PREFACE

ALL the activities of a plant, of an animal, or of man may be grouped in three classes. One class embraces the functions relating to the life of the individual organism. These functions have to do with the processes of eating, digesting, assimilating, taking in of oxygen, producing of energy, and excreting of waste matters. These may be called the nutritive functions, if the term is used in its broadest sense. To the second group of activities belong the functions that have to do with the perpetuation of the animal or plant species, and these are known as the reproductive functions. Living organisms, whether plant, animal, or human, may, in the third place, be considered in their relations to one another and especially to the general welfare of mankind. Thus we may discuss the beneficial or injurious effects, so far as man is concerned, of different kinds of insects or of various types of bacteria; we may learn of the activities of individual men or of groups of individuals which promote or retard the advance of human society; or we might, if we were to carry the study still farther, even seek to learn the ways by which the higher thoughts of mankind, as expressed in poetry, music, and religion, affect the development of the human

race.

In the preparation of this text, the authors have sought to keep continually in mind these three classes of activities, and to unify the study of plant, animal, and human biology by choosing those topics for laboratory work or text description that have to do in a broad sense with one or the other of the three great groups of functions of living things to which

we have just referred. In doing this, they are conscious that many subjects have been slighted or altogether omitted which might well be treated in a year's work in either botany, or zoology, or human physiology.

Again, in the treatment of a given subject, for example, stems, fishes, or circulation, special emphasis might be laid on structure, on function, or on the relation of the given topic to human life. Books both interesting and scientifically worth while could be prepared along any one of these lines, or, if time permitted, all three phases might be equally emphasized. But when we remember that less than two hundred school periods will probably be devoted by the average student to the study of biology, the necessity for adhering pretty consistently to some one plan is obvious.

In the judgment of the authors the kind of biology most worth while for the average boy or girl of fourteen years of age is not one based primarily on structure. Young students are naturally more interested in activities or functions than they are in mere form or structure. Hence, if we wish to work with, rather than "against the grain," we must put function in the foreground of our discussion. Every boy and girl knows, too, that both plants and animals as well as human beings must have food and drink, and that they grow and reproduce their kind. It is relatively much easier, therefore, to unify a course like this along physiological lines than on the basis of morphology, or of homologies of structure, many of which are far too complicated to be made clear to young students.

If properly outlined and presented, there is probably no subject in the school curriculum that can be made of more service to a growing youth than can biology. Biological problems confront him at every turn, and if he is a normal being, he will have asked himself question after question

which an elementary knowledge of biology ought to help him to answer. Some of these questions may be the following: Whence comes the food and oxygen supply used by man? Why are food and oxygen needed in our bodies? Why are some substances beneficial to the body and others injurious? What is the cause of disease, and how is disease transmitted? And if we were to tabulate the biological questions that occur spontaneously to the average pupil in the first year in the high school, we should doubtless find that a great proportion of these questions had to do with the relation of the living world to human life. Is it not clear, therefore, if we are to outline a course in biology that will best fit the interests of the "live material," i.e. the boy or girl who is to take the course, that the central idea or factor must be man; that all the various functions considered must have some relation to human life; and that the course, to be of practical importance, must suggest to the youth better ways of carrying on his own life and of helping to improve the surroundings in which he lives?

In order, however, to treat intelligently such a function, for example, as respiration or digestion, it is of course necessary to know something of the machinery by which each of these processes is carried on, and so there must be at least a minimum consideration of the structure of plants, animals, and the human body. In every case, however, the authors have called attention only to those details which seem to be absolutely essential for an interpretation of the function under consideration. Whenever names in common use are sufficiently accurate for descriptions, these are chosen in preference to scientific terms. Frequently the latter are necessarily used, and so, whenever their meaning is made clearer by referring to their derivation from Latin or Greek, these derivations are indicated in parentheses.

The sections in coarse type contain the material that seems to the authors most essential for any clear understanding of the subject as a whole, while in fine type we have put additional laboratory work and text description which we believe to have an important bearing on the various topics discussed. If both coarse and fine print on animal, or plant, or human biology are used, sufficient material for a half-year course in either elementary botany, zoölogy, or human physiology will be provided.

In the judgment of the authors, plant biology should always be considered first and human biology last in the course for the following reasons: (1) Plants lend themselves far more readily to close observation and especially to experiments than do animals, and so fundamental processes which apply to all living things can be demonstrated scientifically from plant material. (2) Plants are the final source of all the food supply of animals and man, and if the composition and manufacture of the nutrients are taught early in the course, a solid foundation is laid for all subsequent study of nutrition in animals and man. (3) The purpose of the animal study is largely that of showing the adaptations of animal structure to functions and the relations of the animals studied to human welfare. (4) And finally, if human biology comes last in the course, it may be presented in such a way as to review, sum up, and give real significance to many of the facts learned earlier in the course. In fact, as the work proceeds, comparisons will constantly be made between plants, animals, and man to show that the essential differences in the three kinds of organisms consist not in the differences in the functions which they carry on, but in the organs by which the functions are performed.

So far as the order of individual topics under plant, animal, and human biology is concerned, the instructor should