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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 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