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Proof has been read critically by three of my former pupils, J. W. Tietz, F. T. Lacy, and J. W. Ingle. To all of the above my thanks are due.

Thanks are due, also, to Prof. E. B. Wilson, W. C. Stevens, and C. W. Beebe, Dr. Alvin Davison, Dr. Frank Overton, Mr. Spencer, of the New York Aquarium, the United States Department of Agriculture, and the American Museum of Natural History, for permission to copy and use certain cuts and photographs which have been found useful in teaching. Prof. G. N. Calkins of Columbia University kindly loaned the photograph of the Amoeba reproduced on page 182. R. W. Coryell and J. W. Tietz made several of the photographs of experiments. The photograph of the humming bird was used by permission of the Folmer and Schwing Company, Rochester, N.Y. In particular I am under obligation to my former coworker, Dr. C. A. King, for his example of earnest and inspiring work, to Mr. W. P. Hay, Head of the Department of Biology and Chemistry, in the Washington, D.C., High Schools, for his helpful criticisms, and to Messrs. Sedgwick and Wilson, whose General Biology is a monumental work in ele mentary biological teaching.

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Science and Matter. Science deals with that material which occupies all the space around us, the air, the water, and the earth. This material is called matter. Matter is the building material of the world in which we live.

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Matter exists in this world in two distinct states. It may be living, or may have been alive at some previous time, in which case we speak of it as organic matter; or it may never have been alive. The latter state of matter is called inorganic. Hence we find two groups of sciences which deal with matter: the biological sciences, which treat of living matter; and the chemical and physical sciences, which deal chiefly with inorganic matter.

Biology. Biology is the science which treats of matter in a living state. The two subdivisions of biology, dealing with plant life and animal life respectively, are called botany and zoology.

Knowledge in Science gained by Observation. Science has been defined as "knowledge gained by exact observation and correct thinking." First of all, science is a kind of knowledge. It is accurate knowledge. But it is possible to acquire a mass of knowledge not scientific with the sole aid of a text-book. Such knowledge, for example, might be that of the Latin or the German language. Scientific knowledge, according to our definition, must be gained through observation, from the accurate study of a specimen, something that we may see and touch. It is not enough to study a book alone; this may be an aid, but the specimen is, after all, the main thing. If we were to fit ourselves for the trade of a plumber or a carpenter or a mason, we certainly should not depend upon a book for our information regarding our particular

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trade. We should, instead, go to the shop and there learn to work with the tools of our trade. So, in the pursuit of scientific work, we must learn to use the tools with which nature has provided us, our hands, our eyes, and the thinking mechanism, our brains. As Louis Agassiz, the famous naturalist said, "Study nature, not books."

Classification of Facts Observed. The knowledge we gain by observation is worth very little to us or to any one else unless we use our brains to classify it and to apply it. We must find out what different facts mean as related to one another.

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Single isolated facts about the color, coats, or markings found on the coats of a kidney bean mean but little to us if we cannot correlate these observations with others and relate them to scientific truths already learned. A great many men, working for long periods of time, have gathered together a large number of single isolated facts, have correlated these facts, and then have given to the world discoveries of world-wide importance. A careful boy or girl may, by his own painstaking work in science, find out some fact that is new, and in a small way make a discovery. It is one of the most interesting things about science work, that it has in it the spirit of discovery.

Morphology. It is evident that, in order to understand the cause of the regular movements of a clock, it would be necessary to take the wheels apart and to find out the structure of the different pieces composing the works, so as to see how these parts are related to each other. In the study of biology it is usually found best to begin with the study of the form and structure of the parts of an organism; this study is called morphology.

Physiology. After we have discovered in the clock the form and structure of the different wheels and cogs and the relation of one to the other, we are in a position to put them together again and to find out how they move and what causes the movement: to study the use or function of each part. The study of the uses or functions of the parts of an organism is called physiology.

The Experiment. In order to study physiology, and indeed most sciences, we frequently have to make use of an experiment. There are always three steps in a complete experiment. Beginners

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