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As an individual science protozoology only became self- conscious at a quite recent period. The name itself, though already in use between 1870 and 1880, only became current after the opening of the 2oth century that is to say, within the memory of many living zoologists. But the science was really born though not baptized when the first Protozoa were discovered. This far-reaching discovery was made in the latter half of the i7th century. It was made by a man who was neither zoologist nor physician, but who occupied the humble position of chamberlain to the sheriffs of the little town of Delft, in Holland Antony van Leeuwenhoek (1632-1723), an amateur microscopist, who studied at no university, nor under any of the great professors of his day, but whose title to fame rests upon the simple and honest application of his own native genius. This remarkable man made his own microscopes, lenses and all, and turned them upon almost every object which suggested itself to his quick imagination. In the course of his work he examined the water from the leaden gutters of his house, from the well in his courtyard, and also fresh rain-water, snow-water and " the water wherein pepper had lain infused." He found that all these liquids, and many others, were not clear and empty when viewed by the microscope, but teeming with living crea- tures. The discovery was promptly communicated by letter to the Royal Society in London, who published a part of it in the year 1677. Some of the animals which Leeuwenhoek here described can now be identified as Protozoa, and his letter may therefore be regarded as the first page in the history of proto- zoology.

Leeuwenhoek, the father of protozoology, himself studied and described many Protozoa. His observations were soon repeated and confirmed by others, notably by some of the early Fellows of the Royal Society and his fellow-countryman Huygens, the great astron- omer. But for many years protozoology made little progress, and remained essentially what it was originally, an amusement rather than a science. Although many good observations were made and recorded, they were always disjointed, and often distorted by fancy and speculation. Many good zoologists regarded with doubt and misgiving everything seen with the microscope, an attitude of 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