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I wish to acknowledge the courtesy of the Macmillan Company of New York, Macmillan & Co. of London, Henry Holt and Company, P. Blakiston's Son & Co., the Open Court Publishing Company, and the Editor of the Journal of Heredity for allowing me to reproduce figures from their various publications.

S. J. HOLMES.

University of California,

Berkeley, Calif.

CHAPTER I

THE LIVING AND THE NON-LIVING

Biology is the science of life. In its broadest sense it includes all the knowledge that has been amassed concerning living things. Botany, which deals with plants, and zoology, which treats of animals, are therefore subdivisions of the broader field of biology. Through one of those curious coincidences that sometimes occur in the history of science, the word “biology” was coined independently and in the same year, 1802, by two naturalists, Lamarck and Treviranus, in order to have an appropriate designation for the general science of living organisms. Sometimes the term “biology” has been used to signify knowledge of the habits and adaptations of organisms, but in the present volume it will be employed in its wider and more usual meaning

It is much easier to define the science of life than it is to define life itself, and there is probably not much use in attempting a definition of life at the present stage of our progress. Most of us know pretty well what we mean by this term, even if we find difficulty in properly expressing our meaning in words. In his Leçons sur les phénomènes de la vie, which is one of the classics of biological literature, the great French physiologist, Claude Bernard, contends that our efforts to define life are quite as futile, and add as little to our knowledge, as our endeavors to define space, time, matter, and many other common phenomena of our daily experience. We may find it of interest to consider some definitions of life later on, but for most practical purposes we can get along, without the least confusion, with the commonsense notion of the subject which every one has. No one is likely to confuse a dead cat with a living one, at least for long. We may be deceived for a time by an insect that is feigning death, but our doubts are immediately dispelled when the creature begins to use its legs. We may not be able to tell at first whether a seed is alive or dead, but if it develops into a plant, we no longer doubt. It is still more rarely that we confuse animate creatures with the so-called inorganic bodies which have never enjoyed the experience of living. There is a story of a Scotch soldier who picked up a watch and concluded from its movements that it must be alive. With the characteristic thrift of his people, he disposed of it soon afterward and remarked that "he was right glad to get rid of the creature, for she died no long time after he caught her.” But even if the soldier had never seen or even heard of a watch before, he must have been a very stupid sort of person not to have recognized it as a mechanism of human contrivance.

The distinction between the living and the non-living is one of the most fundamental and easily recognizable that is presented by the world in which we live. There is, however, a good deal of debate among scientists and philosophers as to just how fundamental this distinction really is, Many classes of things in this world, although separable without the least difficulty in ninety-nine cases out of a hundred, nevertheless grade into each

a other, or present differences here and there from the usual condition, so as greatly to perplex us when we try to define them with absolute precision. We are not apt to mistake an animal for a plant when we are dealing with their higher representatives, but when we study the simplest plants and animals, the distinction, as we shall find later on, is by no means always easy to make. And although we are rarely mistaken in telling the living from the non-living, we find it much more difficult to set up any absolute criteria by which they may be distinguished. This truth will become more apparent, I think, when we pass in review some of the outstanding differences between living and dead things. The great English biologist, Professor Huxley, specified as the distinctive properties of living matter: (1) its

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