116 
NATURE 
| NOVEMBER 29, 1906 
under the name protista, meaning literally the very first 
things, living things (zoa) being understood. The protista 
would then rank as a separate kingdom, that is to say, 
as a category equivalent to the animal and vegetable 
kingdoms respectively. 
Theoretically, there can be no doubt that to group all 
these primitive living things together as protista is the 
most natural and proper way of dealing with them. We 
should then talk of protistology rather than protozoology, 
and of a protist rather than of a protozoon, which would 
at least be more euphonious. But this method of dealing 
with these creatures is inconvenient and unsuitable in 
practice, chiefly because the group protista comprises such 
a vast array of organisms of different types that no one 
investigator can deal with them all satisfactorily, or with 
the different technical methods requisite for their study, 
and a division of labour has become necessary. Hence 
the bacteria have been assigned to the domain of a special 
science, bacteriology; the botanists claim for their sphere 
of investigations all those protista which are of vegetable 
nature; and there remain, finally, for the zoologist, those 
protista which can be regarded as animals, and which 
are, therefore, termed the protozoa. 
We have now got so far, that the protozoa are minute, 
microscopic forms of animal life. There are, however, 
many minute animalcules which are by no means to be 
considered as protozoa. If we compare the protista with 
higher animals and plants, we find at once a fundamental 
difference. In the body of a protist the living substance, 
the protoplasm, is not divided up into cells, but forms one 
simple mass; that is to say, the whole body of a protist 
is comparable to a single one of the cells that build up, 
in vast numbers, the complex body of a higher animal or 
plant. Expressed briefly in the technical jargon, we may 
say that a protist is a unicellular organism, and that a 
protozoon is a protist of animal nature. Since such 
organisms may be regarded as the most primitive types of 
animal life, the earliest, probably, to appear upon our 
globe, they have been named the protozoa, or “first 
animals.”’ 
We are now in a position to attack the second question 
that was suggested for consideration, namely, what is the 
interest and importance specially attaching to the study 
of the protozoa? This is a matter which can be con- 
sidered most conveniently from two different points of 
view, the theoretical and the practical. In dividing my 
discourse into these two heads, however, I do not wish 
to be understood to imply that there is any real distinction 
between theoretical and practical science. The whole 
history of human progress and culture shows that what is 
theoretical to-day is practical to-morrow. This is such a 
commonplace that it would be superfluous to waste time 
by citing instances. The theoretical knowledge of scientific 
principles must necessarily precede their application; hence 
to discover these principles is, even from the practical point 
of view, the most important occupation of the human 
intellect. This is a point of view which cannot be too 
strongly emphasised, and to which I shall return again. 
From the theoretical point of view the protozoa are of 
the greatest interest on account of their primitive nature, 
and the light which they consequently throw on many 
obscure vital processes. The cells which compose the 
tissues of higher animals have become extremely specialised 
for their particular functions and modes of life, and their 
structural or developmental characters tend to follow certain 
stereotyped patterns and to conform to uniform rules of 
procedure, due perhaps to a common origin and ancestry. 
In the protozoa, on the other hand, each individual is an 
unspecialised cell, capable of performing equally well all 
the functions of life as a free and independent living 
organism, and the structural features or developmental pro- 
cesses of protozoa exhibit the utmost possible diversity of 
character. Only by the detailed comparative study of this 
primitive diversity is it possible to discover the course of 
evolution which has culminated in the relatively uniform 
characters of cell-structure and cell-behaviour in the higher 
forms, and so to elucidate the true significance of many 
obscure cytological problems. Just as the higher division 
of the animal kingdom may be reasonably supposed to 
have originated from protozoan ancestors, so the cytology 
of the higher animals may be said to have its roots in 
NO. 1935, VOL. 75] 
the cytology of the protozoa, and the same is perhaps true 
also of other subdivisions of biological science. 
Turning now to the practical aspects and applications of 
protozoology, we find that these arise from the peculiarity 
already mentioned of many of these organisms, namely, 
that they live as parasites of other animals, and may pro- 
duce diseases in them. For this reason the investigation 
of the protozoa has, like that of the bacteria, become of 
immense importance to medical and veterinary science, and 
for this reason protozoology has taken shape as a definite 
science, and has gained recognition, outside zoological 
circles, just as bacteriology did before it. Formerly it was 
always bacteria that were sought for as the agents of 
diseases. Now it is known that many diseases are caused 
by protozoa, and not by bacteria, and it is suspected that 
this is the case also in certain diseases of which the cause 
is still obscure. 
Although, as I have stated, the practical importance of 
the study of protozoa has only been recognised generally 
in the last few years, nevertheless the actual discoveries 
of important disease-producing protozoan parasites date 
back, in some cases, a quarter of a century. Prof. Koch, 
of Berlin, has directed attention to three great discoveries, 
each of which opened up the way for a new line of in- 
vestigation, and was of the utmost importance in establish- 
ing the true cause of diseases previously mysterious in 
nature. The first was the discovery of the malarial para- 
sites by Laveran in 1880. The second was the discovery 
of the parasites of the so-called Texas fever of cattle in 
America by Smith and Kilborne in 1893. The third was 
the discovery of the parasites of tsetse-fly disease in Africa 
by Bruce in 1895. 
The malarial parasite was first observed by Laveran, 
then an army surgeon, in the blood of fever patients in 
the military hospital at Constantine, in Algiers. Though 
working with inferior microscopical apparatus, Laveran 
described clearly all the principal stages that can be made 
out in human blood. This sensational discovery was re- 
ceived everywhere with coolness and disbelief. At that 
time the cause of malaria was generally believed to be a 
bacterium, which was named Bactllus malariae, and it was 
some years before the bacillus was discredited, and 
Laveran’s parasite established, as the true cause of the 
disease. It still remained a mystery, however, in what 
way this minute organism got into the human blood, and 
the view was put forward that it gave rise to minute 
germs which passed out of the body and were scattered 
abroad, and which, like many other germs of protozoa, 
were able to float in the air. It was supposed that those 
germs were then inhaled by healthy persons, and so gav~ 
rise to the disease. This was simply an extension of the 
old miasma theory, the notion that the disease was con- 
tracted by inhaling the air of swamps and marshes, a 
notion expressed in the word malaria, meaning literally 
bad air. It remained for a countryman of ours, Major 
Ronald Ross, to discover, by a series of brilliant experi- 
ments and observations, the part played by mosquitoes in 
disseminating the disease. It was found, however, that a 
remarkable relation existed between the species of mos- 
quitoes and the species of malarial parasites. The common 
gnats, for instance, belonging to the genus Culex, are 
incapable of transmitting the malarial parasites of man, 
but convey those of birds from one bird to another. The 
mosquitoes which carry the malarial parasites of man 
belong to a different genus, Anopheles, and they in their 
turn are incapable of transmitting the malarial parasites 
of birds. This is one of those remarkable adaptive special- 
isations so often seen in nature. 
Let us now follow the course of infection briefly. If 
a mosquito bite a man suffering from malaria, it takes in 
a drop of blood in which are contained various stages of 
the malarial parasite. The blood is, of course, digested 
slowly in the mosquito’s stomach, and if the mosquito 
be a Culex, all stages of the parasite are digested also: 
but if the mosquito be an Anopheles, certain stages of the 
parasite resist digestion. In the parasite of pernicious or 
tropical malaria, the resistent stages have a form like a 
sausage, and are known commonly as crescents. These 
crescents undergo changes in the mosquito’s stomach which 
give rise to sexual forms, minute, slender males, and re- 
latively large, bulky females. Fertilisation takes place, 
