the Protozoa described and sketched by Miiller mostly from ob-
servations made, as were those of Leeuwenhoek, with the aid of only
a simple lens are easily recognizable now by a protozoologist.
The circumstance that Miiller was able to attempt a comprehen-
sive systematic treatise on the Protozoa implies that a very consider-
able advance had taken place in biological thought since microscopic
organisms were discovered. Many of the earlier workers, like the
uneducated at the present day, believed in spontaneous generation.
They believed, with Aristotle, that many " imperfect " animals were
bred in mud, water, or decomposing matter; and so long as this view
was tenable there was no reason why these misbegotten offspring of
the superabundant vitality of the earth should display any particular
constancy in their appearance or any fixity of form. Consequently,
to attempt to describe and classify the " Infusoria " must have
seemed a futile task to many men of science two hundred years ago.
Spontaneous generation, as a scientific doctrine, was not really
demolished by the admirable experiments of Redi (1668), as is often
supposed, for he disproved it for only the larger and more obvious
animals, such as insects; and the later discovery of microscopic or-
ganisms raised the whole problem once more, but presented it in a
much more difficult form. It was Redi's countryman, Spallanzani,
who, a hundred years later, extended his observations to microscopic
animals, and showed by means of ingenious and exact experiments
that the " Infusoria " spring from living antecedents, and live, grow,
and multiply like larger creatures. Spallanzani helped to lay the
foundations on which Miiller built, though his own work was not
firmly consolidated until, a century later, the last rivets were driven
in by Pasteur and Tyndall.
In the latter half of the 1 8th century many minor contributions
were made to protozoology, and although these were continued dur-
ing the early part of the next century, no considerable advance was
made until about 1830, when the Berlin zoologist, C. G. Ehrenberg
(17951876), began to publish his researches. With amazing per-
severance he studied, described, and named all the " Infusoria " that
he could find : and as he pursued his investigations not only at home,
but also in Egypt, Arabia, Siberia, and elsewhere, the forms which
he discovered were not a few. His chief contribution to proto-
zoology was published in 1838 a monumental folio volume of more
than 550 pages, accompanied by an atlas of 64 coloured plates.
This is still one of the classics of the science. It contained much that
was new and much that was true, everything of note that his indus-
trious reading could find in the works of his predecessors, and withal
a mass of mistakes, to which he clung tenaciously in spite of violent
contradiction and criticism to the end of his days.
Ehrenberg's most dangerous opponent was a Frenchman, Flix
Dujardin (1801-1860). In 1841, with an octavo volume of some 680
pages, but only 23 plates, he undermined the foundations of the big
folio, and thus overthrew, for all time, many of the favourite theories
of his German antagonist. Dujardin's work is also a protozoological
classic. Together with Ehrenberg's volume it marks the end of the
old protozoology of the micrographers and the beginning of the new
science as a special branch of zoology. Rarely does the modern
worker, unless he be a historian, require to consult any earlier trea-
tises than these.
Since the time of Dujardin only one really exhaustive work on the
Protozoa as a whole has been written. This is the great monograph
by O. Biitschli, of Heidelberg, published in 1880-9. It is significant of
the vast modern development of protozoology that up to 1921 no
work on a like scale, by a single individual, had been produced.
It is now, indeed, impossible for any one man even to read all that
has been written on the Protozoa, and the more recent workers have
had perforce to devote their attention to some particular group of
these organisms, or to some special branch of protozoology. To
master a detail of the science is now the work of a lifetime. No one
man could in 1921 claim to be an expert in all protozoology any more
than in all mathematics or all chemistry. The territory already sur-
veyed was so vast that the most he could hope to do was to cultivate
his own small holding properly.
The Modern Science. Since the middle of the ipth century
biological theory and practice have undergone profound changes;
and in more recent years protozoology, with the rest of zoology,
has largely changed its character. This period has seen to
note but a few of its more striking developments the establish-
ment of the Theory of Organic Evolution, the rise of the Cell
Theory, the foundation of Histology and Cytology, and the
unfolding of Physiology and Embryology and Medicine as
experimental sciences. Protozoology has been profoundly in-
fluenced by all these new growths, and has itself contributed not
a little to them. An attempt has been made, and has already
been partly successful, first, to discover all the Protozoa there
are, both living and fossil; then to investigate their structure in
the minutest detail, and to ascertain how they live and develop;
and finally, to understand their relations to other organisms and
their place in nature. Countless monographs have been written
on individual species, on the larger and smaller groups into
which these can be scientifically classified, on collections made
all over the world, and upon the special physiological, medical,
and other problems which the Protozoa, as a whole or in part,
present. But we must content ourselves here with the merest
sketch of the growth and status of modern protozoology.
Before proceeding, we must note some of the peculiar difficul-
ties which differentiate protozoology from the rest of zoology.
The animals with which it deals are, speaking generally, in-
visible to the naked eye. Consequently, they cannot be studied
and anatomized by ordinary methods. The protozoologist has
first to become a master in the use of the microscope, and to
learn its limitations as an instrument of research. When he
i88
PROTOZOOLOGY
has become proficient he must learn or devise methods for
catching, watching, breeding and preserving those Protozoa
that he wishes to study, and must thus become familiar with
a peculiar and varied technique adapted to the investigation
of the lives and habits of animals invisible to the unaided eye.
He must then acquire the power of correctly interpreting what
he sees under these peculiar conditions. If he is an efficient
microscopist and a good observer, endowed with abundant
patience and ingenuity, and if, at the same time, he is a good
zoologist and sound philosopher, then, with experience and
diligence, he may hope some day to become a good protozoologist.
From the very nature 1 of the subject, therefore, it will be obvious
that it is easier to make mistakes in protozoology than in most
other branches of zoology; and there can be little doubt that the
writings on the Protozoa, taken as a whole, contain a larger
percentage of error than those on any other group of animals.
Protozoology is, indeed, still in its infancy, and learning slowly
and painfully by the method of making mistakes.
Protozoology, like most other sciences, is important from two
different standpoints, which may be called the theoretical and
the practical. On the theoretical side we have to consider its
relations to the rest of zoology, and the value of its contributions
to biological philosophy; on the other side, we must consider the
utility of its practical applications, which are chiefly medical.
In other words, we must look at protozoology as a pure science
and as an applied science. It is necessary to distinguish these
two aspects, although they are inextricably blended in reality.
Protozoology was actually applied in medicine before it was
ready; and this led not only to great confusion but almost to
the severance of Medical Protozoology from the rest of the
science. But progress on the medical side has now reacted
beneficially upon the pure science, by bringing to light many
new facts and setting many new problems.
The Pure Science. The theoretical importance of proto-
zoology is not what it appeared to be fifty years ago. It has
not fulfilled some of the high hopes then entertained for its
future. In the earlier period the writer of an article such as
this would have begun, in all probability, by declaring that the
study of the Protozoa would lead to the solution of most of the
outstanding general problems of biology. He would have pointed
out that these animals were of the greatest importance in con-
nexion with the two chief biological generalizations of his time
the Cell Theory and the Evolution Theory and he would prob-
ably have ended by saying that it was only lack of detailed
knowledge which prevented protozoology from answering most
of the fundamental questions of biology. Yet we have now an
abundance of the sort of information then regarded as requisite,
and the great problems are still, for the most part, where they
were. It is both interesting and instructive to inquire how this
has come about.
The cell theory was first definitely formulated, in Germany,
by Schleiden (1838) and Schwann (1839), and was modelled
into its modern form by Max Schultze (1861): that is to say, it
took shape at the time of the reformation of protozoology by
Ehrenberg and Dujardin, when the science was still feeling
for a foothold. According to the cell doctrine, all organisms,
both animals and plants, are built up of structural units, called
'' cells," in much the same way as a house is built of bricks.
Schultze defined " a cell " as " a little lump of protoplasm with
a nucleus inside it," and this definition was generally accepted.
It should be noted that this proposition, so far as the larger
animals and plants are concerned, is not a " theory " at all, but
a statement of fact easily verifiable by means of the microscope.
The body of a rabbit or a cabbage is, for the most part, actually
composed of " cells " as conceived in the definition. The
" theory " was introduced when the proposition was held to
apply to all organisms at all stages in their development. Dujar-
din had shown that the Protozoa are soft-bodied animals com-
posed of " sarcode " the " protoplasm " of later workers
in which no constituent " cells " are discernible. Like " cells "
Protozoa contain " nuclei," but, unlike the large animals, they
show no internal differentiation into cellular units. It was thus
necessary to introduce some new conception if the cell theory was
to become universally applicable.
The extension of the theory, so as to enable it to include the
Protozoa, was made by von Siebold. Each individual protozoon,
he said, is itself a " cell." It is comparable with a single one of
the innumerable units of which the bodies of large animals are
built. The Protozoa are " unicellular " animals, all others
" multicellular." According to this doctrine, therefore, a proto-
zoon is not comparable, as an individual, with a whole multi-
cellular animal, but with one of the cells in its body: or, the
other way about, a multicellular animal is not an individual
of the same sort as a protozoon, but a colony of such individuals.
This conception appeared so plausible owing, it must be sup-
posed, to the backward state of protozoology and cytology at
that date that it found ready acceptance; and, in spite of
the cogent objections which have been raised against it by Huxley
(1853), Whitman (1893), Sedgwick (1894), Dobell (1911), and
others, it has prevailed down to the present day. The cell
theory is still taught to almost every beginner in biology.
He is still told that he is not an individual, but a community of
individuals; and that the protozoon, which he can see with his
own eyes leading an individual existence, is not an individual
such as he believed himself to be but the equivalent of one
little bit of his body.
When the cell theory was being founded, another great
biological generalization was just emerging the doctrine of
Organic Evolution. Charles Darwin's great work, which
appeared in 1859, created a revolution in biological thinking.
Although Darwin's own work, and his statement of the theory,
appear to be unexceptionable, the doctrine miscalled " Dar-
winism " developed along extravagant lines chiefly, as is
now evident, owing to the wild speculations and dominating
influence of E. Haeckel and other German writers. The " cell
theory " was immediately subpoenaed to give evidence for these
" Darwinists." They wrongly believed that the evolution
theory required the presence of some " most primitive " and
" elementary " animals from which all the " higher " forms
had been derived on the earth at the present day; and the shaky
syllables let fall by the cell theory were eagerly seized upon,
interpreted, and ultimately incorporated as incontrovertible
facts in the case of the " Evolutionists." " Unicellular "
organisms such as the Protozoa thus became the starting-
point of evolutionary speculations. The Protozoa were obviously
the " simplest " animals, since less was known about them than
about the others; and they were clearly the " most elementary,"
each individual representing but one of the structural elements
of which the others were composed. Their insignificant size
made them the " lowest " forms on earth, and their position
according to the " theory " at the bottom of the " Scala
Naturae," made them the " most primitive." It thus became
easy to show, by specious arguments and " question-begging
epithets," that protozoology occupied a position of fundamental
importance in biology. By studying the Protozoa the earliest
stages in evolution would be revealed. The beginnings of life
would be laid bare. Physiology and morphology would appear
in their elemental forms, stripped of all confusing detail. And
optimists were not wanting who divined that, by higher and still
higher powers of the microscope, Nature's inmost secrets such
as the origin of life itself would be divulged.
These fantastic dreams have been slowly dispelled by the
" dry light " of reason. It has become clear that protozoology
was placed in a false position by the devotees of the cell doctrine
and the dogmatic evolutionists. Let us look at the fundamental
conception of the " unicellularity " of the Protozoa from another
angle, and see how it appears in the light of modern knowledge.
In the first place, it is clear that the Protozoa cannot properly
be described as " unicellular." Every protozoal animal has an
independent existence. It has its own peculiar structure, exer-
cises its own proper functions, leads its own life often, indeed,
a very complex one. As an animal it is, from every standpoint,
as much an " individual " as a man is. One protozoon is one
whole animal, just as one man is one whole animal. From the
PROTOZOOLOGY
189
standpoint of common sense, no less than from that of modern
zoology, the whole organism is the unit of individuality. But
when we examine a protozoon under the microscope we still
see as Dujardin saw that its body is not differentiated in-
ternally into cells, as is that of a man. Its body is often sur-
prisingly complex in structure, but it is never composed of cells.
It is clear, therefore, that we can contrast the body of a man with
that of a protozoon by saying that the one is cellular in structure,
the other non-cellular. To call it " unicellular," and thus com-
pare one whole animal with a minute differentiated fraction of
another, is obviously absurd. It is as though a man who had
only seen houses built of bricks were suddenly to encounter
one constructed, all of a piece, of concrete; and then, being unable
to find the familiar individual bricks in its fabric, were to declare
that the concrete house is not a house in the sense that the
brick house is but one large and peculiarly modified brick.
When once it is realized that the Protozoa are not, in any sense,
" elementary " or " unicellular " animals, but a group of
peculiarly constructed creatures, adapted in a special way to
particular conditions of life, then it will also be realized that
we have no reasons apart from preconceived ideas derived from
unsound generalizations for believing that they represent
" primitive " or " first " forms of life. That they are not
" simple " we now know. It is true that they display, on the
whole, less visible structural differentiation than most of the
larger animals; but physiologically they are very complex.
That they are able to perform all the chief functions of " higher "
animals, but with fewer instruments, does not make their
mechanism easier to understand; and it is thus hardly con-
ceivable that the Protozoa can ever offer us the easiest way of
approach to physiological problems. They offer us, indeed, the
most difficult field in animal physiology, owing to their micro-
scopic size and apparent simplicity of structure. As a great
physiologist has well said: " Experience and reflection have
shown me that, after all, the physiological world is wise in spend-
ing its strength on the study of the higher animals. And for the
simple reason that in these, everything being so much more
highly differentiated, the clews of the tangles come, so to speak,
much more often to the surface, and may be picked up much
more readily " (Michael Foster). Attempts to found a " general
physiology " on the Protozoa as " cells " and " elements "
are doomed to failure, for they are based upon an unsound
philosophy; and the speculative and deductive efforts in this
direction such as that of Verworn in Germany have slowly
given way before the experimental and inductive methods of
Jennings and others in America and elsewhere.
As a point of historic interest, it may be noted that the father
of protozoology and his immediate followers had none of the
extravagant later notions regarding the " unicellular " and
" elementary " nature of the Protozoa. For Leeuwenhoek the
Protozoa were animals like any other animals, but delightfully
and marvellously little; and he thus saw more clearly and
naturally than many of his later successors.
There are probably few biologists who now cherish any hopes
of seeing the fundamental problems of biology solved by the
study of the Protozoa, though the majority still speak and write
in the optimistic language of last century. For these mental
survivals there is a psychological basis, which seems worth
noting before we go on to consider the true status and value of
protozoology. There is a curious disposition, apparently in-
herent in the human mind, to suppose that by studying the most
minute creatures we can come nearer to first principles. And it
is the same with the study of the larger organisms. As the cytol-
ogist probes into the structure of an animal with higher and still
higher powers of the microscope, he feels that he is gradually
" getting to the bottom " of his problems. He feels that when his
microscope has resolved the larger animals into their smallest
component parts, and has revealed every detail of the smallest
living thing, he will be face to face with fundamentals. It does
not require much thought to realize that this is a fallacy. The
deeper we delve, the more detail we discover. But it is all of the
same sort: we add to the quantity and not to the quality of our
knowledge. With the highest possible magnification we shall
obtain no information which is qualitatively or fundamentally
different from that to be derived from the study of large organ-
isms, and their gross anatomy, with the naked eye.
The mental bias just mentioned seems to be responsible for
many popular and not a few " scientific " notions about the
Protozoa. It appears, for example, to be at the back of the un-
reasonable but common belief that the Protozoa are " elemen-
tary " and " primitive " animals. Although few biologists now
believe in spontaneous generation, yet many are able to believe
that living things must have been spontaneously generated from
lifeless matter in the past; and to those who hold this belief it
still appears self-evident that the organisms so generated were
microscopic. Consequently, these biologists feel that the
Protozoa must, in some way, be nearer than other animals to
" the beginnings of life," and they find no difficulty in conceiving
that the first animals were " Protozoa." In the same way, when
these same biologists come to consider evolution, and the rela-
tions of living animals to one another, they find in the Protozoa
the easiest starting-point for their speculations. The Protozoa
are " the simplest " animals, and the human mind works most
readily from simple to complex conceptions. Consequently,
evolution is pictured as necessarily moving in the same direction
the simply constructed creatures coming first, and the com-
plex developing from them. But it is surely a poor philosophy
which would constrain Nature to order her infinite events in
that particular sequence in which thoughts happen to follow
one another most easily in the mind of man.
What, then, it may be asked, is the theoretical interest or
value of protozoology? Clearly it is this. Biological theory is
sound in proportion to the truth of its generalizations. When
all the facts are known about all animals and plants, we shall be
able to make true general propositions about them. Before we
know the facts our generalizations can be but partial and pre-
mature more or less lucky guesses, based upon incomplete
knowledge. All biological theory is at present in this condition
and therefore the careful study of any animal or group of animals
such as the Protozoa will, if it yields new facts for generaliza-
tion, be valuable ultimately as a contribution to biology. At
present we cannot hope to do much more than collect facts, by
means of accurate observation and apposite experiment. When
we have collected and critically analyzed them, we can some-
times make tentative generalizations of a lesser order. But the
larger and truer generalizations will come later.
It may be said that if this is all that can be expected from
protozoology, then it is no more important than any other
branch of zoology: there is no reason why we should study the
Protozoa rather than any other group of animals. All this is
quite true and reasonable; but there is also a reason why proto-
zoology is likely to yield results of particular interest. The