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animals with their food. Even the meat-eating animals feed in the long run upon those that feed upon plants. How the plants manufacture this food and the relation they have to animals will be discussed in later chapters. Plants furnish man with the greater part of his food in the form of grains and cereals, fruits and nuts, edible roots and leaves; they provide his domesticated animals with food; they give him timber for his houses and wood and coal for his fires; they provide him with pulp wood, from which he makes his paper, and oak galls, from which he obtains ink. Much of man's clothing and the thread with which they are sewed together come from fiber-producing plants. Most medicines, beverages, flavoring extracts, and spices are plant products, while plants are made use of in hundreds of ways in the useful arts and trades, producing varnishes, dyestuffs, rubber, and other useful products.

Bacteria in their Relation to Man. In still another way, certain plants vitally affect mankind. These tiny plants, so small that millions can exist in a single drop of fluid, are called bacteria or germs. Existing almost everywhere about us, in water, soil, food, and the air,— they play a tremendous part in shaping the destiny of man on the earth. They help him in that they act as scavengers, causing things to decay; they help make cheese and butter; they assist the tanner; and the farmer could not do without them; but they likewise spoil our meat and fish, and our vegetables and fruits; they sour our milk, and make our canned goods spoil. More than this, they cause diseases, among others tuberculosis, a disease so harmful as to be called the "white plague." Fully one half of all yearly deaths are caused by these plants. So important are the bacteria that a subdivision of biology, called bacteriology, has been named after them, and hundreds of scientists are devoting their lives to the study of germs and their control. The greatest of all bacteriologists, Louis Pasteur, once said, "It is within the power of man to cause all parasitic diseases (diseases mostly caused by bacteria) to disappear from the world." His prophecy is gradually being fulfilled, and it may be the lot of some boys or girls who read this book to do their share in helping to bring this condition of affairs about.

The Relation of Animals to Man.

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Animals also play an im

portant part in the world in causing and carrying disease. Animals that cause disease are usually tiny, and live upon other animals as parasites; that is, they get their living from their hosts on which they feed. Among the diseases caused by parasitic animals are malaria, yellow fever, the sleeping sickness, and hookworm disease. Animals also carry disease, especially the flies and mosquitoes; rats and other animals are also well known as spreaders of disease.

From a money standpoint, animals called insects do much harm. It is estimated that in this country alone they are annually responsible for $800,000,000 worth of damage.

The Uses of Animals to Man. We all know the uses man has made of the domesticated animals for food and as beasts of burden. But many other uses are found for animal products, and materials made from animals. Wool, furs, leather, hides, feathers, and silk are examples. The arts make use of ivory, tortoise shell, corals, and mother of pearl; from animals come perfumes and oils, glue, lard, and butter; animals produce honey, wax, milk, eggs, and various other commodities.

The Conservation of our Natural Resources. Still another reason why we should study biology is that we may work understandingly for the conservation of our natural resources, especially our forests. The forest, aside from its beauty and its healthgiving properties, holds water in the earth. It keeps the water from drying out of the earth on hot days and from running off on rainy days. Thus a more even supply of water is given to our rivers, and thus freshets are prevented. Countries that have been deforested, such as China, Italy, and parts of France, are now subject to floods, and are in many places barren. On the forests depend our timber, our future water power, and the future commercial importance of cities which, like New York, are located at the mouths of our navigable rivers.

Plants and Animals mutually Helpful. The study of biology also shows us the interrelation existing between plants and animals on the earth. Most plants and animals stand in an attitude of mutual helpfulness to one another, plants providing food and shelter for animals; animals giving off waste materials useful to plants in the making of food. We also learn that plants and

animals need the same conditions in their surroundings in order to live water, air, food, a favorable temperature, and usually light. We learn that the life processes of both plants and animals are essentially the same, and that the living matter of a tree is as much alive as is the living matter in a fish, a dog, or a man.

Biology in its Relation to Society. — Finally, the study of biology should be part of the education of every boy and girl, because society itself is founded upon the principles which biology teaches. Plants and animals are living things, each taking what it can from its surroundings; they enter into competition with one another, and those which are the best fitted for life outstrip the others. Health and strength of body and mind are factors which tell in winning. The strong may hand down to their offspring the characteristics which make them the winners.

Man has made use of this message of nature, and has developed improved breeds of horses, cattle, and other domestic animals. Plant breeders have likewise selected the plants or seeds that have varied toward better plants and thus have stocked the earth with hardier and more fruitful domesticated plants. Man's dominion over the living things of the earth is tremendous. It is due to the understanding of the principles which underlie the science of biology.

II. THE SURROUNDINGS OR ENVIRONMENT OF LIVING

Environment.

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A plant or an animal living on the earth may said to come in contact with air, water, and soil. It may be influenced by light, varying conditions of temperature, of the atmosphere or water, the presence or absence of food materials, and some other things. We shall later see that the sum total of these various factors, acting upon the living thing, may cause great changes to take place in the structure or habits of a plant or animal. The surround

ing forces which act upon living things form their environment. In order better to understand what a living plant or animal takes from its environment, we must find out something about the air, water, and the soil, for it is with these factors that the plant and the animal are in immediate contact.

Problem I. A study of the common elements in the environment of living things. (Laboratory Manual, Prob. I.)1 (a) Nitrogen.

(b) Oxygen and oxidation.

(c) Hydrogen.

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1 Sharpe, A Laboratory Manual for the Solution of Problems in Biology, American Book Company.

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space occupied by the air in the jar. When the water reaches this height, it goes no higher, and, no matter how many times the experiment is repeated, the phosphorus invariably goes out when the water displaces one fifth of the air in the jar.

Evidently, the burning of the phosphorus uses up some gas within the jar, which supports the flame, and the gas which remains in the jar, occupying about four fifths of the space, does not have the power of maintaining the flame. The former gas is called oxygen; the latter, nitrogen. These two gases form the principal constituents of the air in the proportion seen in the experiment.

Chemical Elements. All the materials of this universe, both living and lifeless, are classified by chemists as either chemical elements or chemical compounds. A chemical element is a substance which has never been decomposed into anything simpler in composition. Examples of such elements are oxygen, making up about one fifth of the atmosphere; nitrogen, composing nearly all the remainder of pure air; carbon, an element that enters into the composition of all organic matter; and over seventy others of more or less importance to us in the study of biology.

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Nitrogen. The physical properties (those which we determine through our senses) of nitrogen are its lack of color, taste, and odor. Its chief chemical characteristics are its inability to support combustion and its slight tendency to combine with other substances. We shall later find that nitrogen is one of the most important chemical elements found in living matter. In spite of this, animals and most plants are absolutely unable to take any nitrogen from the air, no matter how much they may need it.

The other element in the air, oxygen, is taken out by the plants and animals. We shall be able to see how, after studying the properties of oxygen.

Preparation of Oxygen. Elements and Compounds. - Oxygen may be prepared by heating half a teaspoonful of chlorate of potash with a little less than its bulk of black oxide of manganese in a test tube over a Bunsen flame or a spirit lamp. After a moment a glowing match inserted in the mouth of a test tube bursts into a bright flame. Evidently the match burns more brightly because of the presence of a gas which has been loosed from the materials in the test tube. These materials are chemical

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