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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