m jj m fellow seed parent Green seed parent First hybrid generation All yellow seed Second hybrid generation Three yellow, one green The Colorado Potato Beetle Leptinotarsa decemlineata V..X/ \ .^i ^ ./ ? /W^ j Parent type \ / Variety torfuosa Potato Beetle Leptinotarsa multitceniata FT FA TARV RI EXPERIMENTS IN EVOLUTION Mendel found that when pure-bred green-seeded peas were crossed with pure-bred yellow-seeded peas, the offspring were all yellow-seeded. When these hybrid plants were crossed with one another, the following generation produced three yellow-seeded plants to one green-seeded plant. Tower found that exposing the young stages of potato beetles to extreme conditions of moisture and temperature produced in the following generations modifications that were inherited. The results of high temperature and low humidity are illustrated at the left of the parent type ; the results of low temperature and high humidity are illustrated at the right >V3 KI 6TVI3M 15131X3 jatg bsid-aiuq narfw ted* fanool ;q by[>938-w6ii3Y bsid-aiuq riiiw b' . sno rtliw b^gaoiD aiaw aJnfifq >rfi nariW . ol aabosd oJBJoq lo ^&\ m b^3"a Mp 1 P&l#&l£i HvHro- SCIFe Oxygen 65% : :;.;. Carbon.-; :.•:; gen mm ! ® £§ti 10% i FIG. i. The chemical composition of the human body In addition to the elements named there are nitrogen (N), calcium (Ca), phosphorus (P), potassium (K), sodium (Na), magnesium (Mg), sulfur (S), chlorin (Cl), iron (Fe), and traces of iodin, fluorin, and silicon shall find that, of the seventy-five or eighty elements that have been described by the chemists (see p. 9), from twelve to fifteen are found in the bodies of very nearly all plants and all animals. There is nothing in this list of elements that distinguishes living bodies. Each of these elements occurs in the soil ; some of them occur in the waters of the oceans and lakes; and a few are found in the air. Organic and inorganic. Yet there is an important difference be- tween living and non-living things on the chemical side. Although the elements of the living body are the same as the elements of the LIVING THINGS AND NON-LIVING THINGS 17 environment, they are combined in ways that are peculiar to living things. Certain compounds are found in nature only in the bodies of plants and animals. Some of the more common of these are sugars, starches, fats, albumins, certain pigments, and woody and horny sub- stances. These substances are not themselves alive, for we find them also in the dead bodies of organisms. Formerly substances that could be obtained only from organisms were called organic and were distinguished from other substances occurring in nature, which were called inorganic. The chemists have succeeded, however, in producing large numbers of compounds that naturally occur only in plants or animals, so that this distinction is not used so much to-day. Since the forms and the structures of organisms remain about the same after death, and since probably most of the compounds remain the same, we must find other distinctions between living and non-living. 26. Growth. The fact of growth is universal for living things. This does not mean that every living body is con- stantly growing ; it means that every organism is capable of growth at some time during its life, or that parts of the body are capable of growth. Yet the crystals of many substances also grow, some of them very rapidly, so that we can actu- ally see them grow. Most of us have seen icicles grow. If by growing we mean becoming larger, then crystals and icicles grow just as truly as caterpillars or babies. What, then, is the real difference between the two kinds of growth ? When an icicle becomes larger it does so through the addi- tion of new layers of ice-stuff (water) on the outside. The growth of a crystal proceeds in the same way. A baby, how- ever, does not grow in this manner, (i) The baby grows not by the addition of baby-stuff from the outside, but by the addition of different kinds of stuff — such as cow-stuff (milk), or hen-stuff (eggs), or wheat-stuff (bread). (2) The growth material is not added on the surface, but is taken in. (3) The new material does not remain the same kind of stuff, but undergoes chemical changes and becomes at last baby-stuff. 1 8 ELEMENTARY BIOLOGY (4) The growth of the body goes on not merely by the exten- sion of the surface ; it takes place in all parts at once, inside parts as well as outside parts growing. 27. Assimilation. These differences between the two kinds of growth may be summarized by saying that the icicle grows by accretion, that is, by the adding of material to the outside, whereas the baby and other living things grow by assimilation, that is, through the conversion of foreign material into mate- rials of the body — the " making alike " of stuff that is different. 28. Movement. Most of the animals that we know are capable of moving about and of moving their parts. Many non-living objects also move, as the clouds and the waves. But these objects do not move because of anything that takes place inside ; we recognize that they are being pushed about by outside forces. An examination of living plants and animals shows us that there are movements going on inside the organ- ism, and we can see that some of these inside movements result in the movements outwardly visible, 29. Irritability. A very striking and interesting character- istic of living things is their apparent sensitiveness to outward changes, or irritability. We ourselves perceive lights and colors, sounds, odors, tastes. The movements of the familiar animals show that they are disturbed by much of what happens about them, in a way that is different from the disturbance caused to a cup when it is dropped. A dog does something when he is hurt ; your eye does something when a sudden flash of light is presented ; even a geranium plant changes its behavior when placed in a sunny window. This sensitiveness of living things is in some ways the most remarkable fact about them. Yet we shall find that sensitiveness is not altogether con- fined to living things. There are certain chemical compounds that are in some ways even more sensitive than plants and animals. Some compounds are so sensitive to mechanical dis- turbance that they will produce a violent reaction when they are dropped — as in the case of dynamite. This substance is LIVING THINGS AND NON-LIVING THINGS 19 sensitive also to heat ; if a hot poker is applied to a stick of dynamite, the results are said to be more disastrous than the consequences of poking a vicious dog. 30. Fitness. There is one respect, however, in which the sensitiveness of living things differs from the 'sensitiveness of non-living things. In most cases the living body responds to a disturbance by doing something that will probably save it from further injury. The non-living body, when sufficiently disturbed to do anything, does something that generally results in its further injury or destruction. Thus, when a dog's tail is pulled,, he will try to run away, or he will bark or snap at the " thing- holding-tail." These responses are, on the whole, of a kind that will save him from further damage. Indeed, we cannot imagine how living beings would continue to live generation after generation if they had the habit of doing things that tended to injure or destroy them. In contrast to this kind of behavior, think of what the stick of dynamite would do if touched with a red-hot poker. There is nothing here that looks in the least like "trying-to-save-itself." 31. Origin. We do not know anything about the first appear- ance of life upon the earth. But we do know that every plant and animal now living had its origin in the body of some other plant or animal. In general, non-living bodies do not reproduce each other, but, so far as we know, living things can be produced only by other, similar, living things. 32. Summary. We have seen that growth, movement, and irritability of a certain kind may be present in non-living bodies, but in no case have we found any non-living thing that has all of these properties. Some have one, some another. Living things are characterized by having all three. We may say that it is the combination of these properties that distin- guishes living bodies from the non-living and from the dead. CHAPTER V THE LIVING STUFF 33. Plants and animals. We have spoken of plants and animals as "living things" ; yet animals seem to most people to differ from plants about as much as they do from non-living things. Plants are just as much alive as animals are. They are just like animals in those very points that distinguish living things from non-living. That is to say, they are capa- ble of growth, they are capable of movement, and they are irritable, or sensitive to various kinds of disturbances — just as animals are. And each organism originates from some other organism. Yet it is true that there are differences between plants and animals. In the matter of growth, plants are even better growers than animals, taking both classes as a whole. This means that generally a ten-pound plant can grow into a twenty- pound plant more quickly than a ten-pound animal can grow into a twenty-pound animal. But there are great variations in the rate of growth among animals as well as among plants. We may also say that in general the plants use up a larger share of their total income for growth, while animals use up a larger proportion of their income as fuel ; that is, more of an animal's income is oxidized, releasing energy in the form of heat or of motion. In the matter of sensitiveness, also, the animals seem, in general, to be ahead of the plants, although we shall find that there are some extremely sensitive plants and some extremely unresponsive animals. It is difficult to see why, in spite of all the differences, plants and animals should still be so much alike. How is it THE LIVING STUFF 21 that bodies organized in such very different ways come to be so much alike in those three points that are said to distinguish living things from non-living ? 34. Protoplasm. The answer to this question is to be found in the fact that in the bodies of all organisms there is a peculiar substance (or rather a mix- ture of substances) which seems to have all the qualities of living bodies. This seems to be the stuff that can grow ; this is the stuff that moves ; this is the stuff that is irritable. When seen under the microscope this living stuff seems to be a slimy, or jellylike, substance — something like the white of egg in appearance. Under a more powerful microscope it sometimes ap- FIG. 2. Protoplasm moves Pears to have man7 minute bubbles in The arrows indicate the stream- k or to consist of an extremely fine ing of the protoplasm within network. This stuff is called proto- the cells . plasm, and in all essential respects it seems to be alike in all plants as well as in all animals. It is the protoplasm of a plant or of a kitten that grows. It is protoplasm in the body of the Venus's FlG- 3> Diagram of a cell flytrap Or OI a Snake that moves when the organism springs upon its victim. It is the protoplasm of the geranium or of the worm that is sensitive to the light. mass Of the cell content consists of the protoplasmic network, t^ie coarser-grained nucleus. Within the protoplasm are more solid bodies, and droplets of more liquid substances 22 ELEMENTARY BIOLOGY FIG. 4. Various kinds of animal cells /,-flat epithelial cells, like those lining the cavity of the abdomen in man and other animals ; 2, co- lumnar epithelial cells, like those lining the air pas- sages, with hairlike projections of protoplasm, called cilia \ 3, muscle cells, unstriped, like those in the walls of the intestine and of blood vessels; 4, shapeless cells of naked protoplasm, like those of Ameba or of white blood corpuscles; j, cells con- taining fat globules, like those in adipose tissue ; 6, bone cells surrounded by hard deposits of limy material ; 7, a nerve cell, or neuron (a, the cell body with its branching outgrowths, or dendrites; b, the longest outgrowth, the axon, ending in c, the terminal branches) 35. Cells. It has been known for a long time that the body of every plant and every animal is made up of a large number of tiny lumps of protoplasm, each of which is shut off from its neighbors by a more or less definite mem- brane, or wall. A single bit of protoplasm with its wall is known as a cell. This name suggested itself to those who first studied the structure under the microscope, because of its resem- blance to the cells of a honeycomb. When we look at a living organ- ism, we do not see the protoplasm ; we see the walls of thousands of these cells. In the larger plants and animals the outer layers of cells are usually quite dead— that is, the protoplasm is no longer present, only the dead wall re- maining. This is true of our own skin, of the bark of trees, and of the hide of the horse. THE LIVING STUFF With the aid of a microscope we can easily make out the forms of many kinds of cells taken from the bodies of plants and animals. We may note that cells of differ- ent kinds differ from each other not merely in size but in shape as well. Some cells have thicker walls, some thinner walls. Some seem to have various kinds of solid bodies floating about within the covering ; others have few or none of these. Some have smaller and some larger bubbles of clearer liquid. In some plant cells the protoplasm can be seen to move about. The cells of certain water plants are espe- cially favorable for show- ing this (Fig. 2). 36. Nucleus. There is one special portion of the protoplasm that deserves particular no- 6 FIG. 5. Various kinds of plant cells /, epidermal, or skin, cells of a leaf, showing the outer wall greatly thickened, and the cuticle ; 2, co- lumnar cells, like those of the palisade layer of a leaf pulp ; 3, moving ciliated cells, like those of typhoid bacilli; 4, swimming spores of a water mold ; j, budding cells, like those of the yeast plant ; 6, guard cells inclosing a breathing hole, or stomate, on the surface of a leaf; 7, a pollen tube growing out of a pollen grain tice. Near the center, or off to one side, we can generally find a por- tion of the protoplasm that seems to be denser than the rest. This is called the kernel, or nucleus. Because of the transparency of the protoplasm it 24 ELEMENTARY BIOLOGY may be difficult to distinguish the parts in many plant and animal cells. It has been found convenient to stain masses of cells with various kinds of pigments or dyes, to make the structure stand out more distinctly under the microscope. When certain dyes are used, the nucleus becomes particularly distinct, since it absorbs these dyes more readily than do other parts of the cell. And within the nucleus we can sometimes see fine little rods or strands (Fig. 3). 37. Numbers of cells. The cells that you have seen under the microscope may have suggested the question whether a body has a definite number of cells. Most plants and animals that you have seen probably have indefinite num- bers, and these run into the countless millions. There are some living things, however, that have a very definite and limited number of cells. One of the simplest animals is the one-celled ameba, which lives in stagnant pools and other wet situations. Under the microscope it appears to be an irregular lump of jellylike matter, in which various granules and bubbles can be made out. There is a nucleus, and all around it movements are constantly taking place. The shape of the mass of naked protoplasm is constantly changing, resulting in sluggish movements of the animal. The slimy mass swallows particles that may serve as food, and it crawls away from contained particles that are no longer of service. The animal is sensitive to physical and chemical forces in the environment, and responds to disturbances by contractions of the protoplasm. 38. Tissues. In the bodies of the plants and animals large enough to be seen without a microscope, there are usually many different kinds of cells. Masses of similar cells together constitute what is called a tissue. Thus, in our own bodies there are muscle tissue, bone tissue, brain tissue, gland tissue, connective tissue, and other tissues. In the body of an ordinary plant we may recognize bark cells, wood cells, pith cells, skin cells, and other cells (see Figs. 4, 5). CHAPTER VI THE CONDITIONS OF LIFE 39. All activities dependent. We may imagine objects of all kinds existing by themselves, but we cannot imagine them doing anything except in relation to other things. The stars in space influence each other in their movements, and every- thing that human beings do depends upon the conditions under which they live. In order to discover the relations of the outside conditions to the activities of a living being, particularly of a plant, we may begin with the characteristic changes that take place in passing from winter to spring. In the winter most of the plants of the preceding season are dead, and those that are not dead are, with comparatively few exceptions, either bare of all foliage or reduced to one of several kinds of "resting states." There are roots and stems lying dormant under- ground, and there are millions of seeds that look as lifeless as pebbles — until circumstances favorable to life activity appear. 40. Sprouting of seeds. How is it that the seed sprouts in some cases, and not in others ? Seeds of many different kinds are kept in boxes or jars for months at a stretch, or even for years, and there is no sign that any of them has sprouted ; yet if some of the seeds are placed in the earth, many of them will sprout in a few days. Just because the gardener or farmer places his seeds in the ground, and they then sprout, we are likely to jump at the conclusion that the soil somehow causes the seeds to begin their active growth after their long rest. But this is not a sound conclusion. The soil is a mixture of many kinds of stuff, some of which may have something to do with the sprouting, and 25 26 ELEMENTARY BIOLOGY others of which may have nothing at all to do with it. In order to find out just what it is that causes the sprouting, we must consider the effect of each of the various factors of the seed's surroundings by itself. 41. The environment. Now, in what ways do the conditions surrounding a seed in the ground differ from the conditions in a box or a jar ? There may be a difference as to temperature,, or as to the air, or as to the amount of water, or as to the light, or as to some of the chemical substances present in the soil. Experiments have been made with every one of these factors, and we also have a great deal of experience that will help us to answer this question in part. Most of us know that seeds kept in jars will not sprout, whether they are kept in the dark or exposed to light. It is therefore safe to conclude that putting seeds in the ground brings about their germination not on account of darkness, but on account of some other factor. We also know that seeds kept in a warm place and seeds kept in a cool place will both fail to sprout, as long as they are in our jars or boxes. The soil may be cooler than our storeroom, or it may be warmer ; but it is not this that makes them sprout in the ground. Per- haps the soil keeps some of the air away from the seed ; but filling a jar with seeds and closing it up tight will not make them sprout. So it cannot be the absence of air by itself, nor the presence of air by itself, that causes the seeds in the ground to germinate. If we consider the chemical substances present in the soil, our usual experience tells us nothing at all. Perhaps there are certain substances there that cause the sprouting. We might find out by trying some of them. The chemist can tell us what there is in the soil, and he can also prepare the different kinds of stuff in a pure condition. But if we place the seeds in boxes containing the various ingredients of the soil, such as sand, clay, and various salts, we shall find that none of the seeds sprout. The failure of the seeds to sprout under these THE CONDITIONS OF LIFE 27 conditions may suggest that the one or many substances that perhaps can cause sprouting would fail under the conditions of the experiment because the dry substances cannot get into the seeds. We should therefore try these substances in connection with water. That, however, at once raises the question whether water by itself has any effect on the sprouting of seeds. 42. Relation of water to sprouting. We should therefore proceed to experiment with pure water. An experiment in which some seeds are placed with various amounts of water, while other seeds from the same lot are kept under similar conditions of air, light, and temperature, but without water, will easily convince us that one of the conditions necessary for starting the germination of the seeds is the presence of a certain amount of water. We shall find also that some kinds of seeds will fail to sprout if they are completely covered with water, although other kinds will sprout under water. The seeds in the first class are not injured by water; the liquid simply prevents them from absorbing sufficient quantities of air. 43. Relation of temperature. It may be that other factors also play a part. For example, seeds in the presence of water may sprout at one temperature but not at another. From actual experience with seeds of different species of plants we know that some kinds may be safely sown earlier in the spring than others, and that some seeds will fail to sprout when it is too cold or too warm. By means of a systematic experiment in which groups of seeds with water are placed in a number of different places having different temperatures, we may satisfy ourselves that there is a limit in the range of temperature for the sprouting of every species of seed, and that there is a point at which the sprouting proceeds most quickly. 44. Relation of air. It may also be that the presence of water at a favorable temperature is not enough to cause the seeds to sprout. The air may perhaps influence the activity of the young plant after water is absorbed. Experiments may 28 ELEMENTARY BIOLOGY be planned to show whether, in addition to water, air also is necessary for sprouting. In the same way we can go on and try out the possible influence of light. 45. Summary. Since it is possible to get seeds to sprout without any soil at all, and without any of the ingredients of the soil other than water, it is safe to say that none of these ingredients is essential to germination. They may indeed be essential to the later growth of the young plant ; but that is another story. We may learn from these experiments that the sprouting of a seed depends upon an adequate supply of water, upon a supply of air, and upon the temperature remaining within certain limits. We may learn that the soil, in which most seeds do actually sprout, is not itself necessary for sprouting ; and that the light, which is really of great importance to life, has nothing to do with sprouting. CHAPTER VII AIR AND SOIL IN RELATION TO SPROUTING 46. Sprouting and transformation of energy. The fact that air is in some way necessary for sprouting suggests that the activity of the plant is in some way similar to the process of burning. Further experiments show closer resemblance — for example, the fact that it is the oxygen of the air that is con- cerned in both processes, and the fact that in both processes the transformation of energy results in the liberation of heat. Moreover, in both cases there is set free a quantity of an oxid — in this case carbon dioxid, as in the case of fires using car- bon or carbon-containing materials as the fuel. When we com- pare these three conditions with what we find in familiar animals, — our own bodies, for example, — we see a similarity that suggests the possibility of all living things carrying on the same fundamental process. And, indeed, it is proper to speak of the young plants in the sprouting seeds as " breathing," and to speak of the chemical changes going on inside the living matter of plants and of animals as " oxidation." There are very many different chemical processes going on in living things. Oxidation is only one of them. But it seems to be nearly universal, and it seems to be the one that makes avail- able to living matter the energy for its various other activities. 47. The soil and the young plant. We saw that seeds can sprout without depending upon the soil. Yet we know that the soil is essential to the growth of plants. This means that although the young plant in the seed is for a time independent of any soil materials, there comes a time in the course of its development when further growth is possible only on condition of receiving various substances from the soil. 29 30 ELEMENTARY BIOLOGY From experiments in which the various materials that make up soil (such as sand, clay, and the various salts) are used separately and in combinations, we learn that it is not the sandiness of the soil, or the color, or merely the water in it that makes the growth of plants possible. We find that it is something in the soil that can dissolve in water. 48. The salts of the soil. These soluble substances in the soil are the salts, of which there are many different kinds. Are all these salts related to plant growth, or only a certain few — or perhaps only one ? These questions have been answered by means of carefully planned and carefully conducted experiments. In these experiments plants were grown in solu- tions of soil minerals from which now one element and now another was omitted. It is found that the omission of some elements will absolutely prevent the further growth of the plants, whereas the omission of others will make no perceptible difference. From the results of such experiments the following table has been constructed : ELEMENT OCCURRENCE IN PLANTS SPECIAL FUNCTION Aluminum Calcium Chlorin Iron Magnesium Manganese Phosphorus Potassium Silicon Sodium Sulfur In lower parts In leaves and stem In lower parts In leaves and stem In seeds and leaves In lower parts In seeds In actively growing parts In stems and leaves In stems and roots In all growing parts No function. Related to the formation of plant cells ; " makes plants hardy." No function, so far as known, although present universally. Related to the formation of chlorophyl (see p. 54). Related to the formation of seeds. No function. Related to the activities of leaves ; takes part in the formation of proteins (see p. 56). Related to the formation of starch and sugar, and to the growing process. No special function. No function, although present almost universally. Necessary to the formation of proteins. AIR AND SOIL IN RELATION TO SPROUTING 31 This shows not only whether a given element is found to be necessary or not, but also in what particular way it is related to the life of the plant. 49. The composition of plants. Another method used for determining what there is in the soil that the plant depends upon for its activities has been to analyze the plant to find out of what it is composed. Such an analysis shows that certain elements are present in the plant body, and we know that some of these elements are present also in the soil. It is therefore reasonable to suppose that the plant derives these elements from the soil. It does not follow, however, that everything taken by the plant from the soil is of use to the plant. The most common elements found in plants are the following : Carbon Sulfur Potassium Oxygen Phosphorus Sodium Hydrogen Calcium Iron Nitrogen Magnesium Chlorin (Compare with the composition of the human body, Fig. I, p. 1 6, to see how much we are like the plants.) Other elements may also be found in some plants, as silicon and iodin ; but it is doubtful whether these are essential to the life of the plant. Indeed, not all of those given in the above list may be absolutely necessary, but most of them cer- tainly are. Since a large part of the plant's life consists of growing activity, the material for growth or for building up the body must be a first condition of life. The materials taken from the soil by the growing plant are some- times called plant food. Strictly speaking, these are not food, as we shall see later (see p. 50) ; they are merely some of the materials out of which plants manufacture their food. CHAPTER VIII SEEDS AND SEEDLINGS 50. The structure of seeds. On examining the outside of any seed we can usually find a scar that was left when the seed broke away from the little stalk by which it was fastened inside the fruit. Very often we can also find a tiny hole through the seed coat. This hole is called the micropyle. The seed may absorb water through this hole, but it does not seem to be of any importance in the mature seed. (See p. 302 and Fig. 134.) The coat of the seed, which sometimes has more than one layer, is apparently a protective covering, although in some species of plants the protection is furnished by the fruit in which the seed is borne. When the coat of a seed is removed, we find the part of the seed that is really important in the life of the plant. In fact, we may say that the^seed contains a young plant. The embryo is really a small, young plant; and we may say that the seed is a young plant (embryo) plus its protective covering. 51. The embryo. That the embryo is a plant can be seen from a careful comparison with the parts of any ordinary plant. Now, what are the parts of a plant ? Ordinarily we see above the ground only the stem and the leaves, but most of us know that under the ground is the root. In most plants the stem and the root are branching organs ; in some plants the leaves also divide or branch. All of the stem system, together with the leaves, we sometimes call the shoot. So we may say that the plant consists of root and shoot. But sometimes we find flowers on a plant, or fruit. The flower is really a special kind of shoot (see pp. 300 ff .), consisting of a very short stem with many special kinds of leaves crowded closely together, and with 32 SEEDS AND SEEDLINGS 33 certain other special organs that have to do with the making of seeds. One of these organs, when ripened, becomes the fruit. Now, in the embryo of a bean or a peanut or a pumpkin seed, it is very easy to find the parts corresponding to the root and the parts corresponding to the shoot. The two fleshy parts that make up the bulk of the embryo are really special kinds of leaves. If we bend them aside carefully in the embryo of a seed that has been soaked in water, without breaking them off, we can see that they are attached to a short stalklike piece. One end of this rod tapers to a point; this end corresponds 4 FIG. 6. Embryos of plants /, diagram showing relative positions of the parts of the embryo ; 2, embryo of peanut ; 3, embryo of pea; 4, embryo of pine ; C, C, cotyledons ; £, epicotyl ; //, hypocotyl to the root. The other end of the main stalk may be enlarged at the tip into a tiny knob or bud ; in the bean embryo we can make out two little leaves folded neatly over each other. This end of the stem corresponds to the shoot. The two fleshy leaves are called seed leaves, or cotyledons. The part below the meeting point is called the rootlet, or the radicle, or the hypocotyl, which means " below the cotyledon." The part above is called the first bud, or the plumule, or the epicotyl, which means " above the cotyledon " (Fig. 6). Although the cotyledons are considered to be leaves, they do not in all plants become flat and green like the more familiar leaves. In many cases they do not even come above the ground during the young plant's development, as in the pea plant. 34 ELEMENTARY BIOLOGY The function of the cotyledons seems to be confined in most cases to the holding of reserve food, which is drawn upon by the baby plant until it is developed far enough to get food for itself. In some kinds of seeds the cotyledons are very thin ; in such cases we usually find that there is a mass of food material packed in all around the embryo. A mass of food thus placed about the embryo is called the endosperm, which means " within the seed." The grains and the castor seed are good examples of seeds that contain endosperm (Fig. 7). When we compare the embryo of a grain, such as the corn, with the other embryos that have been mentioned, we find one great difference in the structure. The grain has but a single cotyledon. This is rather large, though not fleshy, and only the tip comes out of the seed covering as the first leaf. The base remains in contact with the endosperm and serves as an " absorb- ing organ," withdrawing food material from endosperm and transferring it to the growing plant. There are many plants, besides the grains, that have but one cotyledon in the seed. This fact would not seem to be of any great importance by itself, but it is connected with so many other characters, such as the veins in the leaves, the structure of the stem, the structure of the flower, and general habits of life, that we sometimes designate one of the main divisions of seed-bearing plants as the monocotyls, meaning the " one-cotyls," and another as the dicotyls, or " two- cotyledon " plants. Among the dicotyls are included most common weeds and cultivated plants, outside of grains. The seeds of the plants belonging to the pine family (fir, spruce, hemlock, etc.) have usually several cotyledons, and this family is accordingly designated as the polycotyls in some books, this name meaning " many cotyledons " (see Fig. 6). FIG. 7. Seeds with endosperms /, asparagus; 2, poppy; j, pine; 4, maize, or Indian corn. (All shown in longitudinal section) SEEDS AND SEEDLINGS 35 52. Food in seeds. The concentrated food found in the seeds of all plants is of interest to us in three ways : First of all, we may infer that this food is actually used by the young plant until such time as it is able to provide for itself. That this is a sound inference may be tested by separating from several seedlings the " food reserve." Next, we can observe that the cotyledons in such plants as the beans and peas do actually shrivel away as the plant becomes larger ; and that the _ K — » g FIG. 8. Young plants emerging from seeds On the left, squash; on the right, bean. In the squash a little outgrowth on the hypocotyl keeps the seed coat in place while the cotyledons are carried aloft. C, C, cotyledons ; E, epicotyl ; H, hypocotyl ; gg, ground line contents of the corn grain also disappear as the seedling de- velops. Finally, by means of chemical experiments we can see that the changes taking place in the " food masses " of the seedlings are of the kind we should expect to find if the food were actually being transported to the growing portions. (See p. 79.) A second question that may arise is that of the origin of the food which we find in the seeds. It is enough for the present to consider that, as the developing seed obtained the materials for its growth from the parent plant upon which it originated, the reserve food that we find within the coat of the seed was probably also obtained from the 36 ELEMENTARY BIOLOGY parent plant. How the parent plant makes its food we shall learn in the lessons on food-making (see p. 53). Another point of interest in regard to the food in the seed is that of its availability for human use. This will be discussed later (see Chapter XXII). 53. Seedlings. If we examine a few seeds that have been planted two or three days, we may see that the hypocotyl has emerged and is assuming the appearance of a root. At the other end of the embryo we may see the unfolding epicotyl. If we examine different stages of peas, squash, oats, corn, bean, and so on, we shall be able to see a great variety of methods by which the young plant crawls out of its covering and establishes itself in the soil (Fig. 8). Large seeds, containing a large amount of reserve food, are apparently at an advantage, since they may develop more root and more shoot before they are overtaken by the necessity of providing themselves with food. We should therefore expect that plants with large seeds would be, on the whole, more successful in establishing themselves in a new territory than plants with small seeds. We shall find, however, that the best spreaders in the plant world are those with rather small seeds. The speedy and secure establishment of the individual plant is of great advantage, but even more important is it that seeds be well scattered. And in this respect the small-seed plants with very numerous seeds have a decided advantage. CHAPTER IX EXTERNAL FORCES AND PLANTS 54. Gravity and growth. The force that acts most contin- uously upon living things is doubtless gravity. Temperature varies constantly, and the light is intermittent as well as vari- able. We do not know much about the relation of electrical conditions of the atmosphere to living things, and chemical conditions we can consider only as they arise in connection with particular kinds of substances. But gravity seems to be constant without regard to hours or seasons. It is therefore interesting to find how living things, and especially plants, behave in relation to this force. The question often occurs to people who have planted seeds, or who have watched others do so, Does it make any differ- ence which side of the seed falls uppermost ? We know that the lower end of the hypocotyl becomes root, and that roots usually live in the earth. What would happen if a seed were placed in the ground with its hypocotyl pointing skyward ? We can easily find out by means of experiments that permit us to watch the development of the young plant under condi- tions that make gravity act upon hypocotyls from different directions. Incidentally we can discover that the shoot of a plant is also sensitive to gravity, but that it responds in quite the opposite way from the root. That is to say, the shoot tends to grow away from the earth, whereas the root tends to grow toward the earth. 55. Tropisms. To many of us this sensitiveness of the plant will come as something unexpected, for we do not commonly think of plants as sensitive beings. The turnings that a plant or an animal shows in response to the one-sided action of 37 38 ELEMENTARY BIOLOGY some external force is called a tropism, from a Greek word mean- ing " to turn." The response to gravity is called geotropism, or " earth-turning." We may distinguish the behavior of the root and the shoot by calling the former positive geotropism, and the turning away from the earth, negative geotropism. 56. How the plant moves. The plant has no muscles, nor any structures that may be compared to muscles. The turning of the root or of the stem is not the same kind of movement as that which takes place when you turn your head or bend your body. The curvature is brought about by a growth. The shoot or the root grows more rapidly on one side, or the growth is stopped on one side, so that it grows in a curved line. 57. Light. That plants are sensitive to light is well known to all who have had an opportunity to observe either house plants or garden plants. A careful measurement of the growth of plants left in the dark, and of similar plants exposed to daylight, shows very definitely that withholding light from a plant accelerates its growth. But since darkness is a purely negative condition, it would seem that light actually restrains the plant's growth. This is so different from what we com- monly believe, that it is worth studying more closely. 58. Phototropism. Another response of plants to difference of illumination is shown when we leave them exposed to a one-sided illumination. Such an experiment will convince us that a plant is sensitive to light just as it is to gravity. The turning of a plant axis in accordance with the direction of the illumination is called phototropism. Most of our common plants are positively phototropic in the shoot, and somewhat negatively phototropic in the root. 59. The influence of water. The turning of leaves and stems toward the light and the turning of roots and stems according to the direction of the " earth's pull " are evidences of the living organism's irritability. It has been shown that the plant is also sensitive to various chemicals, and we can determine for ourselves that it responds to water. EXTERNAL FORCES AND PLANTS 39 These experiments must not be interpreted to mean that the roots somehow know that there is more water on one side, or that they have any way of choosing to go toward the water. .We may say merely that the plants are influenced in their behavior by these various external conditions. 60. Fitness. In the three sets of responses studied — namely, the responses to gravity, to light, and to water — we can see an advantage to the plant in behaving as it does. The tendency of the root to grow downward will, on the whole, bring the roots of the plants into the soil, where the conditions for getting water are more favorable than they are on the surface of the soil. We can see that the responses of the shoot to gravity and light are, in the long run, likely to bring the plant into situations favorable to its further development. But it does not follow that everything that the plant does is of advantage. We saw that light actually interferes with the growth of the plant, and yet, on the whole, the plant turns toward the light. Is not this response injurious and suicidal ? But we shall find later (p. 73) that the light is of great importance in the life of the plant in ways connected not with growth but with the making of food. Many of the responses of animals — even of higher animals, including ourselves — are just as mechanical as some of these simpler responses of plants. They are mechanical in the sense that they result from the structure of the organism, and do not involve anything in the nature of thinking or desiring or choosing. Like the responses of the plants, most of the ani- mal responses that we are likely to notice are of a kind that help the organism in keeping alive — for example, by prevent- ing injury or by helping in the obtaining of food. But among animals, as among plants, we can find responses that seem to be of no value whatever to the life of the organism, and some that are even injurious under certain circumstances. CHAPTER X ABSORPTION FROM THE ENVIRONMENT 61. All cells absorb. The surface of a young root is made up of cells packed so close together that even with the most powerful microscopes we are unable to see any breaks through which water can pass. Yet it cannot be doubted that water does pass through, and we may be sure that materials pass through the walls of the cells. 62. Diffusion. Illuminating gas and the vapors of odorous substances spread through the air very rapidly, by a process called diffusion. Diffusion takes place also in liquids. Dif- fusion represents a form of energy, since it is capable of overcoming gravity, as we can see in the fact that sugar or salt diffusing in water is actually lifted from the bottom of a vessel and distributed to all parts. This attraction between water and certain kinds of soluble substances helps us to understand what happens in roots as well as in other parts of living things. The substance of which the root's cell walls are made up is called cellulose. This substance cannot dissolve in water, but it can absorb water in much the same way as glue or gelatin does. Now water can diffuse through cellulose, although the cellulose cannot dissolve or diffuse in water. Substances that can dissolve in water can thus diffuse through the cell wall. 63. Diffusion through a membrane. When different sub- stances dissolved in water are separated by a layer of cellulose or gelatin, they may diffuse through the separating membrane. Such diffusion is called osmosis. This process takes place in the walls of cells, since the watery liquid on one side of such a membrane is not the same as that on the other side. Thus 40 ABSORPTION FROM THE ENVIRONMENT 41 there is always a double current : some materials are always passing out of a live cell and other substances are passing in. In this way protoplasm receives from the outside its supply of water, salts, and food. And it is by this process that materials in the cell pass out. Gases as well as liquids diffuse through the cell walls. 64. Osmosis in living things. The cell wall of a root cell is seen to separate the protoplasm from, the surrounding soil water. Income through the root hair is therefore by diffusion through the cell wall, or by osmosis. But the protoplasm within the cell wall is not a uniform mass of substance. The surface layer of protoplasm, the "protoplasmic membrane," also offers obstacles to the free diffusion of liquids and gases in solution, so that osmosis takes place here also. Indeed, there are many substances that can pass through ordinary cell walls of plants, but that cannot pass through the protoplasmic membrane. Common sugar is an example of such a substance. Some substances diffuse in water more easily than others. Some of the solids with which we are acquainted do not dis- solve at all. Of the substances that dissolve in water and can diffuse, some will diffuse more quickly through cell walls than others — and some may not pass through at all. Of the sub- stances that can diffuse through cellulose, some can diffuse through protoplasmic membranes more quickly than others — and some cannot diffuse through such membranes at all. As a result of these differences, cells exposed to the same material surroundings may not be equally affected. Not only do living things absorb materials from the outside world by osmosis, but within the body of every plant and every animal consisting of many cells, materials may pass from cell to cell, or between cells and various body juices, by this process. CHAPTER XI ROOTS OF PLANTS 65. Structure of roots. We have already seen the general appearance of roots, in the seedlings of the plants we used for our earlier experiments, in the carrots, beets, and turnips used at home, in the roots of trees that have been pulled up to clear the ground, etc. The root hair is a single cell formed by the outward prolongation of one of the skin cells (Fig. 9). The root hairs are the actual absorbing organs. Each root hair lives but a short time, and then shrivels up. As the tip of the root grows on, new root hairs are formed. The older skin cells . of the root die, and their contents dry out. Together with the shriveled root hairs, these skin cells form a protective covering through which water does re 0 FIG. 9. The tip of a young root re, root cap ; h, h, root hairs not pass very readily. As the plant becomes older and uses up more water, the absorbing area of the root is increased by the formation of many side roots and by the branching of the roots. But it is always in the region near the growing tip of the main root and of the many branch rootlets that absorption takes place. 42 ROOTS OF PLANTS 43 66. Wood and bark. If we examine the root of a plant freshly removed from the ground, we shall find that there is a soft, easily broken outer layer covering a tougher central por- tion. This central part, running lengthwise in the root, is called the wood or the central cylinder. In a fleshy root like a carrot or parsnip we may distinguish the central cylinder from the bark, or cortex, in both a cross section and a longitudinal section. In very thin slices made lengthwise through the growing tip of a young rootlet we are able, with the help of a micro- scope, to see the character of the cells (Fig. 10). The cortical, or bark, layer can be distinguished from the wood layer by the fact that the cells of the former have about the same diameter in one direction as in another, whereas the cells of the central cylinder are considerably longer than they are wide, and their long diam- eter is parallel with the long diameter of the root. 67. Growing layer. The layer of cells lying between the wood cylinder and the bark is called the growing layer. It is only the cells of this layer that are capable of producing new cells by the process of cell division. The younger bark and wood cells are capable of increasing in size, but they cannot give rise to new cells. Growth in length is the result of the formation of new cells by a special growing layer near the tip of the root. e c FIG. 10. Diagram of root structure e, epidermis, or skin ; c, cortex, or bark ; g, cambium, or growing layer ; w, wood cylinder, consisting of fibers and vessels ; /, pith 44 ELEMENTARY BIOLOGY 68. Vessels and fibers. In the cortex, transportation of material probably takes place by diffusion from cell to cell. In the central cylinder, however, we can find that liquids are moved bodily through the long tubes or vessels that act as the main channels in the transportation of materials taken in by the root hairs. Through some of these tubes materials are also brought down from the stem to the growing layer of cells. In the central cylinder we can find that many of the cells, instead of forming ducts, become thick-walled and stiff. These " fibers " give the cylinder its toughness and rigidity. Bundles of fibers and vessels are sometimes called fibro-vascular bundles, the \sxmfibro meaning " of fibers," and vascular meaning " of vessels," or tubes (Fig. 10, w). 69. Forms of roots. The structure of roots is fairly uniform for different kinds of plants. But roots nevertheless appear in very many different forms, from the thin, stringy roots of grains to the massive fleshy or woody roots of beets or trees. These differences are found to be closely related, in many cases, to the conditions under which the plants live. Thus, fleshy roots are often associated with the biennial habit. In such plants as beets, carrots, and parsnips the first season of the plant's growth is spent in manufacturing food and deposit- ing it in the root. The next year comparatively little foliage is produced, but a stalk bearing flowers (which in turn develop into fruit, bearing seeds) uses up practically all the food that has been left over from the previous season. In contrast with this habit of life we find the plants that sprout, grow ifp into maturity, and die, all within one season. These annual plants have, as a rule, rather delicate, or fibrous, roots. Trees and woody shrubs, which continue to live year after year, develop massive shoots. Corresponding to this fact we may note that such plants also develop elaborate, strong roots. From this we may see that the structure of the root and its functions are closely related to each other and to the character of the plant. There is a connection, on the one hand, between ROOTS OF PLANTS 45 the structure of the root and the size of the plant that it anchors, and, on the other hand, between the size of the root and its food-accumulating, or its absorbing, activity (Fig. n). 70. Tap-roots. In many plants the main root continues to grow downward into the soil as long as the plant lives and as long as the tip of the root remains uninjured. Such a main descending root is called a tap-root. The fleshy roots that have ^ FIG. ii. Forms of roots 7,tap-root of dandelion ; 2, fibrous root of buttercup ; j, bundle (or " fascicled ") root of dahlia ; 4, fleshy root of beet been mentioned are all tap-roots ; and a number of trees, as certain kinds of maples, also produce tap-roots. When a tap- root is injured or cut off, some of the side roots turn and grow downwards, although in a few cases the tip of the tap-root, when not too much injured, can regrow a new tip and continue the main line of growth. 71. Root pressure. We have found that when osmosis takes place in a root there is likely to be an excess of movement in one direction. We should expect more to come into the 46 ELEMENTARY BIOLOGY root than goes out, since, on the one hand, we know that the growth of a living thing depends upon an excess of income over outgo ; and, on the other hand, we know that the soil water is less concentrated than the juices of the root. The stream of incoming material actually sets up a current of liquid that is forced from the root into the upper parts of the plant. FIG. 12. Sand dunes at Pine, Indiana The roots of the grass Calamovilfa longifolia bind together the grains of sand, gradually leading to the formation of larger and larger soil masses. The sand that has no plants growing in it is blown about by the winds. (From photograph by Dr. George D. Fuller) This can be seen in the flow of sap, as when the sugar maples are tapped for sirup in the spring, and it can also be shown experimentally. 72. Uses of roots. It is because of the habit of depositing food in their roots that many plants are of especial interest to us. Our common vegetable roots can be shown to contain a great deal of food, such as starch, sugar, and proteins. Although our fleshy vegetables contain from about 80 per cent to 90 per cent of water after the skin is removed, they are still worth ROOTS OF PLANTS 47 FIG. 13. Adventitious roots A leaf of bryophyllum removed from the stem will put forth adventitious roots and shoots from the notches on the edge, thus giving rise to new plants fodder for cattle. To some extent sources of drugs and flavoring materials, the most important are the extracts of the licorice root, the sassafras root, and the sarsaparilla root. Because of the close ad- hesion of the root hairs to the grains of sand in the soil, roots are very effective agents in binding the soil, enabling the latter to with- stand the eroding effects of water as well as of wind. For this reason certain kinds of grasses are sometimes planted on sandy strips, to prevent the complete re- moval of the sand by the winds. The hillocks formed by clumps of such plants may continue to enlarge for using for their other contents. In addition to the organic substances and the useful mineral salts that they contain, these vegetables have a relatively large bulk of cellulose, which is help- ful in stimulating the activities of the intes- tines (see p. 117). Fleshy roots are used in large quantities as roots are also used as Among the latter FIG. 14. Prop roots Near the base of the trunk the corkscrew pine (Pandamis) sends out prop roots in a manner similar to that of the Indian corn. (From photo- graph loaned by New York Botanical Garden) 48 ELEMENTARY BIOLOGY years, and to give protection to other kinds of plants until the earth has become compact (Fig. 12). Although roots do not generally put forth buds or shoots, the roots of some shrubs and trees — as certain willows, poplars, and hawthorns — do so, and can be used for propa- gating the species. In some plants the roots will form new shoots if the old shoot is completely removed or destroyed. On the other hand, roots frequently arise from stems or leaves, thus making pos- sible the propagation of plants by means of cuttings. Roots that originate in this manner are called adventi- tious roots. Most of our common house plants, and willows and other trees, can be propagated by keeping twigs in water or wet sand until roots appear, and then transplanting them into soil. If the leaf (or even a piece of leaf) of a begonia or of a bryophyllum be placed on damp earth or sand, tiny roots will be seen growing from various points along the edge in the course of a few days. In these species buds will also be produced, so that after a while we can separate small but complete plants from the leaf, and get these to grow into full-sized individuals (Fig. 13). Blackberry and raspberry bushes are frequently propagated by layering, which consists in bending the flexible stems out and burying the tips in the ground. Adventitious roots are formed on the covered portions, and, later, buds form new shoots. The old connecting stem is then cut away. A similar FIG. 15. Climbing roots The English ivy, like many other climbing plants, clings to its support by means of adven- titious roots that grow out all along the stem ROOTS OF PLANTS 49 process takes place naturally in the strawberry plant, whose creeping stems produce new roots and new tufts of leaves, so that in the course of a season a single plant may spread out and cover a large area. 73. Adventitious roots. At the lower joints of the stalk of Indian corn adventitious roots are formed very early in the life of the plant FIG. 1 6. The banyan tree (Ficus bengalensis} The adventitious roots from the horizontal branches finally attach themselves to the soil. By means of these roots the tree is able to spread over a large area, a single tree some- times extending over several acres of ground (Fig. 14). Adventitious roots usually grow from the stem (though sometimes from leaves), and are most frequently in the nature of sup- porting or anchoring organs. The climbing organs of the English ivy as well as of the poison ivy are adventitious roots (Fig. 15), and in some of the tropical tree-climbing plants the roots are very fully de- veloped as holdfast organs. The banyan tree of Asia puts forth adventitious roots from the horizontal branches (Fig. 16). CHAPTER XII WHAT FOOD IS 74. The material needs of protoplasm. Several classes of materials seem to be necessary to keep protoplasm working. Water, for example, which we have seen to be necessary for the sprouting of seeds as well as for the absorption of mineral matters by the roots of a plant, is a constant and necessary factor in the activity of live protoplasm. It is thus a necessary part of the income of every plant and every animal. Within the cellsj too, water makes possible the movements of the various substances, and their chemical action and reaction with one another. In the larger plants and animals the transfer of materials between various parts of the organism takes place through liquid media, — as blood, sap, bile, milk, — and these consist very largely of water. Certain minerals seem to be necessary parts of the income of living things. Some of these salts, through their chemical actions, appear to start other chemical processes, and are therefore called activators. Other salts, or elements, appear to modify certain chemical processes (just as the bromide used by the photographer makes the development proceed more slowly), and are called regulators. We have already seen that living things generally use oxygen in the course of their activities. 75. What food is. In addition to the water, oxygen, and various mineral salts, every living thing uses various substances as material out of which protoplasm is constructed by the process of assimilation, and it uses substances that can be oxidized within the cells and thus yield energy. Whether everything that an organism takes in from the outside is to be called food or not is altogether a matter of 50 WHAT FOOD IS 51 convenience. It is found less confusing to restrict the use of the word food to such substances as can serve as building material for protoplasm or as sources of energy through oxidation. 76. Food organic. Using the word food in this sense, then, we must notice first of all that food materials are found in nature only in the bodies of living things ; that is to say, they are organic, to use the older term. From a chemical point of view we may divide the foods into two main classes : those that contain nitrogen and those that do not. The foods of the first class are called proteins and are represented in our familiar supplies by such substances as albumen, or white of egg ; the curd or casein that is formed when milk sours ; and the gluten, or pasty substance, in wheat flour or bread. Similar nitrogen-containing substances are found in the cells of mus- cles and are called myosin ; others, found in the seeds of the plants belonging to the bean family, the Leguminosae, are called legumin. Of the non-nitrogenous foods there are two main divisions, the fats and the carbohydrates. The fats are familiar to us in such substances as butter, suet, lard, tallow, olive oil, cotton- seed oil, peanut oil, and others. The carbohydrates comprise all the sugars and all the starches. 77. Food functions. In dividing the foods into the two classes, nitrogenous and non-nitrogenous, we have at the same time separated them in accordance with their true relations to protoplasm. For the proteins are the foods that are necessary for the building of protoplasm ; the protoplasm may be said to consist fundamentally of protein. The fats and carbohydrates are important in living cells as fuel, or oxidizable material. We find accordingly that all seeds contain protein, some in larger proportions (beans, peas, lentils, for example) and some in smaller proportions. In addition to this, all seeds contain either fat (as. the castor bean, peanut, cotton seed, flax seed) or some carbohydrate (as the bean, cereals, date). 52 ELEMENTARY BIOLOGY 78. Summary. We may summarize the materials required by a living cell in this way : 1. Water. The chemical changes that distinguish living protoplasm can take place only in the presence of water. 2. Protein. Out of this, new protoplasm is constructed, resulting either in the growth of the cell or in the replacement of protoplasm that may have been destroyed. 3. Fuel foods. In addition to the protein oxidized in the cell there is usually some other material that is oxidized. Two classes of compounds commonly furnish this fuel : namely, (a) carbo- hydrates; (b*)fats. 4. Salts. Various mineral, or inorganic, compounds are necessary for maintaining the activities of protoplasm. These are of many kinds, although certain of the elements contained in these salts are used by all living protoplasm (see p. 31). 5. In the bodies of human beings and of other animals (and pos- sibly also of certain plants) peculiar juices are produced that have a direct influence upon the activities of cells. The ferments contained in these juices are just coming to be understood. It is sufficient for the present to note that they do affect protoplasm activity, and that some of them are necessary for certain cells. 6. Oxygen. Although this is not usually regarded as part of the food, it is an essential part of the income of every cell. It is the chemical union of oxygen with other substances that sets free the energy by which the protoplasm does all of its work. CHAPTER XIII THE ORIGIN OF FOOD 79. Organic foods destroyed. When proteins, fats, and car- bohydrates become assimilated, they are still available as food for other living beings. But when any of this material becomes oxidized, it is thrown out of the world of living things. The question may then be raised, If livirjg matter can continue to live only at the expense of other living matter, and if living matter is constantly being destroyed (oxidized), how can the total amount of protoplasm be maintained, to say nothing of the amount of live matter being increased ? 80. The making of organic food. It is obvious that some- where in the circle of feeding, new foods must be admitted into the world of living things from the world of non-living things. The answer to the question was found in the discovery that the green parts of plants are active in the manufacture of new organic foods. 81. A manufacturing process. The process by which organic materials are built up (chemically) out of inorganic materials may be compared to a manufacturing process. In every such process certain factors are essential. There must be (i) raw material, (2) tools or machines for working on the material, and (3) energy for driving the tools or machines. In addition to these factors, we can understand that there is (4) a main product and sometimes material left over, called "waste," or, better, (5) the by-product. 82. Factors in starch-making. What are the factors in the process of starch-making ? The raw materials used by the plant are found to be water and carbon dioxid. 53 54 ELEMENTARY BIOLOGY The machines or instruments directly involved are different from the machines with which we are familiar. Instead of having wheels or levers or other moving parts, these machines are chemical engines, each consisting of a lump of protein with some of the chlorophyl that gives the familiar plants their distinctive color. This chlorophyl is the tool, or transformer of energy, in the food-making process (see Fig. 17, and Fig. 23, p. 70). The chlorophyl-bearing particle is called a chloroplast. Carbon dioxid Oxygen Light from Milt T t 4- 1 1 4- 4 1 ^^ 1 1 c c c c c c ^^^ • H • H • H • Carbo- K" §* tChloro- H C H • • C H hy- cf. phyJ • • • c* H • • drate S IcJJ* I N. rl .^^ H • H • • H C ** ^H^ C H • H C • • Chlor(. JJ [ *H° Carb0' phyl vl? 'H.C y" HH HH HH HH HH HH \HX t T t t t T INI 1 1 I I 1 1 I 1 rm 4 I • Oxygen FIG. 17. Starch-making by chlorophyl We may think of photosynthesis as taking place in two stages : in the first the raw materials, water and carbon dioxid, are broken up into their constituents — carbon, hydrogen, and oxygen ; in the second these elements are recombined into carbohy- drates, and the surplus oxygen is set free. The energy for this chemical process is sunlight ; the transformations are brought about through the action of chlorophyl The energy for doing this work is the light from the sun. Although the work cannot go on at too low a temperature, it is the light that is used in the process, and not the heat. 83. Oxygen a by-product. The starch that is formed from water and carbon dioxid by the action of sunlight through chlorophyl contains the elements found in the raw materials — namely, carbon, hydrogen, and oxygen. In starch, as in most carbohydrates, hydrogen and oxygen occur in the same proportions as they do in water. The raw materials taken in by the plant therefore contain an excess of oxygen. This THE ORIGIN OF FOOD 55 element is given off in a free, or uncombined, state during the process of starch-making. 84. Sunlight and life. The process which we have called starch-making is really a group of processes. In some green plants starch is never found, and yet the transformation of carbon-hydrogen-oxygen materials by the sunlight, acting through chlorophyl, goes on in these plants. The common fact in all these processes is that some kind of carbohydrate (usually some kind of sugar) is produced. This process of carbohydrate formation is called photosynthesis, from Greek words meaning "light" (compare photo-grwpti) and "put together." In addi- tion to forming sugar, some plants have a way of condensing the sugar, shortly after it is formed, into starch grains (Fig. 17). Without going deeply into the chemistry of photosynthesis we may note that in the making of carbohydrate the energy of the sun- light has practically broken up a combination (CO2) that is ordinarily formed with the liberation of energy. That is to say, through the action of light, carbon and oxygen have become separated so that they are capable of again combining and liberating energy. Carbo- hydrate may thus serve as a source of energy by becoming oxidized, either in the bodies of living things or in a flame. We may thus see that all the energy that plants or animals use as a result of the oxidation of carbohydrates is derived from the sun's energy. There is more than poetry in the statement that every human act is a transformed sunbeam. 85. Origin of fats. All organic materials appear to be derived directly or indirectly from carbohydrates. It has been found that fats originate in the cells of animals as well as of plants, by a modification of starches or sugars. Fats are characterized by containing a large proportion of carbon and a small propor- tion of oxygen. The chemical process by which carbohydrates are changed into fats is not understood. 86. Origin of proteins. The foods of the third group, the proteins, consist of very complex substances. They all contain nitrogen, in addition to carbon, hydrogen, and oxygen. Some 56 ELEMENTARY BIOLOGY also contain sulfur, and some phosphorus. From careful studies of plants it is supposed that proteins are manufactured by certain cells when these are supplied with carbohydrates and salts con- taining the necessary elements. For example, nitrates contain nitrogen, which the plant can use, phosphates contain phos- phorus, sulfates contain sulfur, and so on. A green plant is therefore capable of manufacturing its own food if it receives, in addition to the water and carbon dioxid, a suitable supply of minerals from the soil. Many plants without chlorophyl, as certain kinds of molds, have also been shown to be capable of manufacturing proteins when supplied with carbohydrates and suitable minerals. CHAPTER XIV THE CHEMICAL CYCLE OF LIFE 87. The carbon cycle. An understanding of the behavior of green plants in relation to food-making shows us how closely the living things in the world depend upon each other. Let us take the case of carbon, which is an essential constituent of all living matter. The carbon in our bodies came from the proteins, fats, and carbohydrates which we ate. We obtained these either from the bodies of plants or from the bodies of animals. In the latter case they were still derived from plants, for the cows or pigs or chickens that we used as food got the carbon in their bodies from the plant food which they in turn ate. Now the plant gets its carbon from the carbon dioxid in the air. (Water plants can get carbon from the carbon dioxid dissolved in the water.) But what is the source of the carbon dioxid ? We saw (p. 12) that the proportion in the air is very small. A few warm, sunny days in August would enable the plants of this country to use it all up, and that would be the end of everything. But the winds are all the time stirring up the atmosphere, so that new supplies of this important material are brought to the plants ; and there are certain rocks — lime- stone and marble especially — that . are capable of yielding a small quantity of this gas when they decompose. But this amount is very small indeed when we consider what is being used up by the plants from hour to hour. There is, however, still another source. We have seen (p. 29) that all living things, while using up oxygen from the air, are at the same time throwing off carbon dioxid. Moreover, every fire throws off quantities of carbon 57 ELEMENTARY BIOLOGY Oxygen dioxid. This carbon dioxid then becomes available as a source of raw material for food in the leaves of plants. Now we must remember that the carbon dioxid from fires and from ani- mals is limited in amount by the work of plants, for the only burnable mate- rial that is available is the organic ma- terial manufactured in the first instance by the green plants. Whichever way we go at it, we see that our lives are dependent upon the activities of the green plants ; and on the other hand, the continued exist- ence of new green plants is made pos- sible by the oxida- tion of their organic substances in the bodies of animals Carbon Dioxid in Atmosphere ' t x FIG. 18. The carbon cycle Fires and all kinds of living things are constantly throw- ing carbon dioxid into the air. Green plants, represented by the tree in the diagram, withdraw carbon dioxid from the atmosphere and return oxygen. The material of the green plant is made up in part of the carbon derived from the carbon dioxid. This material serves as food for animals and as fuel for fires. The animals oxidize this material ; or they are eaten by other animals. Finally, the carbon of larger plants and animals is oxidized by simple organisms, such as bacteria and fungi, and is returned to the atmosphere or in fires. There is, then, a certain balance, or limited relation, between the total quantity of plant life in the world and the total quantity of animal life. If the amount of animal life should diminish very greatly, there would come a time when the growth of THE CHEMICAL CYCLE OF LIFE 59 C0, which may serve as food, secretes an enzym, or ferment, which passes out of the cell e and changes the material to a liquid /. This is absorbed into the cell by osmosis, / cell wall, by osmosis, a liquid containing a ferment capable of digesting the solid or insoluble food material. The liquid re- sulting from the digestion is absorbed by osmosis. This may account for the fact that when meat or cheese rots, it becomes fluid. The rotting in such cases is the work of the ferments contained in the digestive juices secreted by the bacteria (Fig. 27). In higher animals like ourselves a similar process of diges- tion takes place. But instead of every cell pouring out diges- tive juices into its immediate neighborhood, only certain portions of the body produce and throw out such juices. CHAPTER XIX DIGESTIVE SYSTEM IN MAN 115. The human food tube. We receive our food and drink into our mouths. The mouth is the beginning of a long tube inside of which all the digestion takes place. This tube is called the food tube or the alimentary canal or the digestive tract. This tract consists of several fairly distinct regions ; in an adult it is about ten or eleven yards long. It manages to keep inside the much shorter body by being coiled and twisted in parts (see Fig. 28,7, k}. 116. Mouth digestion. Since what we eat is important to us on account of its proteins, fats, and carbohydrates, we may con- sider the digestive processes in relation to these substances. After the food enters the mouth, it is crushed and ground by the teeth. During the process of chewing, however, some- thing else happens. The taste of the food, the movement of the jaws, and the rubbing of the food against the inside of the mouth stimulate the action of the saliva glands (see Fig. 29) so that a quantity of saliva is poured into the mouth and this becomes mixed with the food. The more the food is chewed, the smaller are the particles into which it is broken, and the more thoroughly is the saliva mixed with the particles. As we have already learned (p. 78), the action of the saliva upon the starch in the food changes it into sugar. The other materials in the food are probably not changed, except that salts and sugars are dissolved by the water, of which the saliva contains over 99 per cent. As the amount of ferment is very small, the effectiveness of saliva as a digester of starch depends upon the ferment's reaching every particle of starch, and upon its having sufficient time to bring about the change. 81 82 ELEMENTARY BIOLOGY The thorough mixing of saliva with the food makes it easier for the whole mass to slide along into the throat, and later into the gullet, since the surface of the mass is thus coated with the slippery mucin of the saliva. 117. Swallowing. After the mouthful of food has been thoroughly chewed, it is pushed back by the tongue and passed into the throat chamber, or pharynx (see Fig. 28, b), from which it passes di- rectly into the gullet, or esophagus. The swallow- ing is not merely a falling down of the food from the pharynx into the stomach. It is an active carrying brought about by the suc- cessive contraction of rings of muscles that lie in a series in the wall of the gullet. If you watch a horse drinking water from a pond or from a pail set on the ground, you can see him swallow the water 2ip, and you can see, show- ing through the skin, one wave of contraction after another pass along the gullet, from the head to the trunk. FIG. 28. The digestive organs in man a, entrance to mouth ; <5, the pharynx, — a sort of vestibule with seven passages leading out of it, two to the nostrils, one to the mouth, one to the gullet, one to the windpipe, and one to each ear (the Eustachian tubes, see p.- 240) ; c, the gullet, or esophagus ; d, the stomach ; e, the pylorus, opening .from the stomach to the small intestine ; /, the liver ; g, the gall bladder ; /z, duct from the gall bladder and the liver to the small intestine ; /', duct from the pancreas to the small intestine ; /, small intestine ; k, large intestine ; /, vermiform appendix ; m, rectum ; «, the diaphragm, separat- ing the chest cavity from the abdominal cavity; o, the pancreas. The arrows indicate the course taken by food in passing from the mouth through the alimentary canal DIGESTIVE SYSTEM IN MAN 118. The stomach. Whatever fermentation has been started by the saliva in the mouth continues in the mass of food until this reaches the stomach. Here, however, it stops the moment the acid, or sour, stomach juice comes in contact with the saliva. It seems that the saliva ferment cannot act in the presence of acid. The stomach juice contains a special ferment known as pepsin. Pepsin, in the presence of acid, acts upon the proteins in the food, changing them into soluble compounds of similar composition, known as peptones. Peptones dif- fer from proteins chiefly in this one fact, that the former are soluble in water and are capable of diffusing through membranes, while the pro- teins are generally not capable of such diffusion. In the stomach the swal- lowed substances are thor- Parotid gland Blood vessels Siibmaxillary FIG. 29. The salivary glands There are three sets of glands which produce the parotid, in the cheek, just in front sf of the ears ; the submaxillary, under the angles of the jaw; and the sublingual, under the tongue oughly mixed with the gastric, or stomach, juice by P J the action of the muscles in the stomach wall. The stomach wall contains layers of muscle cells running in differ- ent directions, as well as gland cells in which are produced the particular substances found in the gastric juice (see Fig. 30). As the changing of proteins to peptones goes on, the mix- ture in the stomach becomes more and more liquid and more and more acid. From time to time a quantity of the liquid in the stomach is squirted out into the beginning of the intestine by the opening of the connection (Fig. 28, e) and the contrac- tion of the stomach at the same time. After a while most of the contents of the stomach has been changed to a mixture 84 ELEMENTARY BIOLOGY having the consistency of a rather thick pea soup, and all of it has passed on into the intestine. 119. The intestines. There are two distinct parts, or divi- sions, of the gut among the highest animals. The first part is called the small intestine, and in human beings it is about one inch in diameter and about twenty-four or twenty-five feet long. The small intestine opens rather abruptly into the large intestine, which is about two inches in diameter and about five feet long (see Fig. 28, y, k). The wall of the intes- tine is rather thin and soft. You have probably handled a piece of pig gut or calf gut, which is used as sausage casing. In the living animal the wall of the intestine is not so hard and stiff as we sometimes find it in the sausage casing. This wall is made up of FIG. 30. Glands of the stomach The gastric juice is poured into the stomach through tubes, a, which are lined by a layer of delicate cells ; it is produced by special gland cells, b, from materials brought by the blood in fine vessels, c several layers of tissue. The inner lining carries very small glands, and the outer layers contain muscle cells. To this extent the wall of the intestine is like the wall of the stomach. The muscle cells of the gut are arranged in rings, so that when they contract they simply reduce the diameter of the intestine at any given point. The contraction starts at the forward end — that is, the end nearest the stomach — and passes backward along the whole length of the small intestine, aided by longitudinal muscles. As a result of this wave of contraction some of the thick mixture of food and digestive juices is moved along, a short distance DIGESTIVE SYSTEM IN MAN 85 at a time. This movement is called peristalsis and is very similar to the swallowing movement of the gullet. When a food mixture passes from the stomach, it contains all of the fat that it contained when it first entered the mouth, since neither the saliva ferments nor the gastric ferments have any effect upon fats. It contains all the sugar that was there to begin with, together with all the sugar that was formed by the digestion of starch in the mouth. It contains whatever starch was not digested. It contains the peptones formed by the gas- tric digestion (in solution), and particles of proteins that were not digested. In addition there is a quantity of water, mineral salts, the remains of the gastric and salivary juices, and the fibers and cell walls of the food material, which have not been acted upon in the mouth or in the stomach. In the intestines many changes take place in the character and composition of this mixture. Near the beginning of the intestine (Fig. 28, h, i) there is a small opening connected with two small tubes, or ducts. One of these is connected with the largest gland in the body, the liver \ the other is connected with another very important gland, the pancreas (Fig. 28, o). 120. The pancreas. The juice secreted by the pancreas contains three important kinds of ferments : 1. A ferment that converts starch into sugar. 2. A ferment that digests proteins into simpler compounds. Any starch that has been swallowed before the saliva has had time to transform it into sugar, and any protein that has passed from the stomach without being digested by the pepsin, will now be digested by the action of the pancreatic ferments. 3. A ferment that acts upon the fats in the food, breaking them up into glycerin and fatty acids, which latter combine with other substances to form soaps. The soaps and the glycerin dissolve in water and diffuse through cell membranes. 86 ELEMENTARY BIOLOGY The pancreatic juice thus contains all the kinds of ferments necessary for digesting a whole meal. 121. The liver. The juice produced by the liver is called the bile, or gall. i . It does not contain any ferments that seem to be impor- tant in digestion, but it does have an influence on the absorp- tion of the fatty acids and soaps by the cells of the. intestine. 2. The bile seems further to have some ef- fect upon the activity of the pancreatic ferments. When the contents of the stomach pass into the intestine, the mixture is acid ; the bile neutral- izes the acid and makes possible the activity of the other ferments. 3. The bile is made up chiefly of materials FIG. 31. The lining of the intestine that are of no further The tiny projections from the lining of the small USC to the body ma- intestine the vUli give ,the appearance of very fine t ^ th t ^ b velvet. Absorption takes place through the outer layer of cells. Within each villus are fine blood ves- Converted in the liver sels and lymph spaces ; from these the absorbed food j .1 ,1 rnwn is transferred to circulation system and &rG then thrown into the intestine, from which they are removed from the body. The liver is thus also an excretory organ. 122. The intestinal juices. The juices secreted by the glands of the intestine contain no ferments that are of great importance in digestion, although they do contain a great deal of sodium carbonate, which neutralizes the acids resulting from the digestion of fats by the pancreatic juice, and probably also other acids resulting from other DIGESTIVE SYSTEM IN MAN chemical changes in the gut. There is a ferment in the intestinal juice which converts cane sugar into simpler sugars, but this change may also be brought about by the acids of the stomach, and possibly also by the alkali in the intestine. 123. Absorp- tion. The lin- ing of the small intestine is like delicate velvet. Very small out- growths project into the cavity, so that the sur- face exposed to contact with the food mixture is increased sev- eral hundred times. Each of these tiny pro- jections, called a villus (plural, villi)t has a rather complex structure, as is shown in the di- agram (Fig. 31). The villus seems to be a special absorbing and transforming organ. The mixture in the intestine we now know to consist of many crystalloids in solution, many colloids in the process of being converted into crystalloids, and solid substances that are not capable of changing under the conditions that exist in the gut. The crystalloids are absorbed into the cells of the villi, so that as the mass moves along in the intestine, more and more Large Intestine and Rectum FIG. 32. Digestive system in fish and in bird The main features of the digestive system are alike in all back- boned animals. In the birds there is a curious pouch connected with the gullet, — the crop, — in which food may be retained in- definitely and later either swallowed or regurgitated through the mouth. The glandular portion of the stomach, or proventriculus, is distinct from the grinding part, or gizzard 88 ELEMENTARY BIOLOGY of the digested matter is withdrawn into the villi. From the surface cells of the villi the absorbed material is passed on, by osmosis, to the blood vessels and to the lymph vessels. Chem- ical changes take place in the course of the transfer, so that the material taken into the blood is not in exactly the same state as the material absorbed from the intestine, although, of course, it is made up of the same elements. By the time the dinner you have eaten has reached the end of the small intestine, most of the proteins, fats, and carbo- hydrates that it contained have been absorbed by the villi and passed on into the blood and lymph. There is left in the in- testines at this point chiefly the undigested (for the most part indigestible) fibrous and cell-wall material of the plant or animal tissues eaten, and the modified secretions of the various glands that have poured into the food tube all along the way. This mass of refuse now passes into the large intestine (Fig. 28, k). 124. The large intestine. In the large intestine the fer- ments of the digestive juices may still continue to act for some time. But gradually, as the mass proceeds along the canal, it becomes drier, through the continued absorption of material by the lining of the intestine (there are no villi in the large intestine), so that toward the end the only chemical changes going on are those produced by the millions of bacteria that are present in the intestines of all animals. The mass of material that has accumulated toward the end of the large intestine is of no further use to the body, and should be removed from time to time. Birds, having no large intestine, throw off the refuse about as fast as it passes from the small intestine to the rectum (Fig. 32). Other animals and infants throw off the refuse automatically. But older children, absorbed in their games or other activities, are apt to postpone emptying the bowels, and thus become irregular. This neglect of the bowels often brings about serious conse- quences, so that it is important for us to acquire regular bowel habits while we are still young (see p. 1 18). CHAPTER XX HEALTH AND FOOD STANDARDS 125. Conditions of health. The normal, healthy digestion and absorption of food depend upon (1) the secretion of digestive juices by the glands ; (2) the fermentative action of the substances in these juices ; (3) the muscular contractions of the gullet, the stomach, and the intestines. We can do nothing to control these processes directly, either to hasten them or to stop them. But indirectly we can do a great deal to control them. First of all, we can decide what to eat, how to eat, and when to eat. Then we can decide for ourselves what kind of habits we will have with regard to the behavior of the large intestine. And, finally, we can do a number of things that are not directly connected with feeding, but that have important bearings on the healthy behavior of the digestive system. All these controls come to us from a better understanding of the biology of nutrition and digestion. 126. What to eat. Since all living beings consist essentially of proteins, fats, and carbohydrates, it would seem that almost any plant or animal stuff would be suitable for food. But we know from experience that some of these things are not pleasant to the taste, or are even disagreeable, and that others are poisonous. Some substances, while neither unpleasant nor injurious, contain so little usable or digestible material that they are not worth eating. In the course of ages human customs have selected the plant and animal materials in any given region that are most valuable for food. We are all the time 89 90 ELEMENTARY BIOLOGY discovering useful food plants and food animals that are stran- gers to us but familiar to people in remote parts of the earth, and neither our instincts nor our customs tell us the best way to use them. Even in regard to the older kinds of food we are almost as ignorant ; for while we know that the flesh of an ox is better for food than the hoof or the hide, and that the grain of the wheat is better than the leaf or the root, experience has not taught us what proportions of meat and grain and fruit are the best for maintaining efficient health, and certainly we have to learn that one combination of foods is best for one person, while another combination is best for another person. 127. Dietary studies. When the study of dietaries was first begun, it was assumed that what people actually eat is on the average the best thing for them to eat, both as to kind and as to quantity. Accordingly students made careful records of the meat and bread and butter and vegetables and fruits and cheese eaten by thousands of people. They calculated the amount of protein, fat, and carbohydrates contained in these dietaries, and sought thus to establish, from the averages, a standard of what healthy people require day by day. By this method Carl Voit in Germany and Professor W. O. Atwater in this country concluded that a person doing a moderate amount of work needs about four ounces of protein daily, to take the place of the proteins oxidized in the cells of the body. But later experiments, in which the amount of protein taken in and the amount of nitrogenous waste given off were carefully measured, lead to the conclusion that an adult weighing about one hundred and sixty pounds requires hardly more than two ounces of proteins in every twenty-four hours. Since protein is the most expensive material in our food, and at the same time the one that is most severe upon the organs of the body, especially the liver and kidneys, it is a matter of great importance to know whether two ounces will suffice or whether four ounces are necessary. We should therefore try to understand the basis upon which these diverse standards are established. HEALTH AND FOOD STANDARDS 91 128. Units of energy. To measure the energy expended by the body, or to measure anything else, we must have a unit. We measure length in inches or yards or miles. In a similar manner we are able to measure energy by work done. But as different forms of energy do different kinds of work, it is necessary to find some common unit for measuring. For example, motion can be measured by the quantity of matter moved and the distance through which it is moved, as one ton raised five inches, three pounds raised two feet. The unit of measuring this kind of work may be the foot pound, or the amount of energy it takes to raise one pound of matter one foot. If a pint of water at room temperature (about 18° C., or about 65° F.) is placed in a pan over a burner, it will gradually be- come warmer, until it reaches the boiling point. It takes a cer- tain quantity of heat to change the temperature of the water from 65° to 212° (the boiling temperature of water). For a quart of water it would take twice as much heat to do the required work. As a unit of heat energy we might use, for example, the pint degree. The unit adopted among engineers is the quantity of heat necessary to raise one kilogram of water (a little more than a quart) from the temperature of o° to the temperature of i° C. This unit is called a calorie. In dealing with fuel or the conversion of fuel energy into other forms, it is customary to record energy in terms of calories. In deal- ing with mechanical work it is customary to record energy in terms of foot pounds, or horse-power hours.1 1 The fuel values of proteins, fats, and carbohydrates are as follows : CALORIES PER GRAM CALORIES PER POUND 4.1 i860 Carbohydrates • . 4.1 i860 Fats q.-z 42IQ From these figures it will be seen that a given quantity of fat contains more than twice as much latent energy as the same quantity of protein or carbohy- drate, and that the latter two classes of compounds have the same fuel value. 92 ELEMENTARY BIOLOGY 129. Respiration calorimeter. The work of the human body or any other animal body can be measured in terms of calories by means of very delicate apparatus that has been developed in recent years. In a large chamber that is completely inclosed so FIG. 33. The respiration calorimeter In the large chamber a man can live for several days or weeks under conditions that give an accurate account of his body's income and expenditure, in the way of matter as well as in the way of energy. A, door and window ; B, door for food etc. ; C, tank for catching water circulating through the walls of the chamber ; Z>, observer's table, with devices for measuring and regulating temperature etc. ; J5, rubber bag to equalize the air pressure within the chamber ; F, apparatus for circulation and purification of air in the chamber. From photograph furnished by Office of Home Economics, United States Department of Agriculture as to prevent the escape of heat, a person may live for several days or weeks at a time under conditions that allow us to measure every particle of material that goes in or that comes out, as well as the amount of heat that is given off by the body (Fig. 33). With this apparatus exact records are made of the work a human being does in the course of a day, measured physically HEALTH AND FOOD STANDARDS 93 as calories or foot pounds instead of in terms of useful product, as words written, nails driven, or yards of carpet woven. 130. Our daily needs. From experiments with the respira- tion calorimeter it has been determined that a person weighing about one hundred and fifty to one hundred and sixty pounds and doing a moderate amount of physical work expends about 2800 calories a day, whereas a person engaged in a sedentary occupation, as a clerk or bookkeeper, would not use up more than 2400 calories. The higher of these figures is considerably less than the standard set by Atwater, which was over 4000 calories for the moderate worker and 4500 for the hard worker.1 These experiments have been supplemented by others made by college professors on themselves and their colleagues, on college athletes and other students, and on soldiers. We thus learn that most people eat too much food, and especially too much proteins. The protein standard established by Professor Chittenden at Yale, for adults doing various kinds of work, is just one half that announced by Voit, namely, about two ounces in twenty-four hours. In the experiments, students, professors, and soldiers not only kept up their weight on this basis but really did more and better work, and were in better health generally, than under the larger protein allowance. The amount of protein used up in the course of a day de- pends not upon the amount of muscular work done but upon the rate of growth and upon the weight of the body (not count- ing the fat). A stonemason or miner does not need more pro- tein than a shoemaker or stenographer of the same weight, but he does need more fat or carbohydrates. 1 Even Voit's standard gave 3000 calories for the moderate worker and 3500 for the hard worker. A comparison of Voit's figures with Atwater's leads one to suspect that the American workers ate more food than the German workers, probably because food was at that time cheaper in this country, or wages rela- tively higher. In the end we shall have to depend upon experiments to tell us just what is the wisest thing to do in regard to eating. CHAPTER XXI FOOD REQUIREMENTS 131. Selection of food. When we go marketing, or when we look over the bill of fare at a restaurant or hotel, we do not select proteins and calories ; we select cuts of meat, vegetables, fruits, cheese, bread, and so on. Suppose that you had for breakfast a large banana, a glass of milk, two slices of bread and butter, and an egg. How much protein is there in such a breakfast, and what is the total fuel or heat value of the food ? We should have some means of translating the products of the food factories and the kitchen into terms of proteins and calories. This is furnished by tables that have been prepared by experts working for the government, for hospitals, and for manufacturers. We can make use of some of these results to guide us in our own selection of food. 132. Food composition. From the table of food composition on page 95 we can get an idea that some of the food ma- terials which we use contain more nutrients than others, and that some contain a larger proportion of proteins, or of fats, or of carbohydrates. We can get these ideas more readily from charts and diagrams. The United States Department of Agriculture has issued a series of charts in which the compo- sition and fuel value of a large number of articles of food are shown in colors. A few of these are reproduced in Fig. 34. 133. Fisher's table. Professor Irving Fisher of Yale Univer- sity has prepared a list of common articles of food, with a state- ment of how much it takes of each kind to give approximately one hundred calories, and the proportion of this furnished by the protein. A portion of this table is reproduced on page 96. 94 FOOD REQUIREMENTS COMPOSITION OF VARIOUS FOOD ARTICLES 95 PER CENT OF PROTEIN PER CENT OF FATS PER CENT OF CARBO- HYDRATE PER CENT OF WATER CALORIES PER POUND Milk, whole 3.6 4.0 4-7 87.0 T.2<; Buttermilk -? O O S 48 QI O i6s Butter I.O 8q.o I I.O ivjj 361 ^ Cheese full cream 2C.Q •3-1 7 2 4 ^4 2 TQcn Eggs edible portion U.8 IO 1 7 no Beef, porterhouse, edible portion Beef dried 20.0 7O.O 20.0 6.=; 6O.O CA 7 I27O 84O Bacon smoked Q.4. 67.4 188 -7Q-IO Ham, lean, edible portion . . . Lamb leg roast 25.0 IQ.S 14.4 12.7 6o.O 67 I 1075 QOO Chicken, broiled, edible portion Salmon, California, edible por- tion . ... 21.5 17.8 2-5 17.8 74-8 636 505 IO8O Brook trout ... . IQ.O 2.1 77 8 44^ Oysters solids 6.2 1.2 ^ 7 87 o W3 2T. C Bread, homemade . Q.O I.I J-/ CA Q •2-3 2 I 24C Bread brown e.4 1.8 0^-y 47 I A'i (j IO CO Corn meal, granular . Q.2 I Q 12 C "*yf i6c c Oatmeal boiled 2.8 O tj TIC **o 84 i; »W33 281; Rice boiled 2.8 O I *ro 72 ^ ^wj r in Macaroni cooked •VQ I c T5 8 l**3 78 A J1(J J.I c Beans, string, cooked .... Beans, baked, canned .... Cabbage, edible part .... Potato, boiled 0.8 6.9 1.6 2 r I.Q 2-5 o-3 O I 29.1 19.6 5.6 95-3 68.9 91.5 41i 95 600 i45 Apple, as purchased . '•j O 7 O 7. 10 8 75-5 6-3 7 Banana, edible part . I 7 06 °3-3 ^vu Figs, fresh . . . T C 18 8 75-3 780 Figs, dried *o 4-J O 1 /y.i TQ Q 300 Dates, dried, edible portion . . Orange, whole .... •J 2.1 06 W-J 2.8 O I /4'^ 78.4 c c 154 67 /« 14/b 1615 Watermelon, whole . O 2 O I 0*5 °j4 l/U fin Peanut, edible part 2C8 18 6 *'i 37o 2 c8n Walnut, California soft shell, edible portion Sugar, granulated .6^.0 1 6.6 634 "•4 16.1 IOO.O .2 2-5 3285 1860 96 ELEMENTARY BIOLOGY SELECTIONS FROM DR. IRVING FISHER'S 100-CALORIE PORTIONS TABLE SIZE OF PORTION WEIGHT OF PORTION (OUNCES) CALORIES FROM PROTEIN CALORIES FROM FATS CALORIES FROM CARBOHY- DRATES Milk, whole . . . Small glass 4-9 19 52 29 Buttermilk .... ii glasses 9-7 34 12 54 Butter One pat •45 o-5 95-5 Cheese, full cream . i£ cubic inches .82 25 73 2 Efifffs One large 2.1 •32 68 jr Beef, porterhouse . Edible part, small i-3 32 68 steak Beef, dried .... Ordinary serving 1.9 67 33 Bacon, smoked . . Small serving •5 6 94 Ham, lean .... Edible portion, i-5 44 56 average serving Lamb, leg, roast . . Ordinary serving 1.8 40 60 Chicken, broiled Edible portion, 3-2 79 21 large serving Salmon, California . Edible portion, I;S 3° 70 small serving Brook trout . . . 2 small servings 3-6 80 2O Oysters, half shell . i dozen 7- 49 22 29 Bread, homemade . Thick slice T-3 T3 6 81 Bread, brown . . . Thick slice i-5 9 7 84 Corn meal, granular 2.5 level teaspoonfuls . .96 10 5 85 Oatmeal, boiled . . i£ servings 5.6 18 7 75 Rice, boiled . . . Ordinary cereal dish 3-1 10 i 89 Macaroni, cooked . Ordinary serving 3-9 H 15 7i Beans, baked, canned Small side dish 2.7 21 18 61 Beans, string, cooked 5 servings 16.7 15 48 37 Cabbage Edible portion II.O 2O 8 72 Potato, boiled . . . i large 3-6 I I i 88 Apple, as purchased Two 7-3 3 7 90 Banana ..... Edible portion, i large 3-5 5 5 90 Figs, dried .... i large i.i 5 95 Dates, dried, edible 3 Jarge I.O 7 9i Orange, as purchased i very large 9.4 6 3 9i Watermelon . . . Whole 27.0 6 6 88 Peanut Edible part 1 7 double 6^ 20 6-\ 17 .^j f>0 '^cf I 1 Luiig 1 77.0^ FIG. 57. Relation of smoking to physical growth The first column shows the average advantage of non-smokers (indicated by white space) over occasional smokers (indicated by shaded space). The second column shows the average advantage of non-smokers over regular smokers. These measurements are from the physical-training department of Yale University Similar records kept at Amherst College and at Columbia showed similar differences between smokers and non-smokers. In some of the investigations the ages of*the students in each class were also compared. It appears that in any given graduating class the smokers are on the average older than the non-smokers. This prob- ably indicates the extent to which smoking — possibly in association with other unhygienic habits — retards a young person in his progress. Whatever differences of opinion there may be as to the harmfulness of smoking for adults, there is no difference of opinion as to its effect on the young. For this reason the government of Japan some years ago prohibited the sale of tobacco in any form to minors, and some of our states have done the same. The United States Military Academy and the United States Naval Academy forbid the use of tobacco CONTAMINATED AIR 167 by the students. Many railway companies and other large employers refuse to take on young men who smoke. In Min- neapolis one hundred of the leading business men agreed not to give employment to young men who smoked. Many officers in the army and navy and probably most of the railway officials and large employers and business men smoke. Yet they realize that they can get better service from young men who do not smoke. It is not a matter of sentiment or prejudice with them ; it is strictly a matter of business. 199. Economic and so- , _ r~ I 95 Non-smokers Rgfl cial problems. Aside from the injury that to- bacco-smoking doeS tO | 10 Non-smokers S>>^y growing young people, FIG. 58. Smoking and scholarship the economic Side Of Showing the proportion of smokers (shaded space) the Question is Simplv anc^ °^ non"sm°kers (white space) among the stu- dents of highest rank (first bar) and among students Whether, tor the Same of ordinary rank (second bar) at Yale University expenditure of human effort as is required to raise,1 cure, handle, manufacture, and distribute the tobacco and various smokers' appliances, people could get more fun out of life. Certainly those who enjoy smoking, numbering into the hundreds of millions, feel that they are getting their money's worth in this form of enjoy- ment, and it is impossible to say to them that those of us who do not smoke are having more pleasure or satisfaction. The social and aesthetic sides of the question can be seen more definitely. 1. The smoke of tobacco is distinctly offensive to the non- smoker. To be sure, we can get used to that, we can learn to stand it, but in and of itself it is a nuisance. 2. The perspiration of the smoker frequently becomes modified so that it is distinctly objectionable. 1 Over a million acres of good land are worked in this country every year for raising tobacco. Over two hundred and twenty-five thousand persons are engaged in the manufacture and sale of tobacco products, besides the farmers and the makers of pipes, boxes, labels, and so on. 168 ELEMENTARY BIOLOGY 3. The breath of the smoker, too, is frequently offensive. 4. The discoloration of the teeth does not add to one's attractiveness, nor does the discoloration of the fingers of cigarette smokers. Perhaps no smoker makes himself offensive on all of these scores ; but, taken together, these objections to the practice are just as real and just as serious as questions of cost or even of health. There is a real fire risk involved by the wide practice of smoking ; this can be measured by insurance experts, and can be controlled and reduced by police and educational measures. CHAPTER XXXII FIRST AID AND HYGIENE IN RELATION TO BREATHING 200. Air needed continuously. We can go without eating for days or even for weeks. Water has to be taken into the body more frequently. But we cannot go without breath- ing for more than two or three minutes or, at most, four or five minutes. 201. Suffocation and drowning. When, for any reason, the gas exchange in the lining of the air chambers in the lungs is stopped for several minutes, suffocation takes place, and death may result. Suffocation may be due to the replacement of air by some other gas, or it may be due to the exclusion of air.1 The replacement of the air in the lungs by water is called drowning. Suffocation and drowning are commonly fatal, but in very many cases life may be saved by prompt and persistent action. It is necessary (i) to empty the lungs of the water or foreign gas and (2) to reestablish the breathing movements. When a person has been drowned, the first thing to do is to place the body, face down, in a position that will cause the water to pour out of the lungs. A child may be lifted up by the feet. Breathing movements should be begun at once. In the Schaefer method of artificial respiration the victim is laid face down, with the arms stretched forward beyond the head ; the head is turned to one side and supported on a cloth, to leave the nostrils and mouth unobstructed. The operator kneels, strad- dling the subject's thighs and facing his head, and with the 1 Breathing may also be stopped by a severe electric shock, which acts on a group of nerves that control the breathing movements. The treatment should be the same, whatever the cause of the suffocation. 169 ELEMENTARY BIOLOGY thumbs over the small of the back and fingers over the lowest ribs, alternately compresses and releases the chest by swinging forward and back, at the rate of from twelve to fifteen times a minute. The movements should be kept up until natural breath- ing begins, but should not be given up in less than an hour. FIG. 59. Sylvester method of artificial respiration, — expanding the chest After drawing out the tongue and placing the patient on the back with a block or roll under the shoulders, to keep the chest raised and the head thrown back, kneel behind the head and grasp the arms just below the elbows. Draw the arms slowly backward over the head, and hold them there about one second While these movements are being carried out, the victim's tongue should be pulled out and kept out, to prevent it from slipping back into the throat and obstructing the windpipe. The Sylvester method of artificial respiration is shown in Figs. 59 and 60. In case of asphyxiation, or suffocation by gases or by electric shock, the same procedure should be followed, except that it is not then necessary to take special steps for emptying the lungs of water. FIRST AID IN RELATION TO BREATHING 171 Under the supervision of the United States Bureau of Mines squads of miners are instructed in the resuscitation of people who become asphyxiated by gases or by electric shock. This bureau conducted a series of experiments to determine which of the mechanical resuscitating devices was FIG. 60. Sylvester method of artificial respiration, — contracting the chest After the arms have been held above the head about one second, push the elbows slowly for- ward and downward until they are in the position shown. Press the elbows firmly against the chest and hold them there about one second, to drive all the air out of the lungs. (Photographs and instructions, Figs. 59 and 60, from United States Bureau of Mines) best for various purposes. It was found that more reliance could be placed on quick action by men who understood how to establish respiration than on most of the machines, and it is always safer to begin work by hand than to wait for the best machine. One of the devices is illustrated in Fig. 61. 202. Summary on breathing and ventilation. Since we carry on our breathing without needing to think about it, most people have given very little attention to the subject of air and 172 ELEMENTARY BIOLOGY breathing. It is only in comparatively recent times, therefore, that we have come to realize what a close connection there is between our breathing habits and breathing conditions, on Head cap Check valve -xte^fc •^^•0*^"" ^Supply tube FIG. 61. Oxygen inhalator This apparatus for the resuscitation of persons overcome by suffocating fumes or gases was developed by the United States Bureau of Mines. It is used with the Schaefer method of artificial respiration, and supplies oxygen for about thirty-five minutes, which is usually sufficient to restore normal breathing. (Photograph by United States Bureau of Mines) the one hand, and our health, happiness, and efficiency, on the other. The most important things that have been discovered by the new attention to these details are the following : i. Outdoor air is better than indoor air in every way. a. It is better for playing, even in the cold and rain ; suitable clothing will make up for these. HYGIENE IN RELATION TO BREATHING 173 b. It is better for work, since a person can accomplish more in a given time when breathing outdoor air than when breathing indoor air. c. It is better for health, even to sleep out of doors. 2. Nose-breathing is in every way better than mouth-breathing. a. Where mouth-breathing is due to adenoids, these growths should be removed. b. Where mouth-breathing is due to bad habits, these habits should be corrected. 3. Deep breathing is better than shallow breathing. a. Where shallow breathing is due to improper clothing, the clothing should be changed. b. Where shallow breathing is due to habit, correct habits should be acquired through exercise, outdoor games, work, etc. 4. Dust is a source of danger to the health of the body and to the lungs in particular. a. Mechanical dust, soot, and smoke (including tobacco smoke) coat the lining of the air sacs and reduce the breathing surface. b. Hard dust may scratch the lining of the air sacs and thus increase exposure to infection. c. Dust carrying microbes is a direct source of danger. d. Chemical dust and fumes may poison the blood. 5. A person suffocated or drowned is not to be given up for dead before every possible effort to resuscitate him has been made in vain. 6. Ventilation is necessary not only to keep down the proportion of CO2 and to keep up the proportion of oxygen in the air, but also to (a) regulate the moisture, (b) regulate the tem- perature, (c) keep the air moving, (d) remove disagreeable odors, (e) remove gases and fumes, (/) remove dust. 7. Alcohol, by congesting the capillaries, decreases the breathing efficiency of the lining of the air sacs. CHAPTER XXXIII TRANSFER OF MATERIALS IN PLANTS 203. Exchange of materials in living cells. One-celled plants and animals that move about in the water constantly come into a new environment, from which they get supplies of water, oxy- gen, minerals, and food (or food-making materi- als). In larger animals and plants the life of the cells, or at least of the innermost ones, can be maintained only by means of an internal transportation system. This brings to the cells their supplies of food, FIG. 62. Vessels in a plant water, oxygen, etc., and carries from them, to be removed to the exterior, their waste products. 204. The conducting systems of a plant. In all those plants that have a body which is made up definitely of root, stem, and leaf the diffusion of water and of dissolved substances between parts of the body is supplemented by the transportation of material in mass. We have seen that water and salts absorbed by the root-hairs diffuse through the cells of the root cortex and then move bodily through special vessels (see p. 44). These sap-carrying vessels are of sev- eral kinds — some long, some short, some consisting of single 174 Section of squash stem, showing phloem vessels, P/i, which conduct food down from the leaves, and two kinds of wood vessels, W, which conduct water and salts up from the roots. The two kinds of wood vessels are those with spiral thickenings in the walls and those with pitted walls TRANSFER OF MATERIALS IN PLANTS '75 Medullary ray cells, some consisting of series of cells from which the end walls have disappeared, leaving long, continuous channels. Some of these vessels are illustrated in Figs. 10 and 62. The sap-carrying ves- sels of the root are continuous with similar tubes found in the stem. The tubes are found arranged side by side in bundles rather than in single strands (Fig. 62). The bundles of vessels and fibers, in many plants, are easily sepa- rated from surrounding cells, or pulled out. In celery these bundles make up the "strings," and when you pull up a FIG. 63. Structure of a woody stem Cambium leaf of the plantain, you can see the so-called nerves that stick out of the broken end of the leafstalk, which are also fibro vascular bundles. In woody plants the fibrovascular strands are Diagrammatic view of piece of two-year-old black- berry stem, showing the central pith, surrounded by the wood of the first and second years, the bark, and the epidermis E. The wood is made up of wood fibers, Wf, and vessels, Pd, representing a pitted duct. Between the wood and the bark is the grow- ing, or cambium, layer, and this is connected with deeper layers by means of the Medullary rays, Mr. Under the skin are some green cells, Gc, and running through the bark cells C are bast fibers, Bf, as well as bast vessels, which are not shown compacted into solid cylinders ; these make up the successive layers of wood (Fig. 6 3). The fibrovascular bundles branch and divide so that they reach into all the twigs and leaves. In the leaf they branch again and constitute the so-called veins, or nerves, of the leaf blade (Fig. 64). The food sap does not pass through the same tubes as the water and salts from the roots. The manufactured food ELEMENTARY BIOLOGY probably goes from the leaves by way of another set of vessels called phloem, or bast. These vessels are generally larger in diameter than the wood vessels (xylem) and are characterized by having pores in the end walls. These end walls, with their perforations, are sometimes called sieve plates (Fig. 65). The toughened fibers associated with the bast vessels make up the bast fibers. In the leaves the bast and the xylem vessels are closely compacted in the veins. In wood we have masses of xylem vessels and fibers. Bast fibers are commonly used in the form of linen, hemp, and jute fibers. In woody plants the bast is located in the bark. 205. The ascent of sap in trees. Investigators have long been puzzled by this problem, and we are not yet sure that we understand it. How- ever, it is certain that the water does rise, and that it goes through the xylem vessels. 206. The descent of sap. From certain common observations and from the results of experiments we may reasonably infer that sap descends. We know that organic food is formed in the leaves, and that it is accumulated in the roots and in underground stems of many plants. There must therefore be a current of material passing downward from the leaves. A tree that is girdled, that is, one that has a ring of bark removed, will continue to live for the rest of the season. This shows that the removal of the bark does not interfere with the ascent of water and salts from the roots to the leaves. The following spring, however, when the opening of the buds with the rapid expansion of leaves and twigs depends upon the food accumulated during the previous season, the tree will be found FIG. 64. Veins of a leaf Leaf of apple of Sodom (Solatium aculeatissimum), of the potato fam- ily, showing network of veins TRANSFER OF MATERIALS IN PLANTS 177 dead. Although water and salts may still be able to reach the upper parts of the plant (since the channels that served during the previous season are still open), the food that should have been accumulated during the previous summer is lacking. A FIG. 65. Bark fibers and vessel A, a section cut lengthwise, and £, one cut crosswise, showing bast fibers, sieve-plate vessel, and the so-called companion cells found next to the sieve-plate cells 207. Circulation of sap. In plants there is no circulation of materials such as we find in FIG. 66. Ascent and descent of sap The arrows in the diagram are to show that materials absorbed by the roots travel upward through vessels located in the wood part of the stem, and that ma- terials resulting from the food-making processes in the leaves travel downward through vessels located in the bark. When a complete ring of bark is removed from a tree, the plant may live on to the the higher Classes Of animals. end of the summer ; but the buds will not . _ . open the following spring, since they de- There is, it is true, a move- pend upon food accumulated in the roots ment of liquid from the root to the leaves, and from the leaves to the roots ; but the matter conducted along the two sets of vessels is not the same either in amount or in kind. The water that moves from the roots to the leaves is several times as great in quantity as the liquid that moves i;8 ELEMENTARY BIOLOGY from the leaves to the roots, since by far the largest portion of it is transpired and never comes back. The ascending current is for the most part inorganic in its composition ; the descending current is in large part organic. The two currents are nowhere continuous. They are prac- tically independent of each other, except, of course, that the life of the plant can continue normally only when both cur- rents are maintained ; yet one may go on for a considerable time without the other being in action. CHAPTER XXXIV THE BLOOD 208. Blood. In all animals above the corals and sea- anemones, and certain kinds of worms, there is present a circulating mass of liquid which is commonly called blood, although not all kinds of blood are alike. In the clams the blood contains a bluish substance, called hemocyanin, which easily combines with oxygen, and thus carries oxygen obtained from the surrounding water by diffusion into the capillaries of the skin and gills. In the earthworm there is a reddish sub- stance, called hemoglobin, dissolved in the blood, which be- haves in much the same way as the hemocyanin of the clams. The blood of back-boned animals has a rather complex structure, and is associated with an elaborate system of vessels and a pumping organ called the heart. 209. Composition of human blood. When examined with the microscope, human blood is seen to consist of a colorless liquid, called the plasma, and a number of small bodies float- ing in it. The more numerous particles are the so-called red corpuscles. These are very small1 and have the shape of a disc or coin, with rounded edges and compressed towards the middle (Fig. 67). In addition to the red corpuscles, there are also white, or colorless, corpuscles, some barely larger than the red ones, others many times as large. The fluid portion of the blood, the plasma, consists chiefly of water. In this are dissolved various salts, organic substances derived from the digested food, some oxygen, some carbon dioxid, certain ferments, and other organic substances derived 1 On the average, about of an inch in diameter. 3200 179 !8o ELEMENTARY BIOLOGY from various organs and tissues of the body. It is not to be supposed that any given drop of blood will contain all of these substances, or all of them in the same proportions, for, as we shall see, the composition of the blood is constantly under- going changes in the smallest blood vessels, through the walls of which new substances are coming in and others are passing out. 210. The lymph. The blood, consisting of plasma and cor- puscles, fills a set of tubes from which there are no openings ; the system is therefore called a closed blood system, to distin- guish it from the blood system of clams and certain other animals, (QJ in which many of the blood tubes have open _ // en(^s connecting with b various spaces in the FIG. 67. Human blood corpuscles , , .- ., f . body. Outside of the «, fresh leucocytes (white corpuscles) in resting and ui j VPooP1o jn j-Up V,n in moving stage (note granulations and nuclei) ; 6, red corpuscles in flat and in side view man body, filling defi- nite tracts as well as spaces between tissue masses and cells, is a colorless liquid, called lymph. It is from the lymph that the cells obtain their food supplies, water, salts, ferments, and oxygen ; and it is to the lymph that they discharge their carbon dioxid, urea, and other wastes. The communication between the lymph and the blood is by osmosis through the walls of the smallest blood vessels (Fig. '68), and through definite connections between lymph tubes and certain large blood vessels. The lymph, like the plasma, consists chiefly of water, and carries practically the same kinds of substances in solution. In addition, the lymph has floating in it many white corpus- cles, so that it may be compared to blood lacking the red corpuscles. The lymph has been compared to the ocean, in which life probably originated, and from which so many THE BLOOD 181 Water Water Oxygen Sugar etc. BLOOD Protein etc. one-celled organisms obtain their supplies directly. The lymph is an internal ocean from which all the cells of the many-celled animal obtain their supplies. 211. Clotting of blood. When some blood is removed from the blood vessels, whether it is taken out of the body or not, it usually becomes clotted, or thickened. This clotting is brought about by the coagulation, or solidifying, of a certain protein in the blood known as fibrinogen, which means " fibrin-maker." The ferment that causes the coagulation becomes active when the lining of a blood vessel is injured ; or it may be that the ferment is formed only at such times. At the mouth of a small cut this clot soon stops the bleed- ing, and furnishes a protective covering until the wound is healed. 212. Serum. If blood is allowed to clot in a glass vessel, we can see the mass of fibers detach themselves from the walls of the vessel, after the threads have shrunk awhile, and the clot floats at last in a clear liquid that is almost colorless or slightly yellowish. This clear liquid is called a serum, and is practically the same as the blood plasma, but lacking the fibrinogen. Whatever is characteristic or distinctive of the plasma of an individual or of a species will be found in the serum. 213. The white corpuscles. On closer study the white cor- puscles are seen to be cells. In many ways they are like the ameba (see p. 24). They consist of naked protoplasm, have no definite shape, are capable of free movement, are sensitive to chemical and other changes in their surroundings, and are LYMPH FIG. 68. What goes through the wall of a capillary From the blood within the capillary, water, salts, food, and oxygen pass out by osmosis ; from the surrounding lymph, carbon dioxid, urea, and water pass into the blood. White corpuscles work their way through the wall of the capillary, between the cells 1 82 ELEMENTARY BIOLOGY capable of eating small particles by swallowing them intp the protoplasm of which they consist, just like the ameba and other one-celled animals. These ameba-like corpuscles are found in the blood not only of mammals but of all animals that have blood ; and they are very much alike in all, so far as general appearance and be- havior are concerned. Their function has come to be under- stood only in modern times, chiefly through the works of the Russian biologist filie Metchnikoff, the late director of the Pasteur Institute in Paris. To help us understand the functions of these cells it is well to recall that whereas the one cell of the ameba carries on all the functions of a living body, the various cells of a many-celled animal like a butterfly or a baby are differentiated in regard to function as well as in regard to structure. Now, the white corpuscles are in many ways the least differentiated cells in the body. They are the ones that have the general qualities of protoplasm in the greatest degree ; they have not become specialists in any one line. They can move, like muscle cells, but not so quickly or so vigorously. They are irri- table, like nerve cells, but in a slighter degree. They are chemical laboratories, like gland cells, but do not produce specialized juices. They are digesting cells, and so on. With this view of the white corpuscles as undifferentiated, wandering cells we may try to understand just what they do in the body. 1 . As eating-cells (called by Metchnikoff phagocytes -, which means " eating-cells ") they are capable of engulfing foreign particles with which they may come in contact. In this manner they may eat and digest dead particles resulting from the break- ing down of tissue cells, and live cells introduced from without — that is, bacteria that may get into the body in various ways. 2. As sensitive or irritable cells they may respond to a chemical stimulation by producing substances that neutralize or counteract foreign chemicals, as various kinds of poisons. THE BLOOD 183 3. As moving cells they wander about from the lymph to the blood, or vice versa, and even into the intestines, carrying with them dead matter to be eliminated, or crowding in large numbers and producing some special substances that counteract a local chemical disturbance. Because of this peculiar behavior we have come to think of these corpuscles as perhaps the most important agents in keeping the body in health, at least in relation to certain special diseases. White corpuscles probably originate by the division of ameboid cells in the bone-marrow and in particular portions of the lymphatic S)^em, enlarged lymph spaces containing crowds of the white corpuscles. 214. The red corpuscles. * The blood of vertebrate animals contains red corpuscles in addition to the white ones. In all except the mammals the red corpuscle has a nucleus in the center, which makes the disc* somewhat thicker, in the middle, and its shape is elliptical rather than circular.1 The largest red corpuscles are found in the amphibia ; and even with the lower power of the microscope we may easily see the elliptical discs in the flowing blood of a frog's web or a tadpole's tail. The distinctive thing about the red corpuscle is the fact that it contains a substance known as hemoglobin, which readily combines with oxygen or with carbon dioxid, according to the chemical conditions to which it is exposed. The red corpuscle thus acts as a gas-carrier in the blood. Recent experiments made in this country show that there is a special ferment in back-boned animals which causes the hemoglobin to combine with oxygen in the lungs (or gills), and that there is another ferment which causes the oxygen-hemoglobin combination to break up in all parts of the body. 1 Among the mammals, the camel is exceptional in having elliptical blood corpuscles. Among the fishes, the perch and a few others have circular corpuscles. 1 84 ELEMENTARY BIOLOGY The red corpuscles, like the white ones, are really unattached cells. They originate by successive cell divisions of special cells found in the marrow of bones. At first they have a nucleus; but this soon disappears. Lacking a nucleus, these cells cannot remain alive very long. It has been found that the older corpuscles disintegrate, and their hemoglobin is taken up by the liver and converted into bile.1 1 In its chemical composition hemoglobin is somewhat like the chlorophyl of plants. To form either of these pigments, protoplasm must have iron. Iron given as a medicine or tonic is administered to assist in the formation of hemoglobin and red corpuscles. A deficiency in the red corpuscles results in the general weakness and paleness that are characteristic of the condition of the body known as anemia. CHAPTER XXXV THE CIRCULATION OF THE BLOOD 215. The vessels. In all backboned animals the blood is entirely inclosed within a set of tubes, so that everything enters or leaves the blood only by passing through the walls of these tubes (see p. 180). The tubes or vessels reach all parts of the body, so that if we could somehow remove all of a person except the blood vessels, we should have a pretty good imitation of the whole body. There is a special muscular enlargement of the blood vessels known as the heart. Through the contractions of this organ the fluid is kept in constant and continuous motion — not in a back-and-forth motion, as the ancients believed. The vessels in which the blood flows from the heart are called arteries ; those in which the blood returns to the heart are called veins. The main arteries and veins run in parallel courses and branch and subdivide until the smallest tubes are too small to be seen without a microscope. The smallest branches of the arteries connect with the smallest branches of the veins, so that the blood that has been moving into smaller and smaller vessels presently begins to flow into larger and larger ones. These smallest tubes, connecting the arteries with the veins, are called capillaries. The walls of the capillaries are so thin that diffusion is constantly taking place through them ; and the white corpuscles work their way through them, passing between the cells (Fig. 68). All the changes in the composition of the blood take place while the blood is in capil- laries in various parts of the body. 185 1 86 ELEMENTARY BIOLOGY 216. The heart. In birds and mammals the heart is a double muscular organ, the right and left halves being quite distinct from each other, in the sense that blood cannot pass directly from one side to the other. Each half of the heart consists of an upper receiving chamber and a lower pumping chamber. The left heart is somewhat larger and stronger than the right heart. Its ventricle, or pumping chamber, closes up, or contracts, at fairly regular intervals, forcing the contained blood into the largest artery of the body (the aorta), by the branches of which it is carried on to the various organs and tissues. The auricle, or receiv- ing chamber, of the left heart is connected with a large vein that brings blood gathered from the capillaries of the lungs. FIG. 69. Diagram of the human heart The receiving chamber aa, receiving chambers, or auricles • bb, pumping Opens directly into the left chambers, or ventricles- cc, main veins, bringing ventrfcle by means Qf an blood to the heart ; ad, main arteries, carrying * blood away from the heart ; ff, valves, prevent- opening which is guarded ing back-flow from arteries to ventricles; be- , , r fl f^ f tween a and J, valves preventing back-flow from DV a S ventricles to auricles vent the blood from flowing back when the pumping chamber contracts. Another set of valves prevents the blood in the aorta from flowing back into the ventricle when the latter expands again. The left heart thus pumps blood received from the capillaries of the lungs to arteries all over the body. The right heart receives blood from two large veins con- nected with its auricle, or receiving chamber (Fig. 69), and passes it into the ventricle, or pumping chamber. The auricle and ventricle on the right side are also connected with each THE CIRCULATION OF THE BLOOD 187 other, like the corresponding chambers on the left side, and there is a valve preventing the back-flow of blood from the ventricle to the auricle. The right ventricle pumps blood into a large artery that carries blood toward the capillaries of the lungs (pulmonary artery]. 217. The double circulation. The blood stream may be traced from any point back again to the start only by pass- ing through the two sides of the heart and through the pul- monary, or lung, circulation and the general body, or systemic, circulation. Thus, beginning, for example, in the capillaries of the hand, the blood flows into the veins and is gathered into larger and larger vessels, reaching the right auricle ; from this it goes to the right ven- tricle, and when the latter con- tracts, it is forced into the pulmonary artery, which di- vides into smaller and smaller branches, the smallest being the capillaries that lie under the lining of the air sacs of the lungs. As the blood flows on, it is gathered into larger veins that unite to form the pulmo- nary vein, which empties into the left auricle. From the left auricle the blood goes to the left ventricle, and from this it is pumped into the aorta. An artery branching from the main FIG. 70. The double circulation of the blood The arrows indicate the direction of blood flow. The shaded portion represents blood lacking in oxygen. From the right heart (shaded) the blood passes to the lungs, from which it returns to the receiving chamber of the left heart with its carbon dioxid replaced by oxygen. From the pumping chamber of the left heart the blood passes to all parts of the system, or body, and returns to the receiving chamber of the right heart with its oxygen replaced by carbon dioxid 1 88 ELEMENTARY BIOLOGY artery carries blood into the arm, and as the arteries divide, becoming smaller and smaller, we at last reach the capillaries in the hand, from which we started (Fig. 70). The double circulation makes it possible for the carbon dioxid in the body to be completely exchanged for oxygen in a com- paratively short time. In the human body all of the blood passes through the heart (and therefore through the capillaries of the lungs) once in from twenty-three to thirty seconds. The exchange of gases in the air sacs of the lungs has already been described (see p. 148). 218. Changes in the blood. When in the capillaries of the various tissues of the body, the blood absorbs from the surround- ing lymph (by osmosis) carbon dioxid, urea, and other sub- stances that are present in relatively large proportions (that is, compared to their concentration in the blood plasma) ; and it loses by the same process food materials, salts, oxygen, and ferments that are relatively more abundant in the blood than in the surrounding liquids. In certain parts of the body additional changes take place in the composition of the blood. In the intestines, for example, much of the digested food is absorbed into the blood. In the kidneys much of the urea, salts, and other waste substances are taken from the blood. 219. Ferments in the blood. In the capillaries of certain organs the blood receives, in addition to the usual waste products, various special ferments, or chemical substances. For example, from the thyroid gland, which is a Y-shaped, spongy body lying in front of the larynx, the blood absorbs a substance that has an important influence on the development and working of the brain. From the pancreas the blood absorbs a substance that has an important influence on the oxidation of carbohydrates in the cells. From little capsules that lie just above the kidneys the blood absorbs a ferment that influences the muscles of the blood vessels, and also has some effect on the nervous system. The amount of this secretion is increased whenever strong feelings are aroused, such as fear or anger. An increase in the amount in the blood acts upon THE CIRCULATION OF THE BLOOD 189 the liver, increasing the output of glycogen into the blood, and it interferes with the secretions and contractions of the stomach. The adrenin, as this substance is called, also hastens the clotting of blood. In young mammals there is a soft organ lying in the front part of the chest, the thymus, the ferments from which have an important influence on the growth of the animal. Another ferment that affects the growth is absorbed by the blood from a little body lying at the base of the brain. These various ferments are sometimes called internal secretions, because they are absorbed from inside the organ, instead of coming out of the gland through special ducts, as is the case with ordinary glands. CHAPTER XXXVI HYGIENE OF THE CIRCULATORY SYSTEM 220. Care of the heart. Every contraction of the ventricles (the two ventricles work together) sends a wave of pressure through the blood in the arteries. The muscular and elastic walls of the arteries " give " somewhat to this pressure, and this is the pulse which can be felt in any artery near the sur- face of the body, as at the wrist, on the temples, or immedi- ately in front of the ear. From the character of the pulse the physician can often tell a great deal about the workings of the heart and about the condition of the blood vessels. The pulse may be regular or irregular ; it may be strong or weak. A strong heartbeat would ordinarily increase the pressure of the blood inside the arteries ; but if the arteries are flabby, the additional work of the heart may fail to distribute the blood properly to all parts of the body. Cold feet and hands are an indication of inadequate circulation, but the cause of this condition may be in the heart or it may be in the blood vessels. In examining a person the careful physician, athletic direc- tor, or insurance examiner will always listen to the beating of the heart and examine the pulse and test the blood pressure. From the sounds of the heart he can tell whether there is a defect in any of the valves. A leaky heart has to do a great deal more pumping to keep the body supplied than a sound heart, since a portion of every stroke is wasted in pumping blood that goes back into the auricles. A weak heart usually shows itself in breathlessness. If you cannot climb stairs, or take a brisk walk, or play a lively game, without getting out of breath, the trouble is more likely with your heart than with your lungs. In training for athletics 190 HYGIENE OF THE CIRCULATORY SYSTEM 191 one of the most important things is to acquire "wind"; that is, the ability to continue severe exertions without losing breath. This is in fact a training of the heart, as well as a training in correct breathing habits. Under suitable direc- tions one can strengthen his heart considerably by means of graded exercises in walking, running, climbing, etc. Indeed, FIG. 71. Treating a cut When the pressure of the thumb is not sufficient to compress the blood vessels and stop the flow, a tourniquet may be used, made by tying a handkerchief about the limb and twisting it tight by means of a stick slipped under the handkerchief. Of course, the tourniquet or the bandage applied in this way is to be considered an emergency measure, and steps should be taken to have the wound attended to by a physician one of the dangers of the athletic enthusiasm is that a student will overdevelop his heart. But occasional severe strain upon the heart is not the same as training it for hard work, and a person with a weak heart should not be engaged in work that strains this organ severely. 221. Cuts and wounds. Small wounds will usually stop bleeding in a short time because of the clotting of the blood (p. 1 8 1). In view of our modern knowledge about the wide 192 ELEMENTARY BIOLOGY distribution of many kinds of bacteria, however, it is wise to look after even small cuts and scratches. The festering of sores and cuts, which was formerly looked upon as a normal and necessary condition of healing, we now know to be the result of the action of various kinds of microbes, some of which, at least, produce serious blood poisons (see p. 195). To prevent the festering of a sore, and to prevent the invasion of the body by more injurious micro- organisms, it is well to treat every cut with an antiseptic, or sterilizing, solution, such as a solution of carbolic acid or bichlorid of mercury, or with tincture of iodin or alcohol. The cut should then be covered with clean cotton or gauze, to prevent the entry of microbes later. With larger wounds it is sometimes necessary to use special means to stop the bleeding. When the flow of l)lood is too strong, the adhesion of the clot to the sides of the wound may be prevented. When the flow is from an artery (which may usually be recognized by the pulsation), the limb should be tied above the cut ; that is, on the side toward the heart. When the flow is from a vein, the attempt to stop the flow should be made on the side away front the heart. 222. Nose bleeding. In very many cases nose bleeding may be stopped by snuffing cold water. The old-fashioned remedy of dropping a key down the person's back rested on the fact that the chill causes the capillaries to contract. A piece of ice applied for a few moments to the back of the neck will be more likely to have the desired effect. Where bleeding con- tinues after such a simple treatment, it is probable that some small artery has been broken, or that the person's blood is incapable of clotting. An astringent is then advisable. Powdered alum, tannin, or ferric chlorid may be applied on a tuft of cotton. These substances cause the fine blood vessels to contract and thus stop the bleeding. In extreme cases a physician will use adrenin, an extract of the capsules lying above the kidneys (see p. 189). CHAPTER XXXVII THE BLOOD AS A LIVING TISSUE The white corpuscles respond to the stimulation of foreign substances in several distinct ways. Some of these are signifi- cant in fighting diseases ; others have been used in different ways ; still others we can neither understand nor utilize in any practical way. 223. Precipitins. When the white of an egg is taken into the stomach of a backboned animal, it may be digested and eventually absorbed into the blood or lymph, and from this to the cells, where it acts as a food. But if some white of egg is injected into the blood (of a rabbit, for example), it will have a curious effect upon that blood. After a few such injections the blood contains a substance that is not present in normal rabbit blood. This new substance, which cannot be detected by chemical methods, can be shown to be present if a drop or two of the serum (see p. 181) of the treated rabbit is mixed with a drop of water containing some white of egg. Imme- diately there will be a visible precipitate. The substance that is present in this serum, causing the precipitation of the egg albumen, is called a precipitin, or precipitating substance. The formation of the precipitin by living cells is not at all under- stood. An important fact about the precipitin is that it is always specific. That is to say, a precipitin formed in an animal under the influence of white of egg will precipitate only white of egg, but will have no effect on other proteins ; a" precipitin produced under the influence of a protein taken from a goat will precipitate only this goat protein ; and so on. The fact of precipitin formation has been put into practical use in several ways. 194 ELEMENTARY BIOLOGY i. The proteins of different species of animals may be very much alike, so far as the tests of the chemical laboratory show, but the living protoplasm can detect specific differences. Because of the formation of precipitins it becomes possible to determine whether a given drop of blood, for example, is of human origin or from some other animal. This is often important in legal trials. 1890 1895 1900 1905 1910 1911 1912 1913 1914 1915 1916 1917 110 Deaths for every 100,000 of population || 38.0$ of cases) 106 In this year antitoxin was introduced 1 | 18.8J [~153\ nr? |mo J&3 [JUS to P Eg r&9 62 | 36 | 37 | 28 | 22 | 26 | 31 | 24 | 18 | 21 | FIG. 72. Reduction in deaths from diphtheria in New York City (Manhattan and Bronx) The numbers and rectangles on the left show the number of deaths out of every hun- dred thousand of the population. The numbers on the right show how many fatalities there were for every hundred cases of the disease. Note the rapid falling off in both the proportion and the number of deaths after the year 1895, when antitoxin was first used 2. In the examination of food products that contain materials from various sources, it is impossible to determine, by the usual chemical methods, from what organisms the materials were obtained. By means of the precipitin tests, however, it is possible to find out whether a given sausage, for example, contains pork or beef or horse meat. 3. Experiments now under way in laboratories and hospitals make it appear probable that this principle will have wide application in the diagnosis of disease. 224. Antitoxins. In the bodies of various plants and animals are found proteins that act in a peculiar way upon the blood of higher animals. They act as poisons that stimulate the living cells, and especially the white blood cells, to produce THE BLOOD AS A LIVING TISSUE 195 First Day Second Day Third Day Fourth Day Fifth Day or later 0 43 112 159 186 Deaths Deaths Deaths Deaths Deaths specific neutralizing or counteracting substances. This class of poisons is now known by the name toxins (from a Greek word meaning " poison ") and is illustrated by the venom of the rattlesnake and of some other snakes, by a certain protein found in the seeds of the castor-oil plant, and by one found in the bark of the black locust. The substance produced by the live cells under the influence of a toxin is called an anti- toxin, and it is always specific ; that is, it will neutralize the poison under whose stimulation it was produced, but no other. The best-known toxins are those produced by cer- tain bacteria, especially those that cause lockjaw and diphtheria. When a quantity of toxin, not enough to kill, is injected into the blood of an ani- mal (for example, a horse), the cells begin to throw off antitoxin. They will produce more than enough antitoxin to neutralize the poison received. After the poison has all been destroyed, there will be a quantity of antitoxin in the blood. If now a larger quantity of the poison is introduced, some of it will be at once neutralized by the free antitoxin ; and if the animal is in good health, an additional quantity of antitoxin will be produced. In this way it is pos- sible to increase the amount of antitoxin in the blood until there is several hundred times as much as would be necessary to neutralize very many fatal doses of the poison. In preparing antitoxin for diphtheria this is practically the method followed. In two or three months the blood of the horse contains a large amount of antitoxin. Blood is drawn from a vein in the neck and is allowed to clot. The serum now contains all the antitoxin. FIG. 73. Danger in delay Each rectangle represents one thousand children suffering from diphtheria. Antitoxin was admin- istered to all the children on the days indicated. The number of deaths in a group increases with each day's delay in the use of antitoxin 196 ELEMENTARY BIOLOGY The use of the antitoxin is coming to be quite general in the treatment of diphtheria, and in the prevention of diphtheria in the case of people who have been exposed to infection. The efficacy of the treatment depends upon its being introduced early enough to neutralize the poison given off by the bacteria before these have gained any headway in the body of the patient.1 Antitoxin serums have been prepared for tetanus (lockjaw), for snake bite, for scorpion sting, and for castor-bean poisoning. 225. Agglutinins. In the case of typhoid fever it was found that if a few drops of the serum from a patient are mixed with a few drops of liquid containing typhoid bacteria, the bacteria are all clumped together in masses, instead of floating about separately. This illus- trates the formation in the blood of a substance that acts upon bacteria by agglutinating them, or sticking them together. Such substances are called agglutinins, and, like precipitins and antitoxins, they are specific. Although the agglutinins do not kill the bacteria, they probably interfere in some way with their action (see Fig. 74).2 226. Cytolysins. When the blood of a human being or other backboned animal is examined under the microscope, the red corpuscles and the various white corpuscles are seen to float or move about apparently unaffected by one .another. But if some blood of a different species of animal is injected into the veins of a rabbit or mouse, the foreign red corpuscles 1 The gain that has come through the discovery of the principle of anti- toxin formation, and through its application, can be measured in terms of reduced loss of life. This may be measured in two ways : 1. Out of all the population, what is the reduction in the proportion of those that died of diphtheria ? 2. Out of all the people who get the disease, what is the reduction in the proportion of those that die? Both of these sets of facts are given in the diagrams on pages 194 and 195. That it is always safest to use antitoxin as early in the history of a case as possible is shown by facts like those given in the diagram (Fig. 73) based on hospital records. 2 After a mass of typhoid bacteria has been agglutinated by a serum it is still possible to get the same bacteria to multiply in a suitable medium. THE BLOOD AS A LIVING TISSUE 197 are presently destroyed, being dissolved by a specific substance that is capable of dissolving these foreign cells. This cell- dissolving substance is not constantly present in the blood, but is formed after the foreign cells are introduced. These cytoly- sins, or "cell dissolvers," are formed not only in response to the foreign red corpuscles ; they may be formed in the presence of other kinds ~i.j*XCxo, of cells, as of different tissues or of >/7'Jrl£K\ various bacteria; and they are always *"-^r i-N'^"* specific. Thus, the serum of a rabbit that has been treated with human blood will dissolve the red corpuscles of human blood when the two are mixed in a glass, but not the corpuscles of any other ani- mal. The serum of a person whose blood has been treated with dead typhoid bac- a> bacilli swimming about teria will dissolve typhoid bacteria when separately; 6, the same the two are mixed in a glass, but not dumped together' or ag' b FIG. 74. Agglutination of typhoid bacilli other species of bacteria. tion takes place, the person is known to be infected with typhoid germs glutinated. The Wtdal test for typhoid fever consists T-I /• ,-T i .. 11 . in mixing a few drops of These facts are utilized practically in serum from the susp^cted vaccination against typhoid fever, and no person with a quantity of i,, ,1 j. MI i r j typhoid bacteria under the doubt other diseases will be found sus- micr0scope. if aggiutina- ceptible to the treatment. A measured quantity of dead typhoid germs is injected into the body. The specific cytolysin is formed by the action of the live cells. Later, when live typhoid germs get into the body, they are dissolved by the cytolysin already present. Recently a specific cytolysin active against the cells that cause cerebrospinal meningitis has been produced experimentally in monkeys by Dr. Simon Flexner of New York, and the serum of these animals may be successfully used in treating the disease in human beings. The cytolysis test may also be used like the precipitin test in the differentiation of blood stains etc. (see p. 194). 198 ELEMENTARY BIOLOGY 227. Opsonins. Although the white corpuscles (or rather certain kinds of them, for there are really several kinds of colorless corpuscles in our blood) will " eat " foreign particles, including many kinds of bacteria, they will do this only under certain chemical conditions. A substance which thus makes the bacteria "attractive" to the corpuscles is called an opsonin, which comes from a Greek word meaning " to prepare a meal." The activity of the cell-eating corpuscles depends upon the presence of a suitable quantity of the right kind of opsonin. The production of opsonins may be stimulated by the introduc- tion of small quantities of dead or living bacteria of various kinds. 228. Vaccines and vaccination. We have seen that the blood meets the invasion of foreign bodies or foreign substances in several different ways, all of them depending upon the vital properties of the cells of the body, and especially of the white corpuscles. In recent years various methods for increasing the resistance of the body to specific diseases have been developed by using in some cases one of the blood's reactions, in other cases another. All of these are roughly classed as vaccination. In some kinds of vaccination cytolysins are produced ; in other kinds, opsonins or antitoxins. 229. Immunity and susceptibility. Individuals differ so greatly that some are much more sensitive or much less sensitive to certain substances than others. Now we find that sometimes, or in some people, the blood is extremely sensitive to foreign substances of a particular kind. It is a matter of common observa- tion that some people catch colds more easily than others ; some more frequently have boils or pimples ; some are more suscep- tible to typhoid or to diphtheria. Not only do individuals differ one from another ; there are also racial differences. Thus, the dark-skinned races are less susceptible to malaria and to hookworm than the white races ; on the other hand, the white races are less susceptible to tuberculosis and measles than the dark races. THE BLOOD AS A LIVING TISSUE 199 More than this, there are important specific differences. Just as hens are quite indifferent to the action of morphin, and as rabbits are insensitive to atropin, human beings are quite immune to diseases that are serious or even fatal to birds or cattle. Such immunity is called natural immunity, and is inherited. In many cases it probably depends upon the chemical peculiarity of the blood or the body juices ; in others it depends upon the quick response of the live cells to the poisons and other products of the bacteria. But such natural immunity is not absolute ; that is to say, it may be weakened or destroyed by various conditions. We may see from these considerations how important it is to guard the natural immunity of the body against the destruc- tive effects of undue exposure to extremes of temperature, to excessive fatigue (or insufficient rest and sleep) or overwork, and to prolonged hunger or thirst (or improper nutrition). We can also understand the importance of suitable conditions and habits of ventilation and of exercise in maintaining the resist- ance of the body, and the danger of using drugs, alcohol, or other substances that interfere with the action of the blood as a living tissue. Not only the conditions that commonly affect the energy and resistance of the protoplasm, but disease itself may affect the resistance of the body. After pneumonia, for example, one is more liable to catch other diseases than he is ordinarily, and he is more liable to catch pneumonia after typhoid or measles than he is ordinarily. One cannot afford to be sick even a little ; that opens the way for more serious trouble. 230. Acquired immunity. It is a well-known fact that one who recovers from certain diseases is practically immune ; as the common saying goes, "You can't have measles twice." This is true of mumps, whooping cough, scarlet fever, typhoid fever, and smallpox. This acquired immunity is no doubt due to the substances produced in the blood in the course of the disease. In the case of diphtheria and of some other 200 ELEMENTARY BIOLOGY diseases, the antitoxin that is formed gradually disappears, so that after some time it is possible for the recovered patient to have the disease again. Immunity may also be acquired, as we have seen (pp. 196, 197), by artificial means. A passive immunity, lasting a com- paratively short time, is acquired by the administration of an antitoxin. This is called passive because the blood does nothing to combat the disease ; it is simply protected by the substance added to it. An active immunity may be acquired by the use of a vaccine, which stimulates the live cells to produce the substances that cause the immunity. 231. Disease and heredity. We know that certain diseases have a way of " running in families." That is, a given family may show many members that have suffered from the same disease, as tuberculosis. It was commonly believed until recent times that tuberculosis and other diseases are inherited, in the same sense as the color of the eyes or the shape of the thumb is inherited. We know that this is not true. Where tuberculosis runs in a family, two facts are to be distinguished : 1. Where one member of a family has the disease, the other members are more likely to be exposed to infection than they would be ordinarily, and so the disease spreads in that family. 2. Where a person has tuberculosis (or any other disease), the indications are that this person has not a natural immunity to the disease ; in other words, that he is susceptible to it, or has a " disposition " towards it. Now it is this natural sus- ceptibility (or immunity) that is inherited, and not the disease. No matter how much susceptibility one had to a given disease, he would not contract the disease unless he was exposed to the infection by the specific microbes that cause that disease. CHAPTER XXXVIII WASTES AND BY-PRODUCTS OF ORGANISMS 232. The origin of wastes. Every chemical process results in the formation of substances that did not exist before. In the chemical processes that take place in the cells of a liv- ing organism, substances are produced that are directly related to the protoplasm's being ''alive." Other substances, which are of no direct use to the living body or to the living process, are produced incidentally. The latter are called wastes, and may be compared to the sawdust of a mill, or to the smoke that goes up the chimney, or to the ashes that drop through the grate. 233. Removal of wastes from cells. In our study of pho- tosynthesis (p. 54) we found that one of the wastes or by-products is oxygen, which diffuses out of the chlorophyl- containing cells through the cell walls. In our study of ener- gesis (p. 143) we found that carbon dioxid, water, urea, and other substances may be produced. These also diffuse out of the cell. In plants water and carbon dioxid are usually eliminated in the form of vapor. The carbon dioxid given off by the cells of the roots usually remains in solution, forming so-called carbonic acid (see Fig. 42, p. 144). Plants often dispose of their waste substances in a way that seems to be beneficial to them ; and the same is true of animals (see p. 203). 234. Accumulation of wastes in plants. The waste sub- stances produced by plants (outside of water and carbon dioxid) are generally not eliminated from the body. They are usually combined into insoluble or non-diffusible compounds, and in this condition they are accumulated in dead cells in parts 201 202 ELEMENTARY BIOLOGY of the plant where they do not interfere with the vital activities. There are many classes of such waste compounds. i. Among the most common waste substances found in plants are various pigments. These are familiar to us in the autumnal colorings of leaves, in the bright colors of many flowers and fruits, and in the pigments of woods and roots. The red of the radish, of the rose, and of the maple leaf, the yellow of the buttercup, of the pumpkin, and of the carrot, and the blue of the pansy and of the huckle- berry, all are examples of the waste products that are deposited in out-of- the-way cells (Fig. 75). 2. The attractive odors of many flow- ers, preserved for us in perfumes and essences, and the flavors of the many spices are due to essential oils de- posited in out-of-the-way cells of plants. They are found in the flower, the leaf, the fruit, the bark, the wood, and even in the root. 3. Tannins are chemical compounds related to tannic acid which have the FIG. 75. Pigment bodies in plant cells Most of the pigments found in the cells of plants are probably in the nature of waste products. Some pigments occur in solu- tion; others are found attached property of forming hard, insoluble to protein particles, forming j .,T , • T.I definite colored granules compounds with proteins. They are commonly found in the bark of trees, but may also occur in other parts of the plant — often in unripe fruits. 4. The acids that we find in fruits especially, although they are present in other plant organs, are derived from substances diffused out of live cells. Alkaloids (see p. 74) and other poisonous substances found in plants, and the gums and resins, are also waste products. 5 . The excess of mineral matter absorbed from the soil is separated out of the live cells by being deposited in cell walls or by being precipitated as insoluble compounds in certain of WASTES AND BY-PRODUCTS OF ORGANISMS 203 the cells. Sand (silica) is thus found in the scouring rushes and other plants ; and crystals of oxalate of lime are found in hundreds of plants — for example, in the root of the jack-in- the-pulpit and in other sharp-tasting plant parts (Fig. 76). All these waste substances are useless to protoplasm, and many of them are even injurious. But, separated as they are from the living parts of the organism, they may nevertheless be of some value to the plant as a whole, or to the species, in some special relation. Thus, the pigments and odors of flowers may be of use in relation to insect visits (see pp. 310- 314); or essential oils and tannins may be of value in protecting plants from animals and from bacteria or fungi. 235. Excretion in animals. The one-celled animals excrete their wastes just as they excrete carbon dioxid. In the higher animals, those that have blood and lymph, the wastes are dif- fused into these conducting fluids, and are then eliminated from the body through special organs, for the most part.1 236. The kidneys. In the human body, which in this respect is typical of the backboned animals, the kidneys are the special excretory organs. Water and carbon dioxid are, as we have already 1 To a comparatively slight extent the waste products of animals, like the waste products of plants, are accumulated in some of the cells. Thus, many of the pigments found in animals are no doubt to be considered as in the nature of wastes deposited in the cells of the skin, or even in the interior of the body. Much of the lime found in the skin of such animals as the starfish or the sea lily, and the coral framework of the coral polyp, no doubt fall into the same class. FIG. 76. Crystals found in plant cells /, in seed of castor-oil plant ; 2, in bark of a tree ; j, in bulb of squill. Crystals in plant cells often represent an accumulation, or a locking up, of superfluous mineral matter 204 ELEMENTARY BIOLOGY learned (see p. 148), excreted from the lungs ; small amounts of urea, and pos- sibly other organic wastes, also leave the body through these organs. Some water, salt, and urea, with traces of other organic wastes, leave the body by way of the sweat glands, and some reach the intestines and are then thrown out. But most of the waste is eliminated by way of the kidneys. The kidneys, of which there are two, may be considered' as glands whose function is the separation from the blood of certain specific substances (as urea, salt, etc.) and water (Fig. 78). 237. The sweat glands. The sweat is excreted by special glands which consist of delicate twisted tubules surrounded by a network of capillaries (Fig. 77). Waste material, with water and salt, is con- stantly passing from the capillaries into the tubules, and through these out upon the surface of the skin. Ordinarily the water part of the perspiration evaporates as fast as it comes out of the glands, leaving a solid deposit of the wastes. When perspiration is more rapid, we can see the drops of sweat on the skin. When this dries, the solid deposit is left on the outside of the skin, instead of in the mouths of the tubules. FIG. 77. A sweat gland The sweat gland consists of a fine tubule opening to the surface of the skin at one end and coiled up in a knot at the other. The coiled portion is surrounded by blood vessels (capillaries) from which water, urea, and salts are withdrawn into the gland tube CHAPTER XXXIX HYGIENE OF EXCRETION 238. Hygiene of the kidneys. The kidneys work continu- ously. Their work can be facilitated by drinking plenty of water every day, and by emptying the bladder with sufficient frequency to prevent discomfort. Aside from maintaining the general health of the body, there is nothing else that we can do for the kidneys ; and, indeed, nothing else needs to be done. Where a generation ago workers quenched their thirst with beer and other prepared drinks, now employers and managers of factories and shops are finding it worth while to provide an abundance of clean, cool, and palatable drinking water. In some states the law requires that suitable drinking water be supplied in all workrooms. In a similar way the provision of suitable toilet rooms, from being a casual accommodation or conven- ience, is coming to be recognized as a real necessity not only by the opinion of the experts but by the framers of laws. The more progressive cities are also taking steps to provide suitable drinking water for all on the streets and in public places, as well as comfort stations for all who have to be abroad. An understanding of the intimate relation between the wastes of the tissues and the secretions of the kidneys has made possible the development of special methods of diagnosis through the chemical and microscopic examination of the urine. The sugar and the albumin and the uric acid that the expert discovers in the urine are to be con- sidered as indications of the general condition of the body, or of certain organs, and not necessarily of the condition of the kidneys. The effect of alcohol upon the kidneys is related to the fact that it causes a congestion of capillaries. When the capillaries of the kidneys are congested, the excretion of wastes 205 206 ELEMENTARY BIOLOGY is to that extent impeded, and the whole body suffers in conse- quence of the retention of urea and other poisonous by-products of protoplasmic activity. 239. Hygiene of the skin. The skin is more than an ex- creting organ ; indeed, the excretory work of the skin may be considered rather in- cidental. Other functions of the skin are as follows : 1. Protection of the body (a) against mechanical injury, (b) against drying up, and (c) against excessive radiation of heat. 2. Regulation of the body temperature. 3. Perception of sensations of touch, heat, etc. But it is the secretions of the fat glands and the perspi- ration (see Fig. 77) that give rise to the need for special attention to the skin. The accumulation of wastes left behind when the perspira- tion evaporates and the catching of dust in the oil secreted by the oil glands, and in the pores through which the oil is secreted, call for periodic bathing. The best way to remove the accu- mulated wastes and dirt is by means of hot water and soap, with the help of a brush. But hot baths have a debilitating effect, forcing the blood into the capillaries of the skin and thus away from the muscles, brain, and internal organs. A warm bath once or twice a week should be enough for ordinary cleanliness if these supplement a daily cold bath. FIG. 78. Kidneys and bladder a, the main artery, and &, the main vein, in the abdominal cavity, giving off branches to the kidneys, cc ; d, funnel-shaped cavity in which the waste fluid is gathered by the gland action of the kidney ; ee, tubes lead- ing from the kidney to the bladder, /. The left kidney is represented as cut through lengthwise HYGIENE OF EXCRETION 207 For those who can stand it a daily cold bath is refreshing and at the same time an excellent training for the skin in adjusting itself to changes in temperature. The cold bath should never be taken when the body is exposed to cold, however. The best time is immediately upon rising, or immediately after physical work or exercise that has produced copious sweating. But since there are many people who cannot tolerate the cold bath, because of the after-effects of the shock, it is not to be generally recommended. Each one must find out for himself whether he can benefit from it. All of us can stand a splash of cold water after a warm bath or, in the morning, over the chest and back ; and probably most of us can gradually learn to stand the cold bath, either by systematically lowering the temperature of the water used, day by day, or by increasing the surface to which cold water is applied with a sponge. In any case the cold bath should be followed by a brisk rub with a rough towel. It is hardly necessary to remind anyone that bathing will not main- tain cleanliness if the underclothing is not changed with sufficient frequency. 240. Exercise. The value of exercise for the muscles, for the heart, for the breathing, for the digestion, and for the work of the bowels has already been mentioned. The value of exercise for the skin and the excretion generally are worth noting. The slow, continuous perspiration, of which we are not aware, leaves deposits of wastes in the tubules of the sweat glands. More rapid perspiration washes these wastes out. Exercise that results in sweating cleans out the pores. In so far as exercise accelerates the flow of the blood, it contributes also to the more rapid removal of wastes through the kidneys. CHAPTER XL EXCRETION AND FATIGUE 241. Getting tired. When you "chin" yourself on a bar four, five, six times, until you can do no more, this does not mean that you will never be able to chin yourself again. After resting awhile, perhaps a day or an hour, or perhaps only ten or fifteen minutes, you can chin yourself again as well as at first. What happens in the first place to make you stop, or what happens during the rest to enable you to do the work again ? A modern explanation is that the waste substances begin to accumulate in the cells as soon as the work commences ; the wastes are formed faster than they can be carried away, and the result is a poisoning of the protoplasm of the working cells. Experiments have enabled us to discover the importance of these wastes. 242. Fatigue poisons. If a muscle taken from the leg of a frog is made to work (by being stimulated with an electric current) until it is too tired to do any more, it may be restored to working power by the simple process of washing it in salt water. The salt water certainly does not supply fuel to the muscle ; on the contrary, it would seem rather to take some- thing away. Moreover, if the salt water that has been used to wash the tired muscle is now injected into a fresh muscle, one that has not been working, the latter immediately becomes too tired to work. This would show that some unknown sub- stance has been taken away from the tired muscle to make it fresh, and that this same substance has been added to the fresh muscle to make it tired. This substance has been called fatigue poison. 208 EXCRETION AND FATIGUE 209 The presence of fatigue poisons has been shown repeatedly by experiments similar to the above. For example, a dog has been kept running until he is very much fatigued ; some of this dog's blood is then introduced into the veins of a dog that has been kept quiet the whole day. Immediately the resting dog shows all the signs of being a tired dog. 243. Fatigue may be general. We have all been taught that "a change of work is the best kind of rest." To a certain extent this is true. When I am reading a hard book .llliiiliiiliUHMiiiiliiiiiiUliiillMiimiiiiiiiiifiiniiilliHiiiiinlnlili Illlllll III III III Hill lllllllllllll 1111111(1111 Illll Illl lnH.llllH a b FIG. 79. General fatigue as measured by the ergograph The ergograph records the frequency and the strength of a pull exerted by a finger, the rest of the hand being held firmly in place. These two records were made by a medical student on the same day : a, before beginning his day's work at college ; £, at the end of the day's work. The height of the vertical lines shows the difference in energy, or strength, of each pull. The difference between the bases shows relative time of application and begin to doze over it, I am not too tired to play a game of tennis or even to read an interesting novel. But beyond a certain point fatigue affects the whole body ; getting tired from study unfits one for muscular work or play. This is shown by certain kinds of experiments that were first carried out in Italy. Records made on the ergograph by any person will show great variations, according to the condition of the body. A record made early in the morning will differ from one made at the end of the day ; a record made after taking a nap in the afternoon will differ from one made at the close of a game of chess (see Fig. 79). Although 210 ELEMENTARY BIOLOGY the people who made these tests did not use the middle finger in their work, this finger showed different degrees of fatigue in accordance with either the physical or the mental work done before the test was made. We have learned from these and similar experiments that ex- hausting physical work tires the brain and the sense organs ; and we have learned that severe mental work tires the whole body. llillillllllllllllllllillllliilllllHIIIIIIIIIIIIllllilllllllllllllllllllllll a A liliiliiliiliiliiiiimiiiliimlMliiiiilliiimiililiiliiiliiMiiiliiliiiiUllliM 6 FIG. 80. The pace and fatigue These two ergograph records were made by the same student on the same day. a was made by pulling as rapidly as possible, and shows rapid accumulation of fatigue ; b was made by a slow, steady pull every two seconds, and although the time was twice as long as in a, and the work performed about four times as much, there is no appreciable evidence of fatigue It is not to be concluded, however, that hard work is to be avoided. On the contrary, hard work is useful physiologically, as well as morally and economically. But we must use this knowledge to help organize our work in a more effective way. EXCRETION AND FATIGUE 211 244. Rate of work. When we come to think of it, we shall recall that getting tired is not altogether a matter of what kind of work we are doing; it is partly a matter of how fast we are doing it. "It is the pace that kills " (see Fig. 80). Physiologically this means that at a certain rate or speed fatigue poisons are formed faster than they can be removed by the blood, and from the blood by the kidneys, etc., and that when the work is done at a certain slower speed, the blood can remove the wastes just as fast as they are formed. When you walk very fast, you may feel tired before you have gone a mile, although you are not out of breath ; if you walk slowly enough (but not too slowly), you may walk ten miles without showing any signs of fatigue. We may therefore conclude that work can be kept up best if we take the right pace. Work that is speeded may give larger returns in a given time — but only for a short time. If the high speed is maintained, the worker will have to stop sooner or the quality of the work will fall off. This principle has its everyday applications in athletics, in play, in housework, in school work, and in industry. 245. Fatigue and efficiency. When Frederick W. Taylor, the founder of scientific management, wanted to increase the output of useful work on the part of some unskilled workers, he did not urge them to work faster. Instead, he carefully experimented to find out how fast the necessary movements could be performed without accumulating fatigue poisons during the hours of work. He actually made the men move more slowly than they had been accustomed to. And in shoveling dirt and carrying pig iron he more than doubled the day's work without increasing the day's fatigue. This principle is so well recognized among the leaders in scientific manage- ment of works that the efforts of the experts are directed to devising plans that will prevent fatigue on the part of the workers. These plans usually contain two sets of factors, one mechanical and the other biological. 212 ELEMENTARY BIOLOGY The mechanical problem is to find out the fewest movements that are necessary for performing the work. The biological problem is (i) to arrange the material and the machinery and tools in such relations to the body of the worker as to put the least strain on the muscles, the attention, the sense organs, etc., and (2) to establish a pace that will result in a maximum of output with a minimum of fatigue. In other words, the efficiency of the day's work will depend not only on the nutrition and respiration and training of the worker but also to a very large extent upon the prompt elimination of wastes from the working cells. CHAPTER XLI FATIGUE AND THE WORKER 246. The hours of work. No matter how slowly one works, it is impossible for him to keep on working indefinitely without rest. How many hours a day should a person work ? How many hours a day'may one work and play and still maintain his health ? There was a time when mill workers had to be at their tasks sixteen and eighteen and even more hours a day. They lived, but they died young. The shortening of the work- day has certainly played a large share in the lengthening of the work life. With an excessive length of working day the body never has time to catch up with the elimination of wastes. Fatigue ac- cumulates from day to day, and sooner or later the machine is clogged beyond further use. It is, then, a question whether it is more economical to work long hours for a few years or to work short hours for a longer period. From the point of view of making the other person produce profits for me, it has often seemed best to work him for all he is worth, and then, when he is used up, to get someone else. But from the point of view of the worker and from the point of view of society this is certainly poor economy. Especially true is this when it comes to considering the work of children (see Fig. 81). The injurious effects of long working days upon the worker is coming to be realized by the workers and by society at large. This realization shows itself in two ways : 1. The workers are constantly demanding a shorter and shorter workday. 2. Legislation is constantly readjusting the legal workday on a shorter and shorter basis. 213 214 ELEMENTARY BIOLOGY This official demand for shorter hours rests chiefly on two considerations : i. The human stock must be preserved from the evil effects of overwork, and the ordinary methods of bargaining about hours and wages cannot be relied upon to secure what is fair for the workers. 2. In certain occu- pations the fatigue of the worker is a direct menace to the public. This is especially true in such occupa- tions as railroading of all kinds, elevator- operating, work on boats and ferries, and the work of drivers and chauffeurs, tele- graph and telephone operators, etc. I 1 Be 6 U ys .3 . Gi •Is Be , ys it , 01 •Is B 5C 1 ys ;! Gl -Is B 5 — , ,5.3 i6 Girls 63 5 59 4 54 8 51 5 11 1 It lb. Four- Room Three- Room Homes Homes Two- Room Homes One- Room Homes FIG. 8 1. Environment and physique Dr. Leslie Mackenzie had the weights and heights taken of all the school children (73,848) of Glasgow. The diagram shows the average weights of boys (solid lines) and of girls (broken lines), divided according to the kinds of homes the children lived in. All the studies made show that poor food, disease, overwork, poor housing, and other conditions in the environment produce measurable deteriorations in the physique of growing children In planning our own programs we should keep in mind the rela- tive amount of effort and the relative amount of fatigue connected with each kind of occupation. We cannot get the best results from our work if we have fatigued ourselves with play in the morning; nor can we enjoy our play if we have worked too hard during the day. 247. Work and rest. It is important to find out what kinds of work are most fatiguing, and what arrangements of work and rest, or what alternations of work, will make possible the greatest amount of effective activity, with the least strain on the human body. Short rest periods during the day, the alternation FATIGUE AND THE WORKER 215 of physical and mental work, the distribution of work that requires little or no active attention, are devices for achieving these ends. In many offices and factories it is coming to be customary to in- troduce a " pause " of from five to fifteen minutes in the middle of the afternoon. During the pause no work is permitted, and it is found that the total output is increased rather than diminished in this way. In schools similar ideas are being put into practice. We take a few minutes of physical exercise between study or recitation periods, to stimulate the flow of blood and to fill our lungs. In spite of all the precautions that we know should be taken, many people, and even many children, suffer from chronic fatigue. This condition shows itself in restlessness and irrita- bility, in lack of appetite, in languor and lack of concentration, in sleeplessness or disturbed sleep, in loss of weight, and in a certain drawn expression on the face. When fatigue poisons have got a little ahead of the excretory system, the best thing to do is to take as complete a rest as possible. 248. Fatigue and health. Fatigue poisons affect the gland cells as well as the nerve and muscle cells ; hence the frequent indigestion from meals eaten when the body is fatigued. Fatigue poisons also affect the white corpuscles, and the chemical activity of the cells generally, so that, when fatigued, a person is more liable to catch colds, as well as other infectious diseases. We may well conclude that it does not pay to become chronically fatigued, although there is nothing better than getting " good and tired " every day, and then getting over it again by the next morning. This means that sufficient sleep is one of the prime necessities of healthful and efficient and happy living. People whose day's tasks are too long are most likely to get their fun in time taken from sleep. It is during sleep that the working and growing cells can make up for the losses of the day's work ; it is also during sleep that the excretion can catch up with the day's accumu- lation of wastes. 216 ELEMENTARY BIOLOGY 249. Standardizing work conditions. At the outbreak of the Great War the sudden need for a rapid increase in the produc- tion of all sorts of supplies and munitions led the managers of industry to increase the number of hours of work and to "speed up " the workers in factories. They also arranged to continue work on Sundays and holidays. This was especially true in England. After some months of this intensive activity it was found that the high rate of production could not be maintained, and that there was a great deal of ill health and of physical breakdown among the workers. A commission was appointed to inquire into the health of munition workers. Among the impor- tant discoveries made by this commission were the following : 1. The increase in the number of hours of work was bad both for the health of the workers and for the effectiveness of their work. 2. The continuous work, day after day, without weekly rest days, was bad for the health of the workers as well as for the standards of production. As a result of this and of similar investigations many fac- tories in Europe and in this country have established new methods of determining the speed at which work shall be done. They have divided the day's work into short shifts, or "tricks," which permit fatigue products to be eliminated, instead of forcing them to be accumulated in the bodies of the workers. As a consequence, production has been increased, accidents have been reduced, and the health of the workers has been greatly improved. CHAPTER XLII NERVES AND THE REACTIONS OF ORGANISMS 250. What we cannot help doing. No child can keep his face composed and look unconcerned when he is properly tickled. He bursts out laughing, or he draws away the tickled part, or he does both. And when he does any of these things, he cannot help it. When something suddenly approaches your eyes, you wink, and you cannot help it. 251. Reflexes. Movements of the kind mentioned, which take place without any intention or desire on the part of the agent, in direct response to some disturbance or stimulation, are called reflexes. Some reflexes are useful, as winking, or sneezing, or coughing, or withdrawing the hand when " it hurts." Reflexes need not result in movements. The " funny-bone " reflex carries with it a definite sensation. Indeed, that is about all that we are aware of when the funny bone is struck. This suggests that there are some reflexes that are not altogether confined to movements. We have already come across reflexes that do not involve movements at all. The increased flow of gastric juice in response to the stimulation of pleasant food, and the watering of the mouth on the mere sight of pleasant food, are examples of reflexes that let themselves out in glandular activity. 252. Using an animal's reflex. If you ever catch a fish with a hook and line, you depend upon a reflex for your success. The fish responds to the vision of certain kinds of objects by snapping at them with his mouth. You simply have to make sure that you have the right kind of bait, and that it is properly fastened to the hook, and that it is dropped into the water at a suitable depth. Your "luck" depends upon the fish seeing the bait, and the reflex does the rest. 217 218 ELEMENTARY BIOLOGY 253. Reflexes and tropisms. Reflex actions of animals differ from the tropisms which we studied in the young plants, both in the greater speed with which they are Executed and in the kind of structure which brings them about. The reflexes all depend upon certain connections of nerves, muscles, and special perceiving organs, such as the eyes, ears, tongue, etc. To understand the mechanism of the reflex we must therefore know something about these three kinds of organs. 254. The muscle. We all know in a gen- eral way that the muscle is the " thing that makes us move." We also know the appear- ance of a muscle from having handled and eaten the flesh of animals. When thousands of millions of such cells contract at the same time, we can see that the whole mass will be considerably shortened. An ordinary muscle of the body, such as draws up the forearm or one of the fingers, is essentially a bundle of several masses of muscle cells, together with connec- tive tissue, blood vessels, and nerve connections (Fig. 82). 255. Kinds of muscle. The muscles that are most familiar to us are the skeletal muscles (those attached to bones of the skeleton) of such animals as we use for food — chicken, lamb, ox, etc. We have already learned that there are other mus- cles, however, such as the muscles of the heart (p. 186) and of the diaphragm (p. 149). The muscles connected with the skin manifest themselves to us in the facial expressions of those we see about us, in the movement of the ears (of which FIG. 82. Contraction of a muscle The movement of an organ, as the forearm, is brought about by the contraction of a muscle. The mass of muscle cells becomes shorter and thicker, the parts to which its ends are attached being brought closer together NERVES AND THE REACTIONS OF ORGANISMS 219 Dorsal root many of us are still capable), in the twitching of a horse's skin when it is annoyed by flies, and in winking. And, finally, we may recall the muscles of the esophagus, the stomach, the intestines, and the blood vessels. Some muscles are called volun- tary and some in- voluntary, but all muscles contract in response to a stimulus received from a nerve cell. 256. Nerves and nerve cells. The nerves that are found run- ning to all organs of the body are compounded of many nerve fibers. Many such fibers, bound together by connective tis- sue and associ- ated with blood vessels and lym- phatics, constitute FIG. 83. Diagram of the spinal cord A, left half of cross section, showing impulses entering the dorsal root and outgoing impulses passing out by the ventral root. B, the neurons connected with the gray matter of the cord give off branches passing up and down the cord and transmitting nervous disturbances by way of the collaterals. In the gray matter of the cord, branches of afferent neurons carry impulses up and down and pass them on, by way of the collaterals, to efferent neurons and to the brain a nerve. For our present purpose we are not so much concerned with the nerves as we are with the nerve cells which compose them. The nerve cell consists of (i) the cell body and (2) certain processes, or outgrowths (fibers] ; together these make up a unit of the nervous system. Such a unit is called a neuron, and may be compared to a muscle cell as a unit of a muscle, or to a 220 ELEMENTARY BIOLOGY FIG. 84. Reflex arc Stimulation of the re- ceiving end a of an afferent nerve A leads to a discharge of energy to all parts of the neuron, including the fine terminals, or den- drites, d. The discharge passes over to con- nected nerves, as the efferent nerve E, byway of the interlacing den- drites,orsynafise,s. The discharge in E leads to the stimulation of the organ with which it is connected, as a muscle M. The disturbance passes from a to the spinal cord, where it is reflected by one of the side branches, or collat- erals, c, of A, through the synapse s into E, leading to a movement by the contraction of M gland cell as a unit of a gland. It always acts as a unit (see 7 in Fig. 4). The cell bodies are found chiefly in the cortex, or rind, of the brain, in the core of the spinal cord, and in special groups (called ganglia} in various parts of the body. Occasionally single cells are also found. The processes are of two kinds : 1 . The long, slender fiber extending, with other fibers, through the nerves, is called the ax on. 2. Shorter processes, of which there may be several, usually branching irregularly, " like the branches of a tree," are called dendrites (from a Greek word meaning "tree"). In some neurons a stimula- tion, or disturbance, is received by the delicate branching ends of the axon and transmitted to the cell body. In other neu- rons the disturbance is received by the delicate endings of the dendrites and transmitted to the cell body and on through the axon. The axon may be very short, as in the neurons of parts of FIG. 85. Affer- ent and efferent nerves Disturbance of a sense organ S, con- nected with an affer- ent nerve AI, may set up nervous dis- charges in several connected nerves. There may be a mus- cular reflex through the efferent nerve Efr connected with a muscle ; there may be a gland reflex through the efferent nerve £2, connected with a gland ; and there may be a sen- sation, or feeling, through the disturb- ance of a brain cell B, by a discharge through a con- nected neuron A2 NERVES AND THE REACTIONS OF ORGANISMS 221 the brain, or as in some neurons in the gray part of the spinal cord ; or the axon may be very long, like those in the neurons extending from the lower part of the spinal cord to the ends of the toes or through the length of the arm. 257. Kinds of neurons. The follow- ing different types of neurons may be recognized. 1 . Those that bring impulses toward the cord or brain, — the afferent, or sensory, neurons. 2. Those that carry impulses from the cord or brain, — the efferent neu- rons that may stimulate a muscle or a gland. 3. Those that connect afferent and efferent neurons, which we may call associative neurons. 4. Neurons in the brain, many of which are not directly related to reflexes but are related to knowing and feeling and the voluntary control of muscles. 258. Nerve connections in a simple reflex. Suppose that your finger touches something hot. The nerve endings in the skin are disturbed, and the disturbance of the proto- plasm is transmitted to the rest of the neuron in a fraction of a second. The fiber of the affected cell sends FIG. 86. Behavior limited by nerve connections The diagram shows the nerve connections of a simple mus- cular reflex, with collateral con- nections to the brain. Such connections make possible auto- matic reflexes as well as volun- tary movements. If the afferent nerve is cut, as at «1? only volun- tary movement is possible, and there is no sensation. If the efferent nerve is cut, as at e^ neither reflex nor voluntary movement is possible, but sen- sation remains. If the spinal cord is cut high up, as at a%, e2, neither sensation nor voluntary movement is possible, but the reflexes are not affected off a number of branches in the spinal cord (Fig. 83), and the dendrites at the ends of these collaterals form fine networks with dendrites of other neurons. 222 ELEMENTARY BIOLOGY These interlacing dendrites allow the nerve action to pass over from one cell (neuron) to another (Fig. 84). It is possible that in this region the protoplasm of one neuron is in touch with the protoplasm of the next one, so that a disturbance passes from one cell to the next just as it might pass from one part of a single cell to another part. The disturbed spinal-cord cell sets up nerve action in an efferent muscle nerve, with the result that the arm or hand is drawn back. At the same time, it sets up nerve action in a fiber leading to a brain cell, with the result that you become aware of the pain. But the movement has taken place before you realize what has happened. The nerve disturbance from the finger to the spinal cord along an afferent fiber is reflected out through an efferent fiber, which stimulates the muscle to action (Fig. 85). 259. Afferent and efferent neurons. If a certain part of the sciatic nerve (which is the main nerve trunk running down the leg) were cut, destroying the afferent fibers (alt Fig. 86), one might walk on carpet tacks or hot iron and not know it (unless he happened to be watching his feet), and accordingly one would not jump to escape the damage. Under these cir- cumstances a person would still be able to move his legs or to jump if he wanted to. On the other hand, if another portion of this nerve were cut, — the portion carrying efferent fibers (elf Fig. 86), — one would remain just as sensitive as ever to carpet tacks or hot iron or tickling, but he could not move his legs, no matter how hard he tried. And they certainly would not move of themselves, for the reflex arc would be broken in the part connecting the spinal cord with the muscles. 260. Reflexes without consciousness. If the brain of a frog is removed or injured, or if the spinal cord is cut near the brain (az, e2, Fig. 86), the animal will still be able to perform a large number of reflex actions similar to the one described. Thus, a frog with its brain destroyed will scratch with its leg at a spot on the skin that has a drop of acid placed upon it. NERVES AND THE REACTIONS OF ORGANISMS 223 The sense impression produced by a touch on the skin travels along the axon of an afferent nerve. This disturbance is shunted off, or reflected, through a synapse into one or more efferent neurons to the corresponding muscles and results in a movement more or less suitable for the occasion. But this shunting takes place in the spinal cord or in the lower parts of the brain that do not involve feeling or consciousness or willing. No matter how useful or purposeful such actions appear to be, we must understand that reflexes do not represent the animal's desires or intentions. In many animals, including man, these reflexes may be called forth during sleep or during unconsciousness produced by ether or chloroform. Under such circumstances it is certain that the movements are not intended, not " done on purpose." CHAPTER XLIII TROPISMS AND THE BEGINNINGS OF SENSE 261. Tropisms. In the absence of neurons in the simplest animals we cannot speak of their reflexes. Most of the reac- tions that have been studied are classed as tropisms. Tropisms have been ex- plained as resulting from the unequal contraction of the protoplasm on oppo- site sides of the body, under the influence of unequal, or one-sided, stimulation. 262. The general reaction of lower animals. Many organisms are not sym- metrical, and many, like the Paramecium, or slipper animalcide, make progress in a given direction by moving spirally around the line that represents this direction (see Fig. 87). In response to any disturbance or change in condition, such animals always make the same movement (see Fig. 88). As a result of this "general reaction" to all kinds of stimuli, the animal man- ages to escape many dangers, and to get into situations that are frequently advantageous (see Fig. 89). 263. Chemical sense in lowest organ- isms. The simplest animals, like the roots of many plants, are sensitive to many kinds of chemical disturbance. We cannot suppose that an ameba, for example, has the feeling of sour or sweet, or 224 FIG. 87. Movement in Paramecium In the Paramecium, as in many other free-swimming protozoa (one-celled animals), the organism moves forward by spinning about its own axis and at the same time swinging in a spiral path TROPISMS AND THE BEGINNINGS OF SENSE 225 that the Paramecium has an idea of nice or nasty. But it is very plain that the protozoa are repulsed by the presence of sand grains and attracted by the presence of various kinds of bacteria. They will swallow the bacteria and pass the sand grains by. There is no doubt, however, that the difference between their reaction toward food and their reaction toward inert matter or toward injurious matter is due to a certain relation between the chemical constitution of the protoplasm and the chemi- cal constitution of the outside substances. We should hardly be any more justified in saying that the ameba likes meat juice than we should be in saying that water dislikes oil. In one case, as in the other, the reactions de- pend upon certain relations be- tween the chemical compositions of the two reacting substances. Water does not choose to dissolve sugar and to leave sand undis- solved ; neither can we be sure that a protozoan chooses its food, notwithstanding the fact that it does take some kinds and reject other kinds of objects or materials. It is only when we come to the higher animals that we may begin to speak of choice in this sense ; and even among the highest animals most of the selecting and rejecting depends entirely upon reflexes and instincts rather than upon thought or feeling ; that is, they depend upon the structure of the organism and upon the composition of certain organs rather than upon a conscious purpose or discriminating taste. FIG. 88. General reaction In many one-celled animals every stimu- lation brings about the same response. In the Paramecium the animal, when it runs into an obstacle, whether physical or chemical (0), immediately reverses its movements, backing off a little way, turning to one side, as shown by the arrows, and starting off along a new path 226 ELEMENTARY BIOLOGY In the simple organisms the response, like the irritation, concerns the whole cell; that is, the whole organism. We cannot separate the part of the animal's structure that is irritable in the sense in which a neuron is irritable, from the part that is irritable in the sense in which a muscle fiber is irritable. Nor can we separate the perceiving part from the contracting part, although of course we may readily believe that in the complex mixture that we call protoplasm there are some contractile arrangements of mate- rials and some irritable combinations. In the Vorticella (Fig. 90) and other one-celled animals it is indeed possible to distinguish a strand of highly contractile substance. In the Hydra (Fig. 91) we can see the beginning of separation between irritable region and contractile region. FIG. Original Path $9. " Trial and error lowest animals When a simple animal, like Para- mecium, runs into a region un- favorable to its existence, the stimulus causes a reversal of its movements, with a change of di- rection. On moving forward in the new path, 2, it may again meet the same obstacle. The same reaction is repeated. After a number of trials the animal is likely to find a clear path. This behavior gives the appearance of trying again after each failure until success is attained 264. Organs of touch. In our- selves, as well as in the other higher animals, the sense of touch is de- pendent upon the presence of special nerve endings in the skin, and their connection, direct or indirect, with other neurons (see Fig. 92). In some parts of the body the touch organs are much closer together than" they are in others; for example, they are set very close together in the skin of the tips of the fingers, and compara- tively far apart on the back of the hand. It seems that we perceive hot through the stimulation of certain end organs in the skin, and cold through the stimulation of certain others. 265. Organs of taste. On the upper surface of the tongue, on the palate, and in other parts of the lining of the mouth TROPISMS AND THE BEGINNINGS OF SENSE 227 and of the pharynx there are little projections called papilla, which contain the nerve endings of the neurons connected with the cells that feel taste. The wry face that one makes on tasting something disagreeable is a reflex in which the afferent nerves of taste and the mus- cular nerves controlling the muscles of the lips, tongue, and cheeks form the arc. There are also associated in this kind of reflex other neurons that stimulate the salivary glands. Your mouth waters on tasting something sour, but this watering is not related to the digestive process. It may not be unreasonable to consider this excessive watering as useful in the sense that it helps to dilute the acid, or to neutralize it (since normal saliva is alkaline), or to wash it away. The nerves are capable of perceiving four distinct tastes: sweet, sour, bitter, and salty. When we perceive the vari- ous flavors in substances that we place in our mouths, we are really getting FIG. 90. Vorticella «**%£%£ dark area Stimuli that act upon the nerve endings This one-celled animal lives in in the nose. We can readily convince watfr' attached ** lts s^lk * * 3 rock or twig. When disturbed Ourselves Of this by trying tO distinguish, the animal contracts suddenly. without the USe of Sight or Smell, the taste Running through the stalk is a of various substances having distinct flavors. A blindfolded person, holding his nose to prevent currents of air pass- ing through it, cannot distinguish ground coffee, for example, from sawdust, or vanilla flavor from raspberry. When we speak of the " taste " of good food, we generally mean the odor. 266. Organ of smell. The nerve endings in the lining of the nose, and of the air passages extending back from the nose into the pharynx, are of two kinds : some are sensitive to touch ; others are sensitive to odors. This sense of smell is a 228 ELEMENTARY BIOLOGY FIG. 91. Simple tissues in a simple animal The Hydra is among the simplest of many-celled animals, consisting of a hollow bag whose wall is made up of two layers of cells. There is apparent a division of labor between the inner layer of digesting cells and the outer layer of protecting cells. In a section of the wall we may see that the outer cells, a, have elongations, b, at their bases, which are highly contractile, and that in- terspersed among these cells are smaller ones, c, which are highly sensitive and extended into delicate threads and expansions, , larva (not shown for j) (in / and 2 the larva is similar in general form to the adult ; successive stages are attained by molting, £2) ; c, pre-adult (in 4 and 5 this is a resting stage, or pupa) ; d, adult. The larva of the wasp develops within the caterpillar buried by the mother wasp 280 ELEMENTARY BIOLOGY 329. Transformation. When we compare a chick as it comes out of the shell with the contents of the eggshell before hatch- ing begins, we cannot conceive that the little speck on the side of the yolk has become the chick, with its many organs and its many kinds of cells with their many peculiar functions. And yet all that comes out of the eggshell must have been there at the beginning of the incubation period. We are so familiar, however, with the fact that chicks come from eggs, that we are content to ac- cept the changes that go on inside, on the supposi- tion that, since they are so gradual, everything is possible. A study of the develop- ment of insects will give us an idea of how sharply limited the stages in an individual's life may be. When a locust or a cock- roach comes out of the egg, it is very much like the parent, except that it is very small and lacks wings (Fig. 115, i(b^ b$). By a series of meltings the animal advances not only in the matter of size but in the development of the wings and other organs. When the egg of a moth or of a butterfly hatches out, the young animal that emerges is not at all like the parent ; it looks more like a worm (Fig. 117, b, b). It has no wings; its mouth has biting jaws that work sideways ; its coloring is different. Indeed, if we did not know that it came from the egg of a butterfly, or that it would in time become a butterfly itself, we should never suspect, from its appearance, that it had anything to do with butterflies. We may well believe that during all the months of outward inaction some- thing was going on inside the case of the pupa, just as FIG. 1 1 6. Molting cicada In many jointed-legged animals (Arthropoda) the growth takes place at intervals between molts. The hard outer skeleton breaks open and the soft skinned animal crawls out. After a while the shell hardens and growth stops again b c Tussock moth (Notolophus) b c Hawk moth (Hyloic-us kalmiae) b c Yellow swallowtail, or tiger butterfly (Papilio turnus) a b c d Fritillary (A rgynnis) FIG. 117. Development of Lepidoptera (moths and butterflies) The egg, «, hatches into a wormlike larva, or caterpillar, b. The larva feeds voraciously and grows very rapidly. On reaching full growth it curls up, secretes a hard covering, and goes to sleep. In this resting stage, or ptipa, c, it may remain for months, giving no outward sign of life whatever. At the end of the resting period the cover of the pupa breaks open, and out crawls the fully formed insect, d. In some species the two sexes have distinct forms or color patterns in the adult stage. In the tussock moth the adult female is a sluggish, wingless animal 282 ELEMENTARY BIOLOGY b d Fish, Chinook salmon (Oncorhynchus tscha-wytscha) Batrachian, frog (Rana palustris) Batrachian, newt (A mblystoma punctatum) FIG. 1 1 8. Development in some backboned animals a, egg ; a1; fish ready to break out of egg ; <£, first free-swimming stage (tadpole in batrachians) ; b-±, later stage in fish, still showing yolk sac ; c, more advanced stage (in batrachians, tadpole just before the appearance of hind legs) ; d, later stage ; e, adult form during the three weeks of hatching something was going on inside the eggshell of the chick. The development of an individual through a series of well-marked stages is called a metamorphosis, which means ' ' trans-formation . ' ' DEVELOPMENT 283 In the large class of Insecta the development is characterized by more or less complete metamorphosis (see Figs. 115 and 117). In the life history of the frog and the salamander we find a metamorphosis that is as well marked in some ways as that of the insects (see Fig. 118). A complex animal, developing from a single cell, passes through a number of stages that are different from the finished form, on the one hand, and from the simple beginning, on the other. This is really all that metamorphosis means when applied to living things in general. It is another name for devel- opment. But when we use the latter term, we have in mind the process, whereas when we say " metamorphosis," our attention is fixed on the forms, or stages. 330. Metamorphosis in man. The changes that take place in a human being from day to day are comparatively slow, and the form of the infant is in general very much like that of the adult, so that we do not commonly think of the metamorphosis of human beings. But if we compare the proportions of a baby with the proportions of an adult, we can see that the changes are real even in the outward form (see Fig. 119). A man is something more than a large baby, something different, even in this outward form. We know, of course, that as we become older there are many changes in the internal FIG. 119. Metamorphosis in man A comparison of the infant and the adult shows that after birth the legs of the baby grow more than any other part, whereas the head grows the least. A study of this figure will show other changes that take place in the outward form 284 ELEMENTARY BIOLOGY organs : some organs present in infancy disappear, others not present make their appearance later, and others are present at first in a rudimentary stage and gradually reach maturity. 331. Development of plants. The simpler stages in plant develop- ment are not so familiar to us, so that we do not have the same opportunity to observe the similarities. In our study of the embryo of the seed, we saw that the young plant had all the main parts of a plant body, al though the embryo did not at all resemble the full-grown plant. The embryos of related plants look much more alike than the adult individuals, just as the tadpoles of the newt and the frog look more alike than the adults. Thus, the embryo and Beginning, like an animal, as a single cell, ,, the plant eyen ^ geedli of passes, by a series of cell divisions, , completion of the inter- change. E, same, further enlarged, f, the active germ nucleus fuses with the stationary one. G, same, enlarged. In the meantime the macronucleus has broken up and disappeared. After the restoration of the micronucleus through fusion, F, the two in- dividuals float apart. H ': The new micronucleus, d, breaks up into two. 7, each portion splits again. /, after a third division. K: Four of the nuclei become new macronuclei and four remain as the micronuclei. The rest of the protoplasm divides and four individuals result, each with a micronucleus and macronucleus. (From Calkins, after Hertwig, and Maupas) 298 ELEMENTARY BIOLOGY Among some of the fresh-water algae the swimming cells produced are of two sizes. In such cases the smaller cell is usually more active in the water ; the larger cell has more food material. In the common rockweed, or bladder wrack, of our -o- FIG. 130. Conjugation in Spirogyra Cells lying close together put forth processes, or projections, toward each other, a. As these processes finally come in contact, l>, the two threads with their crosspieces have the appearance of a ladder when looked at through the microscope, c. The cell walls at the points of contact are dissolved, probably by the action of a ferment, and there are thus formed continuous passages between the cells of one thread and the cells of the opposite thread, d. In the meantime, however, changes have been taking place inside the cells. The spiral ribbon of chlor»phyl seems to break down, d; the mass of proto- plasm in each cell draws away from the cell wall ; and the protoplasm from one of the cells of each pair moves into the connecting tube and passes completely into the opposite cell, e. Here the two masses of protoplasm unite into one, and a thick cell wall is formed around the new combined protoplasm, f. The cell with the thick wall, inside the old dead cell wall, may apparently remain in a resting state indefinitely, or may begin the next day to put out a thread of new Spirogyra, giving rise to millions of cells in the course of a few days seacoast, the gametes are produced in special organs found on certain of the bladderlike expansions of the plant body. When formed, the gametes are discharged into the water and have nothing more to do with the parent plant (Fig. 131). 348. Fertilization. When the two gametes are so unlike as to be distinguishable, the process of conjugation is sometimes SEX 299 called fertilization. The essential thing about fertilization is the uniting of two different nuclei into one. What the meaning of this process is in the life of organisms we do not know with certainty. We know some of the effects of the process, and we can tell what conditions lead up to it in some cases. FIG. 131. Reproduction in rockweed, or bladder wrack a, expansions of the rockweed containing the gamete organs ; b, section of an egg-bearing organ ; c, the large gamete, or egg, with large, distinct nucleus and food granules ; d, the small gamete, or sperm, having the shape of a pear and bearing motile cilia. Sperms swarm around an egg until one of them unites with the egg. After the conjugation the zygote develops into a new individual 349. Male and female. The gametes that are so unlike as we have seen them to be in the rockweed are distinguished by special names. The large gamete is sometimes called the oo sphere, or egg cell. The small one is called the spermatozoid, or the sperm cell. We sometimes distinguish the two gametes by calling the large one the female and the small one the male. Most of the familiar plants and animals reproduce by means of male and female gametes, forming zygotes. This kind of reproduction is called sexual reproduction, in distinction from reproduction by spores, which is called asexual ; that is, without sex. There are many animals and very many plants that repro- duce both sexually and asexually (see Chapter LXI). CHAPTER LVII FLOWERS 350. What do flowers mean? We know that roots, stems, and leaves, with their various parts, are more or less directly related to the securing of water, food, and air, and to the pro- tection of the organism against possible injuries. When we examine the flower with a view to discovering its possible uses to the plant, we shall find very little indeed. On the contrary, we are likely to find the flower a source of expense to the plant, without any compensation whatever. It takes a great deal of material and a great deal of energy to build up a flower like that of the poppy or the lily ; and so far as we can discover by experiment or observation, the flower does nothing that is of use to the plant. Are we therefore to conclude that the flower has no meaning in the life of the plant ? 351. Structure of a flower. In most common flowers, such as wild roses or tulips, we find certain leaflike parts that are conspicuous because of their color. This set of conspicuous leaflike organs is commonly associated with a less prominent, cuplike arrangement of leafy structures, connected with the base of the flower and partly surrounding the bright leaves (Fig. 132). Although the floral envelope is in most plants the first to attract people's attention, it is by no means the most important part of the flower. In order to understand the essential organs, we must consider them from the point of view of the function of the flower as a whole, which is the making of seeds. 352. The essential organs. In the center of the flower is a structure called the pistil, from its resemblance in many cases to the shape of a pestle (see/, Fig. 132). 300 FLOWERS 301 Surrounding the pistil may be found a number of rather slender stalks, with knobs, or enlargements, on the ends (see d, Fig. 132). These structures are called stamens, from a word meaning "thready." Flowers differ greatly in size and shape, as well as in color and odor. The various parts differ in many ways, but the pistil and stamen are always and everywhere the organs that have directly to do with seed- making ; and their work is essentially the same in all flowers, no matter how varied they may be in form and arrangement. 353. The ovary. On cutting open the ovary of a flower we find that it is a hollow box, with a number of compartments in some species (see Fig. I33)» containing from one to very many tiny rounded bodies that are normally destined to become seeds. These bodies are called ovules. FIG. 132. Structure of a flower The outer set of covering leaves, a, a, is called the calyx ; the single parts are sepals. The inner layer, b, b* is the corolla ; its parts are the petals. The central organ is the pistil; the main body of the pistil, f, is the ovary and contains one or many little structures (ovules) capable of becoming seeds. The tip, e, of the pistil is the stigma • this is connected with the ovary by the style c. Surrounding the pistil are a number of stamens, d, consisting of a stalk, /i, called the filament, and an enlarged capsule, g, called the anther. This con- tains a mass of cells which can be thrown out, i ; these loosened cells are called pollen best, and although there may be no sign that there is anything diseased or out of order with the plants. As time goes on, these ovules en- large, and the ovary also becomes larger. When the seeds are ripe, the ovary has become the fruit. But the changing of ovules into seeds is not simply a matter of growth. Every farmer and gardener knows that it is possible to have a good lot of flowers or blossoms with a very poor crop of fruit, although the conditions for the growth of the plants may be of the 302 ELEMENTARY BIOLOGY The inside of the ovule is a soft mass, made up of many compartments, or cells, containing the jellylike living matter, or protoplasm. One of these cells, usually near the center, is much larger than the others (see es, Fig. 134). This large cell, called the embryo sac, grows and divides and in time becomes the young plant, or embryo, inside the seed. The rest of the ovule becomes the coat, or covering, of the embryo. 354. Fertilization. Before the ovule can become a seed, certain changes must take place in the living matter of the embryo sac. The nucleus of the em- bryo sac must first unite with the nuclear substance of a pollen grain. The uniting of two nuclei is called fertilization. The method by which the two nuclei are brought together is shown in Fig. 1 34. 355. Seed and fruit. After fertilization the embryo sac, which now contains protoplasm from two parents, divides into very many cells. It absorbs food in large quantities from the parent upon which it is borne, and becomes a baby plant, or embryo (see Fig. 120). The walls of the ovule, surrounding the embryo sac, become the seed covers. The ovule with its embryo sac thus changes into a seed. In addition to the food used in the growth of the embryo, the parent plant supplies other food materials that are accumulated either in a mass surrounding the embryo, or within the tissues of the embryo itself. This surplus food may later be drawn upon by the young plant, after it sprouts and before it is able to maintain itself through the work of its own leaves and roots. FIG. 133. Sections of ovaries Ovaries are of many sizes and shapes. They contain but a single ovule in some species of plants, and in other species they bear hundreds. The ovules are definitely placed in one or more compartments of the ovary FLOWERS 303 Fertilization also seems to in- duce changes in other parts of the flower. The petals drop off, and usually the stamens also. The ovary begins to enlarge, and eventually it ripens into the fruit.1 In some plants the calyx of the flower, and even the re- ceptacle, may become fused into the fleshy fruit. We must be on our guard against thinking of the plant as an organism that looks ahead and supplies the later needs of its off- spring. We may say merely that the plants behave in such a way that the later safety and develop- ment of their offspring are made more probable. The baby plant is protected by the mother, as well as nourished, in the sense that the early development takes place within the ovary, and in the sense that in many species hard or spiny coverings are formed which prob- ably prevent injury. 1 In most of the common plants the fruit will not ripen — that is, the ovary will not continue its development — unless fertili- zation takes place. But there are many plants in which a seedless fruit is possible. Seedless oranges, seedless apples, seed- less grapes, the pineapple, and the banana are examples of fruits that develop without the ovule being first fertilized. The plan- tain and the breadfruit develop a more juicy fruit when there is no fertilization. FIG. 134. Fertilization in a flower When a pollen grain,/, alights on the moist surface of a stigma, s, it absorbs water and puts forth a thread of proto- plasm, or a pollen tube,//, which grows down the style into the ovary. The tip of the pollen tube finds its way to the inside of the ovule, o, through a small passageway, the micropyle, m. The large cell in the middle of the ovule, called the embryo sac, es, undergoes a number of changes which result in pro- ducing several nuclei. One of these nuclei at the end nearest the micropyle corresponds to an egg cell. Similar divisions take place in the nucleus of the pollen grain, and one of the result- ing nuclei corresponds to a sperm cell. The cell walls separating the pollen tube and the embryo sac dissolve, and the pollen nucleus unites with the egg nucleus. The newly formed joint nu- cleus, or fertilized egg, begins to divide. The embryo sac develops into a new plant, or embryo ; the ovule becomes a seed ; the ovary becomes a fruit CHAPTER LVIII POLLENATION 356. Function of pollenation. We have learned that flowers are seed-producing structures, and that seed production takes place only after fertilization. But in seed plants (most of which are land plants) the parts of the organism which bear gametes are so situated that fertiliza- tion is possible only after pol- lenation ; that is, the transfer of pollen from the anthers to the stigma. In these plants re- production de- pends in a rather peculiar way upon cer- pollenation, or at any rate handicaps close pollenation tain external factors. 357. Self -pollenation. In many plants the transfer of pollen is brought about by the growth movements of the parts of the flower. The style, in elongating, may bring the stigma into contact with the anthers ; a movement of the corolla may push the stamen against the stigma ; the stalk of the flower 304 FIG. 135. 'Dimorphic flowers of Chinese primrose {Primula) In these plants there are two forms of flowers (hence the term dimorphic). In form A the anthers are high and the stigma is low ; in form B the anthers are low and the stigma is high. The pollen from form A is prepotent for the pistil of form B, and vice versa. That is, long-stamen pollen must reach long-style pistil, and short-stamen pollen must reach short-style pistil, to produce the best or the most seeds. This necessitates cross POLLENATION 305 FIG. 136. Polymorphic flowers of purple loosestrife (Lythrum) In species having three forms of flowers the best seed- production seems to result from the pollenation of a pistil by pollen from a stamen of the corresponding length, which must necessarily be from a different flower. (After Darwin) may bend as it grows, dumping some of the pollen from the anthers onto the stigma. In other cases the anthers are placed above the stigma, so that the pollen is brought to the latter organ by the action of gravity. There are many plants in which the stigma regularly pushes through the ring of anthers and thus becomes pol- lenated. In other plants this kind of pollenation takes place only under special conditions, as in extreme dampness or extreme drought. 358. Close pollenation and cross pollenation. Any process that results in the transfer of pollen from the anther of a flower to the stigma of the same flower is called close pollenation. This designation is used to distinguish the process from cross pollenation, in which pollen is carried from the anther of one flower to the stigma of another flower (of the same kind, however). There are many plants in which close pollenation is impossible. * 359. Obstacles to close pollenation. There are three sets of conditions in plants that interfere with close pollenation. I . Space relations. The relative position of stamens and pistils within the flower may make close pollenation impossible. Or the FIG. 137. Stigma of a grass In wind-pollenated plants the stigmas usually expose a large surface to the wind 306 ELEMENTARY BIOLOGY stamens may be in one flower and the pistils in a different flower, either on the same plant or on a different one. Some common monoecious plants (that is, plants having the stami- nate and the pistillate flowers on the same individual) are birch, hazel, chestnut, oak, walnut, hickory, squash, maize, and the cone-bearing trees. Some common dioecious plants are poplar, willow, box elder, tape- grass ( Vallisneria), begonia, sassafras, and virgin's bower. FIG. 138. Pollenation by water The tape-grass ( Vallisneria) is a dioecious water plant. The pistillate individual grows up to the surface of the water, where the flowers, a, are opened, while the staminate indi- vidual remains beneath the surface. The staminate flowers, b, are detached from the stalks and rise to the surface, where they float about and gather in large numbers in the quiet stretches of water close to solid objects of various kinds. When one of these float- ing stamen flowers comes close to the pistillate flower of the species, the anther is brought into direct contact with the stigma, and thus pollenation is effected 2. Time relations. The stamens and pistils of some species of plants do not ripen at the same time, close pollenation being thus impossible in these species. The pollen ripens before the pistil in maize, in the B mallows, in many species of the aster family, in the creeping crowfoot, and in the sage. The pistils ripen ahead of the stamens in the common plantain, in the potentilla, or cinquefoil, and in the oriental grass known as Job's tears. POLLENATION 307 3. Physiological relations. In some species of plants it is found that when the pollen is placed on the stigma of the same flower, it will either not germinate at all or it will produce, on the whole, poorer seeds than those produced by means of pol- len taken from another flower. This physiological difference in favor of outside pollen is called pre- potency, and was demonstrated by Darwin in several species of plants. Prepotency is com- monly associated with the presence of two or three lengths of styles and of filaments. In the flax, cow- slip, Chinese prim- rose, bluet, and other species there are two forms (Fig. 135). In the purple loose- strife (Ly thrum sali- carid) and in certain species of Oxalis (re- lated to our sorrel) there are three lengths of stamens and three lengths of pistils (in different flowers) corresponding to them (see Fig. 136). In buckwheat, in most orchids, in certain species of day lily, and in certain species of the bean family the pollen will not germinate at all on the stigma of the same flower. There are, then, many species of plants in which close pollenation cannot take place, or in which it is not very effec- tive if it does take place. How, then, do these plants produce seeds, or, rather, how do they secure pollenation ? In other words, how is pollen carried from flower to flower? FIG. 139. Pollenation by birds Saber-billed humming bird pollenating flower with trumpet- shaped corolla. (From exhibit in American Museum of Natural History, New York) 308 ELEMENTARY BIOLOGY 360. Wind pollenation. The most common moving agency that is able to act between plant and plant is the wind. The abundance and the dryness of the pollen produced by many of the common trees, and the frequency with which pollen may be found in the dust at certain seasons of the year, would lead us to suspect that the wind is an effec- tive agent in this matter (see Fig. 137). A study of conditions on farms that pro- duce corn, wheat, oats, and other grains shows that these plants, as well as many others, depend entirely upon the wind for their pollenation. Indeed, it is sometimes necessary to take special precautions to prevent the wind from bringing to a group of plants an undesirable kind of pollen from a remote field. 361. Water pollenation. Another agent that is effective in distributing pollen for plants is water. This, of course, is con- fined to plants that live in the water. FIG. 140. Pollenation by insects In the lady's slipper and in many other flowers, in- sects alighting on the co- rolla crawl into the interior, guided by the form and the markings. In many flowers the arrangement of the parts is such that the in- sect must brush against the stigma in going in, and against the anthers in pass- ing out. As a result the animal carries pollen from flower to flower. Many species of plants, especially among the orchids, depend upon single species of in- sects for their pollenation A good example of pollen transfer by water is furnished by the tape-grass (VaUisneria), which lives near the edges of ponds (see Fig. 138). 362. Bird pollenation. Next to the wind, the most common moving agents that go from flower to flower are flying animals and birds and insects. Now we know that not all birds or all insects can serve plants as pollen carriers ; only those that regularly visit flowers can be considered of importance in this connection. Certain humming birds that visit flowers lap up the sugary fluid, or POLLENATION 309 nectar (see Fig. 139), and rub off some of the pollen in one flower, and when they visit another flower this pollen comes off onto the stigma. Certain tropical flowers are said to be pollenated by bats that come to them for nectar. 363. Insect pollenation. There are hundreds of species of plants whose flowers are pollenated by insects, chiefly bees and wasps of the bee order, and certain moths and butterflies. All of these insects have sucking mouths, and they all visit flowers that contain nectar. Some of these insects also use pollen as food. The bees, for example, carry away quantities of pollen, which they feed to the young in the hives. In gathering the pollen or in sucking the nectar the insects rub off pollen on various parts of their bodies, and then transfer this pollen to the stigmas when they visit other flowers of the same kind (see Fig. 140). CHAPTER LIX ADAPTATIONS OF FLOWERS 364. Colors and odors. In many species of plants the colors and odors of the flowers are no doubt of value to the plants as furnishing aids to insect visits, and thus to the process of pollenation. It is a mistake, however, to suppose either that all colors and odors are of value to the plants in this way or that there is any necessary connection between the existence of these colors in the flowers and the habits of the insects. There are many plants that have colored corollas and that do not depend upon insects at all. And there are other plants that receive the visits of insects without being particularly conspicuous. 365. Nectar. While many insects will visit plants for the nectar, there are many plants that produce nectar in positions that make it impossible for the visits of insects to be of any use to the plants. Indeed, there are certain ferns and some seed plants that produce nectar on the stems or leaves, so that the plants get no benefit whatever from the visits of insects to these nectaries. 366. Fitness. We are not to suppose that the plants produce these queer shapes in their flowers, or the colors or odors, for the special purpose of attracting insects. Nor are we to suppose that the insects visit the flowers for the purpose of carrying pollen, or for any other purpose. Bees will fly toward nectar or honey, houseflies will fly toward manure or decaying fish, moths will fly toward a light, not because they have the idea of getting something they want, but because they are built in a certain way. It is interesting to note in this connection that while insects cannot distinguish objects at any great distance, — say at more than about 310 FIG. 141. Pollenation of the fig The larva of the little wasp Blastophaga passes the winter in the sterile pistil, /1; of the winter fruit, a^ of the caprifig, A. In the spring the adults appear, the wingless male first. After fertilizing the female, the male dies. The female flies out and crawls into the new figs, which are just forming, #2, and loses her wings in the process. This fig carries both stamen flowers and pistil flowers, but the latter have short styles and can bear no seeds. The insect lays her eggs in these sterile pistils, and the young complete their development here. When the new generation of females flies out, there is a new growth of fig buds on the caprifig, as, and also on the true fig, b on B. Some of the females find their way into the caprifig receptacles, and some into the fertile-fig recep- tacles, carrying with them pollen from the spring receptacle of the caprifig, «2. The pistils of the true fig have long styles, /2, which can be pollenated. The styles are so long, however, that the insect cannot lay her eggs on the pistil. On the other hand, the pollen brought into the receptacle of the caprifig, a%, is entirely wasted, since the pistils here are sterile. A new generation of insects develops in this receptacle, and the emerging females find their way into the autumn growth of new figs, in which the winter is spent. The true figs can thus produce fully ripened fruit only in the presence of the caprifig and of the wasp. But the wasp can complete its life cycle with the caprifig alone. The insects that carry pollen (from «2) either waste this pollen and reproduce themselves (in a3) or they pollenate pistils and die without reproducing themselves (in b) 312 ELEMENTARY BIOLOGY two to three yards, — they will nevertheless visit only one kind of flower in the course of a day, or even for many days running. Thus, if a bee starts out in the morning by visiting a red clover, it will visit only red clovers for the rest of the day, or as long as any red clovers are to be had. 367. The interdependence between flowers and insects. In some cases the relation between a seed plant and some insect is so close that it affects the practice of plant raisers. When fig trees were first introduced into California, they produced large, juicy fruit, even without pollenation. But the fruits thus produced are not as satisfactory for commercial purposes as the others : they do not dry properly, and so cannot be prepared for shipping or for preservation. To get the normal fruit it was necessary to find the insect that regularly brings about pollenation. This little wasp, the Blas- tophaga, has a curious life history which is closely adapted to the flowering habits of the fig tree. On the other hand, thousands of fig pistils supply breeding places for wasps without ever producing seeds (see Fig. 141). Thus the insect and the fig tree are of great value to one another, although it is difficult to see what advantage either species has in its dependence upon the other. It is quite impossible for us, at present, to imagine how this relationship came to be established in the course of time. There are many cases of plants that have been transferred from one region of the earth to another, and then failed to bear seeds because of the absence of the suitable insect. When vanilla was transplanted from Mexico and South America to various islands in the Indian Ocean and else- where,* the plants grew luxuriantly, but produced no fruit, although flowers were produced in abundance. Since the plant was raised for the "beans" or pods, there was no profit in the business so long as the fruit failed to develop. It was found that the failure was due to the absence of pollenation, which is brought about in the native regions by certain insects. ADAPTATIONS OF FLOWERS 313 Instead of importing the insects to carry on pollenation, it was decided to hire women and children to go from flower to flower and pollenate by hand (see Fig. 142). In our regular horti- culture it happens occa- sionally that trees or bushes in full blossom fail to yield the expected crop of fruit because of the lack of insects to in- sure pollenation. This is why wise farmers and or- chardmen so often main- tain hives of bees in the neighborhood of their fields or orchards. Even where the honey is not worth getting, the bees are worth having because they insure abundant pol- lenation at the right time. G FIG. 142. Hand-pollenation in the vanilla flower 368. Advantage of insect pollenation doubtful. In a general way the lower fam- ilies of seed plants are wind-pollenated, and the higher families are insect- pollenated. But it must not be supposed that there is any real advantage to plants in depending upon insects to carry their pollen. In many cases we may see that there is an actual saving of pollen. On the other hand, many species of plants, especially among the orchids, are so dependent upon the insect visits that they are dying In the orchids the stamens are fused with the stigma, placing the anthers above the stigma in such a way as to make self-pollenation absolutely im- possible, an, anther ; /, pollen masses ; s, stigma. A, general view of flower ; B, position of hands and needle in artificial pollenation ; C, needle lift- ing pollen masses ; D, anther raised to expose pollen masses ; JS, style raised to show opening in stigma; F, longitudinal section to show relative positions of anther and stigma; G, longitudinal section after pollenation, showing pollen masses in the stigma. All the vanilla beans in the Seychelles Islands are grown with hand pollenation 314 ELEMENTARY BIOLOGY out just because of the inability to produce sufficient seed to replace the old individuals, the suitable insects not being numerous enough. In general, the plants that are most decidedly dependent upon the wind for their pollenation seem to be at least as successful as those that are dependent upon insects. Thus, the grasses and the common catkin- bearing trees and the cone-bearing trees are widely distributed over the surface of the earth, and none of the insect-pollenated plants seem to have any decided advantage over them. The insects that are able to get food from highly specialized flowers, because of their peculiar instincts or structure, may seem to have some advantage over insects that cannot make use of those particular flowers. Nevertheless we find it extremely difficult to understand what advan- tage a species may derive from such extreme adaptation, since such dependence often leads to complete extermination (as in the case of Blastophaga in the absence of fig trees), and in any case means pay- ing a high price for benefits received. CHAPTER LX FRUIT AND SEED DISTRIBUTION 369. Seed as forerunner. We have studied seeds as arising from the ovules in flowers (pp. 300-303), and we have studied them as consisting of young plants with more or less accumulated food and a covering (pp. 32- 36). We can realize the full meaning of seeds in plant life when we con- sider that during the winter the fields are bare and thousands of plants have entirely perished, leaving behind them the seeds as the only living remains. It is these seeds that repre- sent the species of all FIG. 143. Mechanical protection of seeds the annuals during the /, bitternut (Hicona minima), of the walnut family ; months in which active 2' chestnut oak (Quercus frinus); j, sweet gum OVe (Liquidambar styratiflua) , of the witch-hazel family ; plant life is impossible. 4, table-mountain pine (Pinus pungens) And it is from the seeds that these species will be reestablished the following season when the conditions for growth are again favorable. From the point of view of the seed as the forerunner of the new generation, the fruit may be considered in relation to the protection and the dispersal of seeds, since the fruit is the organ within which the seed ripens. 3*5 ELEMENTARY BIOLOGY 370. Protection of seed. As living things seeds are exposed to destruction by other plants or by animals and to injury by inorganic factors of the environment, as excessive low tem- perature and excessive moisture or drought. We find many fruits covered with spines ; others have hard or tough cover- ings or shells ; still others contain bitter or acrid substances. FIG. 144. Dehiscent fruit Seeds are scattered by the opening of the fruit in a definite way. /, chestnut ; ;?, witch hazel ; j, poppy ; 4, pea ; 5, monkshood Seeds that become separated from the fruit are frequently tough-skinned or covered with some other protective layers (see Figs. J and 143). 371. Escape of seeds. The seed attached to the parent plant and surrounded by other structures is of no significance in the life of the species. To be in a position to perform its functions, the seed must get oiit and get away — and the farther away the better, in most cases. Many common seeds escape from the parent plant through the splitting open of the ripe fruit along definite lines or by FRUIT AND SEED DISTRIBUTION 317 the appearance of holes. The pods of the bean family and of the evening-primrose family illustrate this dehiscence, and the poppy is a good example of the formation of pores. Fleshy fruits often drop off, carrying the seeds with them, and the seed escapes when the fleshy part of the fruit is eaten by some animal or rots (that is, is eaten by some plant). FIG. 145. Seeds scattered by the wind /, dandelion ; 2, milkweed ; j, white maple ; 4, prickly lettuce ; j, thistle Many fruits, however, do not permit the seeds to escape ; the fruit and the seed are so closely united that they constitute a structure that acts as a whole — as in the grains, the nuts, and the nutlets of the dandelion family. 372. Seed distribution. In their dehiscence many fruits open so suddenly that they shoot the seeds to a distance of a yard or more. This shooting is commonly brought about by the rapid twisting of the parts of the pod, as in the touch- me-not and the lupine (see Fig. 144). Most plants depend upon outside agencies to scatter their seeds for them. The wind is active in the case of species whose seeds are either very small (the orchids) or have 318 ELEMENTARY BIOLOGY expansions in the form of wings or tufts of hair that furnish a large area in contact with the air (see Fig. 145). Seeds that have hooks, as the cocklebur and beggar-ticks, attach themselves to the fur of passing animals and are carried considerable distances from the parent plant (see Fig. 146). FIG. 146. Seeds scattered by passing animals /, beggar-ticks, or bur marigold (Bidens frondosd) ; 2, burdock (Lappula echinata); j, small- flowered agrimony (Agrimonia patviflora); 4, carrot (Daucus c arotd) ; j, enchanter's night- shade (Circaea lutetiand) ; 6, cocklebur (Xanthium canadensis) ; 7, bur grass (Cenchrus tribuloides) ; 8, spike rush (Eleocharis watd) Seeds that are inclosed in edible fruits are often distributed by being eaten by animals and then discharged from the intes- tines without having suffered any injury. Cherries, black- berries, and other small fruits are commonly distributed by blackbirds, robins, thrushes, and other birds (see Fig. 147). From the point of view of the species, there are three factors in seed dispersal that are of fundamental importance : (i) the number of seeds that are scattered ; (2) the distance to which they are carried ; and (3) the final lodgment in a place favorable to germination and later growth and development. FRUIT AND SEED DISTRIBUTION 319 It is obvious that the more seeds there are scattered, the better are the chances that enough of them will find suitable lodgment to replace the individuals that die each year. On the other hand, to produce excessive seeds would be wasteful, and might under some circumstances neutralize the ad- vantage of num- bers. Thus the orchids, producing relatively many seeds, lose many ; only a very small proportion of them ever develop into new plants. On the whole, the plants that depend upon the wind to scatter their seeds seem to maintain them- selves and to invade new regions more successfully than those that depend upon other agencies for scattering the new plants. Many plants have their seeds distributed by currents of water, — streams of various sizes, or ocean currents, or wind currents acting on the water. Seed plants that grow in swamps or ponds are commonly dependent upon water currents for the dispersal of their seeds. But it seems that many seeds are also spoiled by the water. The coconut, for example, which is often cited as a plant that invades ocean islands by being carried over the sea, is really killed by the salt water. FIG. 147. Seeds scattered by birds Birds eat the fruit and discharge the indigestible seeds. /, thistle ; 2, mistletoe ; j, bird cherry ; 4, red-osier dogwood CHAPTER LXI ALTERNATION OF GENERATIONS 373. Life history of a moss. In the moss plants, the individuals that we ordinarily have in mind when we speak of moss bear at the end of a leafy stem a group of sexual organs. Some individuals carry egg- producing organs ; others bear sperm-producing organs (see Fig. 1 48). FIG. 148. Reproduction in moss a, a leafy moss plant (Hypnum molluscum) ; l>, section cut lengthwise through tip of one of the branches, showing position of archegonia, or egg-bearing organs ; £, single arche- gonium, more highly magnified, showing single large egg cell ; d, enlarged view of antheridium, or sperm-bearing organ, of Polytrichum formosum, discharging sperm cells ; *, greatly magnified view of sperm cells ; /, tip of leafy plant from the archegonium of which a spore plant has grown, showing stalk and spore capsule 320 ALTERNATION OF GENERATIONS 321 When the moss is covered over with water, it is possible for the male gametes to swim about, and some of them find their way into the archegonium. Here one of the sperm cells fuses with the egg cell, and the fertilized egg cell begins to develop into a new moss plant immediately — - that is, while still within the body of the parent. FIG. 149. Alternation of generations in the life history of the moss G, the gametophyte, or gamete-bearing plant ; /, the female gamete organ ; m, the male gamete organ ; e, the fertilized egg resulting from the fusion of egg and sperm ; S, the sporophyte, or spore-bearing plant ; s, spores. The spore always develops into a gameto- phyte ; the gametes (egg) always give rise to a sporophyte. G and 5 represent alternate generations that reproduce in different ways, — the first sexually, by means of gametes, the second asexually, by means of spores But the new plant is very different from the parent plants. It has no leaflike organs or anything to correspond to leaves. It consists mainly of stalk, and at its base it is buried in the tissues of the parent plant, from which it gets most of its nourishment. It is therefore parasitic upon the parent to a large extent. At the end of the stalk a capsule is formed, and when this is ripe, a great many spores are 322 ELEMENTARY BIOLOGY thrown out of the opened top. When one of these spores alights upon a moist spot, it absorbs water, and the protoplasm breaks out on one side ; it then proceeds to develop into the next generation. Here again we must notice that the new plant developed from the spore is not at all like the parent plant ; that is, the plant which produced the spores. At first there is a very delicate, green, branching thread, re- sembling some of the green algae found in water. In a short time a clump of cells, or a bud, appears at some point along this branching thread, and from this develops the leafy stalk that we recognize as moss, and some colorless, hairlike threads that look very much like root hairs. The leafy moss plant, bearing gamete organs at the top, is called a gameto- phyte, which means a gamete plant. The FIG. 150. Reproduction in fern The gamete-bearing plant, a, of the fern, called a prothalhis, is a flat plate of cells, with hairlike roots on the undersur- face. Flask-shaped organs, Z>, each bearing a single egg cell, are embedded on the undersurface, near the notch, with the mouth pointing downward and backward. Near the small end of the prothallus, on the undersurface, are the organs, c, bearing the male gametes. These are dis- charged into the water, and swim about freely, finding their way into the egg organ, where fertilization takes place leafless plant, COn- sisting of stalk and capsule, together with the attachment to the parent, is called the sporophyte; that is, spore plant. By following the history of a number of generations of moss we may see that there is a regular alternation of gametophyte and sporophyte. This is illustrated in the diagram (Fig. 149). 374. Life history of a fern. In the ferns the spores are produced on the underside of the leaves (see Fig. 127, p. 294). The spore gives rise to a little plate of chlorophyl-bearing cells, sometimes no larger ALTERNATION OF GENERATIONS 323 than the nail of your little finger, called a prothallus (see Fig. 150). Prothalli are often found growing on flowerpots in greenhouses. The prothallus corresponds to the gametophyte of the moss, while the plant which is familiar to us as the fern is a sporophyte. The FIG. 151. Alternation of generations in the life history of the fern G, the gametophyte, or gamete-bearing plant ; f, the female gamete organ ; m, the male gamete organ ; c, the fertilized egg. S, the sporophyte, or spore-bearing plant ; s, the spores discharged by the spore-bearing organ. The spore develops into a gametophyte ; the gametes (egg) always give rise to a sporophyte. The alternate generations repro- duce in different ways, — one by means of gametes, or sexually, the other by means of spores, or asexually spore always gives rise to a prothallus, which bears gametes. When fertilization has taken place, the zygote formed develops not into another prothallus but into a sporophyte. The diagram in Fig. 151 shows us the alternation of generations in this group of plants. 324 ELEMENTARY BIOLOGY In some plants related to the ferns the two kinds of gametes are borne on two different individuals; that is, each individual gameto- phyte is either male or female. In such species each spore therefore gives rise either to a male plant or to a female plant, as is the case with the moss. It is impossible in such cases to find any difference between the spores that give rise to male plants and the spores that develop into female plants. 375. Heterospory. But there are other plants related to the ferns in which two different kinds of spores are produced, — a large spore and a small spore. In such species the large spore always develops G-m FIG. 152. Heterospory Plants producing spores of two sizes, Is and ss, give rise to two distinct forms of sexual, or gamete-bearing, individuals, female and male, Gf and Gm. The gametes, / and m, unite to form the zygote, z, which develops into the spore-bearing plant, S. There is an alternation between the sexual (gametophyte) and the asexual (sporophyte) generation ; and there is a differentiation between male and female gametophytes, and, finally, a differ- entiation between two types of spores. The next step would be to have two kinds of sporophytes, S, one bearing large spores and the other bearing small spores ; and, indeed, there are plants in which this condition is found into a female gametophyte, while the small spore always develops into a male gametophyte. There are thus two kinds of spores as well as two kinds of gametes (see Fig. 152). 376. Alternation of generations in seed plants. The pollen grain corresponds to a small spore ; that is, one that gives rise to a male gametophyte. The embryo sac is really a large spore, one that can give rise to a female gametophyte. In seed plants the small spore is scattered, as in ferns and mosses ; but the large spore remains in the spore case — the ovule. The male gametophyte is a very much sim- pler organism than we have found in mosses or ferns ; it is, in fact, the simple pollen tube. It is incapable of nourishing itself, but lives in part on the nourishment stored up in the pollen grain and in part on material absorbed from the stigma. The only distinct organ that it has is the divided nucleus that acts as a gamete. ALTERNATION OF GENERATIONS 325 The female gametophyte is still further simplified, for it never gets out of the spore wall. It is nourished altogether by the parent plant, and its activities are confined to the dividing up of the nucleus, finally separating the portion of nucleus that is to act as the female gamete. FIG. 153. Alternation of generations in seed-bearing plants Gm, the male gametophyte, or pollen tube ; »z, the male gamete, a nucleus at end of pollen tube ; Gf, the female gametophyte, or embryo sac ; /, the female gamete, a nucleus in the embryo sac ; g, the fertilized egg, or embryo sac ; Si, young sporophyte, the embryo in the seed ; S%, the mature sporophyte, a flower-bearing plant ; J1} the large spore, giving rise to the female gametophyte, or the embryo sac. s2, the small spores, or pollen grains, giving rise to the male gametophyte. The spores always give rise to gametophytes, and the gametes (producing a fertilized egg) always give rise to sporophytes. Sporophytes alternate with gametophytes, generation after generation Our common seed plants are accordingly seen to be sporophytes, or spore-bearing plants. The alternation of generations of these plants is illustrated by the diagram in Fig. 153. 326 ELEMENTARY BIOLOGY 377. Alternation of generations among animals. Among some of the animals related to the sea anemone and hydra there is found a fairly regular alternation between generations that reproduce sexually — that is, by means of gametes — and generations that reproduce asexually. Good examples of this alternation are furnished by jellyfish found in the ocean off FIG. 154. The jellyfish aurelia The mature medusa, a, reproduces sexually, the gametes being thrown into the water, where fertilization takes place. The egg develops into an individual having the general form of a hydra, £, and attaches itself to a rock. The animal elongates and breaks up into a number of individuals by means of constrictions, so that it comes to resemble a pile of bowls. Each individual, when separated, turns over and swims away, changing into a medusa, a our coasts (see Figs. 154 and 155). The complete life history includes both kinds of individuals, male and female, and two kinds of generations, sexual and asexual. Alternation of generations is also found in many parasitic animals, especially parasites that inhabit two or more different hosts at differ- ent stages in their development. Thus, the malarial parasite repro- duces in the blood of human beings by sporulation ; that is, by the formation of a large number of spores. But in the body of the mos- quito there are produced tiny protoplasmic structures that unite in pairs ; that is, they conjugate. There is thus present a sexual method of reproduction and an asexual method, and these alternate regu- larly so long as the organism has the opportunity to pass from one host (man) to the other (mosquito) and back again (see pp. 403-407 and Fig. 209). CHAPTER LXII REPRODUCTION IN ANIMALS 378. Aquatic invertebrates. Among the invertebrate ani- mals — that is, those having no backbone — living in the water, such as sponges, corals, starfish, clams, and crayfish, fertilization usually takes place outside the body of the parent. In the cases of many, however, the developing egg cell may be protected by some portion of the mother's body, as when the young hatch in the mantle cavity of the clam. 379. Reproduction in fishes. Among the fishes, the female gametes are usually deposited in quiet places at the bottom of the sea, near shore, or in quiet pools of rivers. Then the male fish swims over the eggs, dropping out a quantity of fluid con- taining the sperm cells. These swim about in the water, fer- tilization taking place in much the same way as in the rock- weed (see p. 299). The fluid containing the sperms is called milt, or semen. A sperm cell of a fish is illustrated in Fig. 1 56, ^. As soon as the nucleus of the egg has fused with the nucleus of the male gamete, the combined nucleus begins to divide, and thus the development of a new fish is started. The female gamete of the fish contains a small amount of food material in addition to the protoplasm. While the devel- opment is under way the young fish lives on this accumulated food. In some species of fish the adults swim about in the neighborhood of the developing fry and protect them against possible destruction by other fish. In most species, however, the sperm and eggs are thrown out by the adult males and females, and then left to themselves. Thus exposed, thousands of eggs are destroyed before they have a chance to develop into fish. Of course, thousands are also destroyed in the case 327 328 ELEMENTARY BIOLOGY of those species that protect their young ; but it is not probable that in these species so large a proportion are lost. 380. Water essential to gametes. As we have seen, sexual reproduction is possible only on condition that two gametes of opposite sex combine. In addition to producing the gametes, the bringing of them together is another problem of life. Moreover, the sperm and egg cells (gametes) are unlike spore cells in that they are quite incapable of resisting drought ; drying kills them very quickly. It is therefore another condition of reproduction that the gametes be pro- tected against drying up. Among the animals and plants that live in the water, or where water may remain in contact with their reproductive organs, a, enlarged view of portion of colony, showing this is simple enough, feeding individuals and reproduction individuals. T> <_ New individuals are here produced asexually, by But m Organisms that budding. l>, the medusa stage, which originates as a bud on the hydra colony and reproduces by means of gametes thrown into the water FIG. 155. Hydromedusa (Bougainvillea ram os a) live on land, or in the air, the older methods of bringing the gametes together will no longer serve. We have seen how this con- dition is met in the case of the flowering plants. Among land animals there are special organs and modes of behavior that make fertilization possible. 381. Reproduction among batrachians. The frogs, which live on land and breathe air in their adult state, go to the edges of ponds and puddles at the breeding season. After the gametes are thrown into the water, fertilization takes place, and the adult frogs pay no further attention to them. REPRODUCTION IN ANIMALS 329 In some species of toads the fertilized eggs are placed in the mouth of the parent, where they are kept until the tadpoles are large enough to swim away. Among the batrachians, — which include newts and salamanders, as well as frogs and toads, — there are very many cases of parental care of the devel- oping young, ranging all the way from abandonment directly FIG. 1 56. Sperm cells of animals /, pig; 2, bird ; j, salamander; 4, ray; j, threadworm (Ascaris) ; 6, lobster after the discharge of the gametes to guarding within the body of the mother until the young are fully formed and able to shift for themselves. 382. Reproduction in the insects. Among the insects, which of all animals are most distinctly adapted to living in the air, the spermatozoa of the male are passed directly into the body of the female through a special duct. The semen is discharged into a receptacle, from which the spermatozoa pass, a few at a time, into another space, wherein the female gametes (the eggs) are fertilized. It is possible for a queen bee to retain a quantity 330 ELEMENTARY BIOLOGY of living sperms for two or three years, or even much longer, and to force these out of the receptacle from time to time as she produces new eggs. Even the insects that normally lay their eggs in the water — as the mosquitoes — have fertilization take place within the body of the mother. 383. Reproduction in vertebrates. Among all the backboned animals, above the amphibians, fertilization takes place within the body of the mother. The eggs begin to develop immedi- ately after fertilization and are retained within the parent's body for a longer or a shorter period. Here they are not only protected against possible injury by enemies, but they are nourished and supplied with moisture and, in some cases, kept warm. The degree to which the new organism is dependent upon the parent during the early stages in its development varies considerably. Among the reptiles — for example, some tor- toises and alligators — the developing egg becomes enveloped in a mass of food material on its way out of the mother's body and is then supplied with a horny shell. The egg is then deposited in the sand, where it hatches under the heat of the sun. In certain lizards, however, the eggs hatch within the body of the mother, and the young leave her body fully formed. Among the birds the fertilized egg becomes covered with a large quantity of food material (yolk and egg albumen), and the whole mass becomes surrounded by a limy shell. Nearly all birds protect their eggs, and they also supply the heat necessary for the hatching of the young. Among the mammals the development of the egg takes place entirely within the body of the parent. The new organism is cared for not only until it leaves the body of the parent but for a comparatively long period after it is born. The length of this period varies almost directly with the level of the family of animals in the scale of development. CHAPTER LXIII INFANCY AND PARENTAL CARE 384. Infancy in lower plants. Among the one-celled plants or animals each cell resulting from a cell division begins to shift for itself immediately, as soon as it comes into existence as a distinct cell. The simplest organisms of any series are detached from their parents and shift for themselves early in life. As we go up the scale we find that more and more do the parents provide for the offspring in the way of food or protection or both. Among the seaweeds, like bladder wrack and many other species, the gametes are thrown into the water, where thousands are destroyed for every pair that fertilize and establish a new individual. In the mosses and ferns the female gametes are retained within the body of the parent plant until after fertilization, and until the new plant has been well started. In the mosses the new plant gets nearly all of its nourishment from the parent plant. 385. Infancy in seed plants. When we come to the highest plants, the adaptation of structure and behavior to the apparent advantage of the species is still greater. The spores are pro- duced in comparatively small numbers and the gametes in still smaller numbers. The fertilized egg is completely protected by rather elaborate structures, and the young plant develops within the body of the parent until it is fairly well along — in most species until the root, stem, and leaves are quite distinguishable. In addition to the nourishment and protection, the parent also supplies a quantity of food that is available after the baby plant is separated from the parent. And in most species we find a 332 ELEMENTARY BIOLOGY still further contribution of each generation to the next in the form of special structures or organs that assist the young plant in getting to some distance from the parent (see pp. 315-319). 386. Advantage of longer infancy to the species. In all these various directions the organisms expend energy in ways that enable the offspring to get a better start in life than would be possible if the spore or the gamete (or zygote) were dis- charged by the parent immediately upon being formed. On com- paring the various groups of plants with respect to the amount of nourishment or protection that parents supply to the young, or with respect to any other services rendered by the parents to the young, we shall see that with the ascent of plant life from the lowest to the highest there is an increase of the dependence of the offspring upon the parent. And with the increase of service rendered by parents to offspring there is at the same time an increased advantage to the species. An advance in the scale of life seems to impose additional burdens upon the organisms. But these are more than com- pensated by the additional advantages. As has already been pointed out, the production of flowers and fruits and seeds is a source of great expense in material and energy to the organ- ism. Yet in any species of plants that produces well-stored seeds, well-protected seeds, and seeds well adapted to wide dispersal every individual gets the full benefit of this addi- tional expendittire of energy at the very beginning of his career. We might even say that, apart from all other considerations, a plant comes to be able to do all of its life's work just in pro- portion as its parent has guarded its youth and has given it a good start. In doing things for posterity a plant is thus merely repaying to the species what was done for it in the past. Of course we are not to suppose that the plants do this or that because they have any feeling of gratitude, or ability to foresee future needs. In speaking of the advantages or disadvantages of various types of behavior on the part of plants, we mean merely to point out that certain kinds of doings may actually contribute to the prosperity INFANCY AND PARENTAL CARE 333 of the species, whereas other kinds of doings would probably lead to the extinction of the species. Some plants behaved in a certain way in past ages, and their progeny to-day occupy the surface of the earth. Other plants behaved quite otherwise, and we know of them only by the traces they have left in the ancient rocks of the hills. FIG. 157. The four-spined stickleback (Apeltes quadracus) The adult fish swims about and through the nest, guarding the eggs while they are hatching 387. Infancy among animals. When we study the pro- longation of infancy among animals, we find that the advan- tages of a protected and cherished youth are even more -marked there than they are among plants. Among most of the lower animals the mother lays large numbers of eggs — in the water, on leaves, in the soil — and abandons them. But toward the upper end of many series of 334 ELEMENTARY BIOLOGY animals we find that much more is supplied for the young. The lobster and crayfish mothers carry the eggs about on their abdominal legs, or swimmerets, and even the young em- bryos until they are able to care for themselves. Among the insects there are some that abandon their eggs as soon as laid, whereas others provide shelter and food for the young. Most fish leave their eggs in the water with- out further attention. There are a few fishes, like the stickleback, that prepare a rather rough protection, or nest, for the eggs (Fig. 157). Some toads carry their eggs about in the mouth until they are hatched. Among the reptiles FIG. 1 58. Wallaby and young The babies are not only protected and kept warm in the marsufium, or pouch, but are also nourished by a milky secretion produced by glands in the lining of the pouch and birds the egg begins its development inside the parent's body, and receives a large amount of food and a protective covering. Most reptiles and some birds leave their eggs to be hatched by the heat of the sun, or at ordinary temperatures. Most of the common birds, however, build more or less elabo- rate nests and care for the fledglings and for the eggs, besides supplying heat for the hatching. The feeding of the young birds by the parents is a very interesting operation to observe, and it shows a very complex development of instincts. 388. Infancy among mammals. When we come to the mammals, the dependence of the young upon the parents is carried even farther. Not only does the egg develop inside the body of the mother until it has acquired the general form characteristic of the species, but it is nourished by the parent INFANCY AND PARENTAL CARE 335 for a long time after birth. Among the marsupials, or pouch animals, like the kangaroo and the opossum (see Fig. 158), the young are placed in an abdominal pouch immediately after birth. In all the other mammals the young suckle from the milk glands of the mother. As we go from the lower mam- mals to the higher, we find that the infancy of the individual becomes proportionately greater or longer. This is true even if we compare different races of mankind. Among the primitive savages children are allowed to run about without anyone to watch them as soon as they can walk ; in a civilized community we sometimes keep close watch over chil- dren, even at their play, for several years. It is easy to see the advantages of a long youth from the point of view of more and more complex civilization. There are physiological dif- ferences also connected with the relative length of infancy. This is shown, for example, by the length of time it takes the individual to reach maturity. The table below shows the duration of the growing period for a number of animals, including man. GROWING PERIOD OF VARIOUS MAMMALS ANIMAL LENGTH OF ADOLESCENCE LENGTH OF LIFE Dormouse Guinea pig 3 months 7 months 4-5 years 6—7 years Lop rabbit 8 Q months 8 years Cat 1—2 years 12— i <5 years Goat 1 15 months Fox 1 8 months j -j j^ years English cattle 2 years 1 8 years Larsre dosrs 2 years Horse xi years •JQ years Hoe . Hippopotamus Lion _ycaio 5 years 30 years Camel 8 years Man 20 2 ? years Elephant 10— T, *\ vears 100 1 20 years PART IV ORGANISMS IN THEIR EXTERNAL RELATIONS CHAPTER LXIV OBSTACLES TO LIFE 389. Life and the environment. To live means to do. Pro- toplasm tends to be active. But the activities of protoplasm de- pend not alone on its own structure or composition ; they depend in part, as we have learned, upon external conditions as well as upon the opportunity to obtain various materials from without. While many of the external conditions are favorable to the ac- tivities of live matter, others are just as decidedly unfavorable. 390. Temperature and life. Observations on various plants and animals show that the activities of life are dependent upon temperature. Warm-blooded animals can endure a wide range of tempera- ture, but the protoplasm of such animals can really endure a rather narrow range only. When such protoplasm is exposed to a temperature several degrees below the normal, or several degrees above, it ceases its activities and may even be killed. On the other hand, the cells of the so-called cold-blooded animals can actually endure extremes of temperature. Many animals can be frozen and then thawed out again without being appreciably injured. In careful experiments fish have been frozen in blocks of ice to a temperature of 5° F. (27 degrees below the freezing point), kept this way for some time, and then slowly thawed out without being 337 338 ELEMENTARY BIOLOGY killed. When cooled a few degrees lower, the fish were killed. Frogs have been frozen to a temperature of — 28° C. (18.4° R) without being killed. Some of the animals without backbones regularly survive even colder temperatures. Many insects that survive the winter in the adult stage have to be frozen and then thawed out again, although many of them no doubt escape freezing by burrowing into the ground. In our experience with winter weather many of us have no doubt frozen an ear or a finger. That did not kill us, but it may have killed some of the cells in the affected part. When frozen cells are thawed out rapidly, they are liable to burst and thus be killed, but with a slow thawing the life of the cells may be saved. That is why a frost-bitten ear is rubbed with snow, to prevent it from warming too rapidly. At the other end of the temperature scale some of the simplest animals (ameba) have been found to survive a tem- perature of I22°F. when slowly heated. But most of them died at that temperature. This does not mean that the animals were unaffected in the gradually heated water until they were killed. Long before this temperature is reached — at about the temperature of our blood — they ceased active motion. 391. Water and life. We have learned that water is an intimate part of the cell contents, and we can realize that life is impossible without it. Yet the amount of water available for plants and animals is constantly changing (except in the oceans and larger lakes and rivers), so that at one time there is drought, — at least relatively speaking, — whereas at other times there is an injurious excess of moisture. Ponds and brooks dry up ; and, so far as the availability of water is concerned, the same condition arises when the water freezes. The soil dries, or it freezes, and the water supply is thus cut off from the countless plants that inhabit the earth. We know that a dry spring or summer may mean a famine, and that some parts of the earth's surface are quite uninhabitable because of the scarcity of water. 392. Light and life. We have learned that light is essential to the manufacture of organic food, that it is the ultimate source of OBSTACLES TO LIFE 339 all the energy which living beings constantly use. The amount of living matter that can maintain itself on a given territory depends largely upon the amount of light available. The tropics, in addition to being warmer, also receive more sunlight and are therefore more closely occupied by living beings than the frigid zones. There is an almost continuous gradation in the density of population1 between the equator and the poles. On the other hand, extreme intensity of light is itself a serious obstacle to the normal processes of living protoplasm. Light interferes with the growing process (p. 38) and may be destructive to protoplasm. We see again, then, that a form of energy that is essential to life may be a source of danger to it. 393. Salts and life. The various mineral salts found in the ocean, in other bodies of water, and in the soil are ordinarily absorbed by living beings through the process of osmosis, and many of the salts take active parts in the processes that go on in living protoplasm. Many of them are apparently indif- ferent in their action, being neither helpful nor injurious ; a few are injurious ; and of those that are essential, some are injurious in large quantities. On the other hand, a scarcity of particular elements, or of compounds containing these ele- ments, will absolutely prevent the growth and development of living things. The kind of life that is possible in each of two regions that are substantially alike as to temperature, moisture, and light will in many cases be determined by the chemical condition of the substratum. 394. Excess of air. The air, which is necessary to practically all living beings either directly, as an immediate source of oxygen, or indirectly, as a more remote source of oxygen (for plants and animals living in the water) and as a source of carbon dioxid, never seems to be injurious when in excess. Indeed, we do not know of any situa- tion where the air is in excess. If we consider high atmospheric pres- sure in deep holes in the earth as such situations, we may not be sure 1 Population refers here, of course, to all plants and animals and not merely to human beings. The statement is not strictly true for human beings. 340 ELEMENTARY BIOLOGY that it is the excess of air that interferes with life there ; no light is available in such places. If we consider artificial conditions produced by the digging of mines or the use of caissons under water, it is indeed true that these conditions interfere with normal life processes ; but they do this not because there is too much air; but because the FIG. 159. The wind as an obstacle to life The wind, often helpful to life and growth, is sometimes a hindrance. In the picture the wind, besides making the tree grow one-sided, and bending over the top branches, has blown the earth away from the roots. (Photograph lent by New York Botanical Garden) human beings that go into these places are not adjusted to the high pressure} Nor is there any place on earth where there is naturally a scarcity of air, except on the very highest mountain tops ; but in these situations other conditions are sufficiently unfavorable to life, so that we do not usually think of the absence of plants and animals in these places as due to the lack of oxygen. 1 The distressing disease known as " the bends," which affects many of those who have to work in the high-pressure atmosphere of the caissons, is very easily avoided by taking sufficient time to enter the working chamber and sufficient time to come out. The disease is not caused by the high pres- sure ; it is caused by the siidden change from high pressure to the normal pressure of the surface atmosphere. CHAPTER LXV THE CONFLICT OF LIFE WITH LIFE 395. The predatory relations. Many of the animals, and most of the plants, that are incapable of manufacturing their own organic food get their food from the bodies of other plants or animals that are already dead. But there are very many animals, and a few plants, that kill their prey. The gentle cow and the soft-eyed deer browse on the herbage, and we never think of them as beasts of prey ; yet from the point of view of the grasses and shrubs that furnish them their food these animals are truly predatory. That is to say, they are direct destroyers of living things. To maintain themselves upon this earth, certain living things must somehow protect themselves against predatory enemies, and this is just as true of plants as it is of animals. 396. The parasitic relation. There are many plants and animals that get their food supplies from the living bodies of other organisms. That is to say, they eat from the living victim, sometimes thereby killing, but not always and not necessarily. Plants and animals that get their food in this way are called parasites. The most common parasites are found among the lowest plants and animals ; but nearly every class of living things has its parasitic representatives. Some two dozen of the common diseases of man, and many diseases of our domestic animals, are known to be caused by the activities of parasitic bacteria in the bodies of the victims. Protozoa as parasites are known to cause malaria and the sleeping sickness of Africa. Most of our common plant diseases are caused by fungi or bac- teria. The hookworm is a serious parasite on man ; and the 342 ELEMENTARY BIOLOGY tapeworm, although perhaps not so serious, is probably more common. Among insects are many related to the wasp and the bee that lay their eggs in the bodies of caterpillars ; when the young hatch out, they begin to feed on the caterpillar (see 5, Fig. 115). Among the backboned animals, certain fishes will attach themselves to the bodies of other fishes and suck the blood from their victims. Most vertebrates get their food either by killing plants or other animals or by taking dead matter (that is, plant or ani- mal remains) of one kind or another ; in other words, there are very few vertebrate parasites. The idea of parasitism extends beyond the means of getting food. The European cuckoo will lay her eggs in the nests of strange birds, thus getting from other organisms at least two direct benefits — the work of building a shelter for the young and the work of keeping the eggs warm during incubation; there is also the feeding of the young through the work of the strange foster mother. This is a case of getting services from another organism, without giving any- thing in return. It is in this sense that we use the word parasitism in connection with higher animals, and especially in connection with human affairs. From the viewpoint of the unwilling hosts to the unbidden guests, parasitism is an obstacle to life ; and every species of living things is exposed to a number of such parasitic enemies. To be able to protect itself against parasites is one of the conditions necessary for maintaining life. 397. The competitive relation. If all the offspring of any plant or animal should reach maturity and reproduce the usual number of young, and if this were continued for several gen- erations, the earth would not be able to hold the resulting population.1 1 A conger eel is said to lay 1 5,000,000 eggs in a year. If each of these eggs hatched and reached maturity, and if each of these individuals repro- duced at the same rate as the parents, the ocean would soon be crowded with conger eels. The same thing is true of all animals. THE CONFLICT OF LIFE WITH LIFE 343 It is evident that survival is impossible for all that are born. Many are killed by the unfavorable conditions of life, many are killed by mechanical injuries of various kinds, many are killed by predatory enemies, and many are killed by parasites. Finally, there are left those who remain to live out their lives. But those do not all reach the full length of years. There are still too many of them to live comfortably in the world. Many of these are now destroyed in their competitive struggle with one another. This idea of competition, borrowed from the forms of business operations that prevailed during the nineteenth century, applies to living things, for the most part, only in a figurative sense. There are really very few animals, and no plants, that are engaged in a direct conflict for the materials necessary to their well-being or to their survival. There are, however, situations in which more individuals are born than can possibly reach full development, and in the course of time we find that some have endured, while others have perished. In a shaded wood, for example, the young seedlings grow at different rates. Some grow fast enough to bring their tops into the sunlight before others do ; they have the advantage of more light. They now grow faster, not only because they are more favorably situated, but because the growth of their " competitors " is retarded by lack of light. It is absurd to suppose that these plants are struggling, in the sense in which two wrestlers or two racers are struggling with each other. No one does anything that is directly related to injur- ing the other or to helping itself as against the other. The result that one survives and the other perishes depends upon certain external and certain internal conditions of the plants, and not upon anything in the slightest degree resembling effort, or offense or defense. It is only when we come to the highest animals — especially birds and mammals — that there is a real competitive struggle that involves direct danger to the participants. A number of wolves, for example, may fight over a carcass. In any farmyard 344 ELEMENTARY BIOLOGY you may see chickens peck at each other when they are feed- ing ; they peck at anything that gets in their way. More conspicuous are the competitions among the highest classes of animals for their mates. Male seals and walruses will fight for the possession of the females ; male stags and other mammals will do the same. In such struggles the individuals are actually exposed to injurious attacks, and the survival of the individual depends upon his superior means of protecting himself. Some birds also fight each other in this competitive way. We may conclude by recalling that every living thing is exposed to a number of obstacles or direct dangers to well- being ; that some of these arise from excess or shortage of certain materials in the environment, and that others arise from the various co-inhabitants of the world. To live, one must be able to overcome these obstacles and to escape these dangers. CHAPTER LXVI PROTECTIVE ARMORS OF ORGANISMS 398. Walls and shields. The simple cell wall that we find in the one-celled plants, and the cell membrane found in many one-celled animals, may be considered to serve as protection against mechanical injury to the protoplasm. At the same time they permit the osmotic transfer of income and excretion. FIG. 1 60. The horseshoe crab This animal is protected by an external skeleton, or armor, of chttin secreted by the skin cells In plants and animals made up of many cells we generally find that the external layer of cells is either modified into a protective layer or supplemented by various protective struc- tures. The outer cell walls of skin cells in plant structures are usually thicker than the inner walls and much thicker than the walls of inside cells. The skin cells of leaves usually have a secretion of a fatlike substance on the outer surface (see /, Fig. 5), called cutin, which prevents evaporation from within as well as water-logging from without. It adds also to the protection against mechanical injury. 345 346 ELEMENTARY BIOLOGY In many plants the outer surface of the leaf or fruit has, in addition to the cutin, a layer of waxy material. This is the bloom that we see on plums and other plant surfaces. In many animals the cells forming the surface layer of the body are small and thick-walled, and many kinds of secretions add to the protective value. The horseshoe crab (Fig. 1 60) produces his armor by secreting a substance that hardens like a varnish in the water. This is very similar to the substance that makes up the exo-skeleton (outside skele- ton) of insects, the chitin (pro- nounced ki'tm), which is also formed by the secretion of the skin cells. In lobsters, crabs, crayfish, and their relatives (the Crusta- cea] the chitin secretion is com- bined with a comparatively large amount of carbonate of lime. FIG. 161. Sea urchin Animals of this branch deposit large quantities of lime in their skin, and produce knobs and spines that form a ^^ ^ ^ ^ protective armor skeleton of these animals their crusty quality. Clams, oysters, and snails have extremely soft skins. (The name of this whole group of animals, Mollusca, refers to the general softness of these organisms.) They receive, however, a great deal of mechanical protection from their shells, which consist of deposits of lime formed by the secretions of a special fold of tissue called the mantle (see Fig. 44). On the clam and on the snail the lines indicate the successive deposits of lime. The inner surface of the shell is often very beauti- ful and iridescent because of the very fine lines that break up the surface. This mother-of-pearl is used extensively for ornamental purposes, — for buttons, knife-handles, etc., — and the shells of many mollusks are used for their hardness and durability in the making of buttons and similar objects, without regard to their beauty. PROTECTIVE ARMORS 347 In the starfish and sea urchins and their relatives (Echino- dermata, meaning " spiny-skinned ") the skin secretes a great deal of lime, which is deposited in the form of definite rows of plates, and in projecting spines (see Fig. 161). We may well imagine that no fish would care to eat a mouth- ful of such spiny creatures as the sea urchin, or to bite and swallow the harsh rays of a starfish. In the trunks of our common trees there is a growing layer that con- stantly produces new layers of wood and new layers of bark (see Fig. 63). The bark cells produced on the outside of this cambium layer soon die, and the walls become corky. As new layers are produced underneath, the old layers are moved farther and farther from the center FIG. 162. Hairs of plants <7, branching hair on leaf of tobacco plant ; l>, hair on leaf of thorn apple ; <:, glandular hair on leaf- stalk of Chinese primrose ; d, marginal tooth on of the plant. On the out- sedge leaf ; '» slandular hair on flower of h°p; /, leaf of apple of Sodom; g, stinging nettle, Side the dead Cells, ex- with tip, greatly enlarged posed to the weather and to mechanical injury from moving animals and other objects, rub off or chip off. The mass of bark is thus a constantly renewed protective layer. Similar in some ways to the bark of a tree is the hide or skin of a mammal. Our own skin, for example, is made up of dead cells on the outside. These are constantly rub- bing off, but are as constantly replaced by new cells from beneath. The growing layer (see bt Fig. 92) gives rise to 348 ELEMENTARY BIOLOGY new cells ; these die as they are moved toward the surface by the newer cells beneath, becoming a layer of dead scales. The skin protects the animals not only against mechanical injury but also against the loss of water and against the absorption of water, for the skin is practically waterproof, being FIG. 163. Mullein in meadow These plants are closely covered with fine, branching hairs, giving the leaf a flannelly texture. We can well imagine that a cow would not care to eat anything that felt like flannel in the mouth, and so we can understand that the hairy growth may actually protect the plants against grazing animals. (From photograph by Dr. H. A. Kelly) more or less oily (see p. 206). It also protects, to a certain degree, against too rapid changes of temperature. In this function many skins are supplemented by layers of fat on the inside and by hairs or fur on the outside. 399. Hairs and other outgrowths. On the leaves and stems of plants the cells of the epidermis enlarge at right angles to the surface. This mode of growth results in the formation of hairs (see Figs. 162, 163). PROTECTIVE ARMORS 349 It seems likely that in many plants the hairs are really related to the moisture. The absence of moisture, or, rather, a shortage of Papilla with blood vessels 4 Sfeatt Branches of shaft forming , baronies Horny layer FIG. 164. Hair of mammals /, human hair follicle, showing mode of growth (the dead shaft is pushed forward by the new growth about the papilla) ; 2, hair of horse ; j, hair of mouse ; 4, hair of marmot. a, base of hair ; £, tip ; £, more highly magnified portion of shaft moisture, is known to bring about the production of hairs in species of plants that ordinarily do not produce hairs when water is abundant. Hairs are also likely to protect many plants against extremely high or low temperature. The hairs familiar to us in common animals and on our own skin are much more complex in structure than are plant hairs (Fig. 164). The feather of the bird may be considered as a highly complex hair. In the manner of of mammals very much, Growing region Pulp FIG. 165. Feather structure The feather of a bird is a skin structure that grows in substantially the same way as a hair or a finger nail growth the feather resembles the hair but in its structure it is of course very 350 ELEMENTARY BIOLOGY different, and each feather has a determinate growth ; that is, there is a definite limit to the size and form which a single feather can attain (Fig. 165). The bristles of hogs and the quills of hedgehogs and porcu- pines are giant hairs. Hairs, quills, bristles, and feathers may FIG. 1 66. Box turtle The exoskeleton consists partly of skin plates and partly of bony expansion. This animal is protected not only by the withdrawal of head and limbs but by the further closing of the hinged breastplate, shown on the right be considered as special kinds of skin growths and may be compared to the scales of the common fishes and of reptiles, and to the plates found in the skin of sturgeon and the gar pike. The shield of the turtle or tortoise is in part a skin structure and in part produced by the skeleton (Fig. 166). The amphibians (frogs, newts, etc.) are the only backboned animals that never produce outgrowths on the skin, although some of the toads have irregular thickenings in the adult stage. CHAPTER LXVII PROTECTIVE PIGMENTS AND APPEARANCES 400. Pigments and light. Animals that live at great depths of the sea, and those that live in caves, — situations in which there is little or no exposure to light, — do not generally show much pigment in the skin. This fact may be interpreted in two ways : 1. Where there is no dan- ger of being injured by light, the species will be able to main - tain itself without acquiring the pigment-forming habit. 2. Where there is no light stimulation, pigment will not be formed. In the human race the dark pigment of the skin is un- doubtedly a protection against the light, as shown by the relative sensitiveness of light- skinned races and dark- skinned races to the influence of the tropical sun. It is also shown by the behavior of the skin of a person who has been tanned and the behavior of the skin of the same person before the tan has formed. A person who does not get tanned is likely to be sunburned with every exposure to strong sunlight. On the other hand, in a person who is dark-skinned, or who has be- come tanned, the pigment acts as a screen, cutting off the rays that are injurious to the protoplasm. FIG. 167. Katydids Microcentrum retinervis (above) ; Cyrtophyl- lus concavus (below). These insects match the color of the foliage upon which they feed ; in some species the resemblance to a green leaf is even more striking than in the two shown here 352 ELEMENTARY BIOLOGY In certain experiments with flatfish that are ordinarily pig- mented on the upper surface and white on the lower surface, the light was supplied from below by means of mirrors, with the result that the fish developed pigments on the lower sur- face and remained white above. From these experiments and from our own experience with getting tanned, we may feel con- fident that at least in many cases the formation of the pigment is due to the stimulation of the light. But we know also that there are many other pigments that are formed without refer- ence to the light, whether they have any protective value or not. 401. Invisibility. In relation to enemies that can see, one of the most obvious means of protec- tion is something to make one in- visible. The jellyfish (Figs. 1 54, #, and 155, b) is so nearly transpar- ent that it is practically invisible in the water. But transparency is not the only means by which an object may be made invisible. The see- ing of objects depends upon the contrasts in lights and shadows ; an object that is colored like the background becomes by that fact invisible. This type of invisibility is so common in nature that some men claim to be able to tell the kind of surroundings an animal naturally occupies from the character of its surface colorings. The green katydid among the green leaves is a common example of so-called protective coloration FIG. 1 68. The underwing moth (Catocala) When they are at rest, the moths of this genus resemble the bark of trees, so that they are no doubt often over- looked by their enemies PROTECTIVE PIGMENTS AND APPEARANCES 353 (Fig. 167). The tree toad and the partridge become lost to the eye, as well as the sand flea and the underwing moth (Fig. 168). It is familiar to all of us that desert animals are frequently tawny in their color, whereas arctic ani- mals are frequently white. There can be no doubt that in relation to certain enemies the resemblance between an animal's color and the background color is often a real protection. At the same time, there is danger of exaggerat- ing the importance of this resemblance to- the organisms, and there is a corresponding danger of trying to prove too much from this resemblance. Thus, the whiteness of arctic animals is appar- ently due in many cases not to the whiteness of the surroundings but to the low temperature. The color of an animal is often due to the char- acter of the wastes pro- duced by the chemical changes going on in the protoplasm. The character of the waste, in turn, will depend upon the nature of the food. A change in diet will therefore in many cases result in a change of color. This is shown in the FIG. 169. The walking stick This animal has startled many a person by walking away from a hand stretched out to grasp a leaf or twig. The insect is related to the locust and katy- did, but it has no wings. Its body and legs are very long in proportion to thickness, and the en- largements at the joints and the irregularity of outline increase the resemblance to bare twigs. Moreover, the color of the animal changes with the seasons, from a bright green in the spring to a deep brown in the fall, thus matching its surroundings very closely 354 ELEMENTARY BIOLOGY brightening of the color of canaries by a regulation of the diet, and by a change in the color of many insects with the change of diet. On the other hand, if a color is to protect, it can do so only in rela- tion to an eye that fails to discriminate. But if the enemy finds his prey by means of smell or some other sense, the color cannot be a protection. People have frequently made the mis- take, also, of supposing that other animals see exactly as we do. What looks alike to us may be readily distinguished by other animals ; and the opposite is also true. Thus, the white spots at the rear end of a deer, or the white stripes on a badger, make these animals conspicu- ous in our sight; but from the point of view to be obtained by eyes that are close to the ground these white spots merge with the light of the sky, and the outlines of the animal are as completely lost as are those of the zebra or the tiger among the stems of the underbrush.1 FIG. 170. Walking-leaf insect This insect, related to the locust and the katydid, resembles the foliage upon which it crawls FlG. 171. Tree hoppers (Membracis binotata) These small insects resemble miniature quail quite as closely as other animals " mimic " their models. Yet there is no conceivable advantage to the insect in this resemblance 402. Protective resemblances. In some animals the mottlings and striping are often very close imitations of particular kinds of backgrounds, and this resemblance is further heightened in many animals by peculiar forms (see Figs. 168, 170). 1 The art of " camouflage " as developed during the Great War rested largely on the observations of naturalists on protective coloration. PROTECTIVE PIGMENTS AND APPEARANCES 355 What is perhaps the most remarkable resemblance between an animal and a part of its surroundings is furnished by the East India butterfly Kattima (Fig. 172). The undersurface of the wings, exposed when this butterfly is at rest, resembles a brown leaf with a distinct midrib and veins passing from this to the edges. Near one end is a dark spot close to a nearly transparent area, resembling very much the kind of spot often produced by the action of some fungus. The details are very sharply defined and almost uniform. If one of us should see a flying kallima come to rest on a twig, he should perhaps have some difficulty in distinguishing the insect among the leaves ; it is pos- sible also that the lizards and birds that feed upon this species are some- times baffled in their pursuit of prey. Yet it is doubtful (i) whether the ad- vantage of this resemblance has had anything to do with its gradual appear- ance as a character of this species, and (2) whether, indeed, it is an advantage (see Fig. 171). FIG. 172. The Indian leaf butter- fly (Kallima} Many arguments concerning the evo- lution of animal life have been based on the striking resemblance between the wings of this insect when at rest and brown leaves. It has been said that the animal looks like a leaf only when it comes to rest with the head up ; but observers who have seen the animal in its native surroundings tell us that it always comes to rest head (/own, on guard against lizards. In this position it is sufficiently con- spicuous to be recognized even by untrained human eyes 403. Warning colors. We saw that some of the wastes produced in living bodies are poisonous (see p. 203), and we can understand that the presence of these poisons in the body of a plant or an animal would make such a body undesirable as food for another animal. Distasteful (bitter, sour, acrid, foul- smelling) substances may thus serve to protect organisms against possible enemies. Poisonous and distasteful substances in an animal body are often associated with conspicuous colors, which have been called warning colors by some naturalists. The idea is that the bright color warns enemies against eating 356 ELEMENTARY BIOLOGY the animal. But this involves some way of educating the enemies as to the meaning of the warning. It is true that many animals instinctively avoid certain kinds of plants and cer- tain kinds of animals, and that some of the avoided species are really injurious. It is also true that animals are often poisoned by eating unsuitable organisms, and that animals often eat organ- isms that are distasteful or that make them sick. A young chick, fresh from the egg, soon begins pecking about for food. A chick finds a worm or a caterpillar and at once eats it. Most of the material thus taken is sufficiently palatable. But presently the chick finds a hornet or a woolly-bear caterpillar. This mouthful is somewhat too much for the chick ; it makes a profound impression on the young animal. The hornet may be killed, or the caterpillar may be killed, but the chick is impressed. She will never eat that kind of food again. The dead hornet or caterpillar has taught the chick a lesson, but can- not get the benefit of the lesson. Other hornets, however, or other woolly bears, are safe, so far as that particular chicken is concerned. The individual sample is thus sacrificed for the benefit of the species. When we consider that every individual has to have his own lesson, we should think this a rather expensive mode of protection, but we may take the idea for what it is worth. One thing is certain, many conspicuous species lack the bitter juice, while others have the bitter juice, and yet lack a conspicuous appearance ; and one species seems to hold its own about as well as another. FIG. 173. Mimicry among butterflies The viceroy, £, belongs to a different genus of butterflies from the monarch, or milkweed butterfly, a ; yet the resem- blance at first glance is so striking that most people will be unable to point out any difference between the two except in size. A close study will show us, however, a number of differences in the pattern PROTECTIVE PIGMENTS AND APPEARANCES 357 404. Mimicry. Growing out of our knowledge concerning the relations of the characters of animals to their safety and danger, a very interesting idea was developed by some naturalists during the last century. This is the idea of FIG. 174. The mimicry of the African swallowtail butterfly (Papilio dardanus cened) f, the male. The female, a, occurs in three distinct forms. Each of these forms pre- sents striking resemblances to butterflies of other genera. Thus, the form cenea, 2 a, resembles Amauris echeria, 2 b, which in turn resembles Pseudacraea tarquinia, 2 c. The form lippocoon, 3 a, resembles Amauris niavius, 3 b, which in turn resembles Euralia •walbergi, 3 c. The form trophonius, 4 a, resembles Danais chrysippus, 4 b, which in turn resembles Diadema misippus, 4 c. The argument that these resemblances bring about advantages may be sound, but too little is as yet known as to what brings about the patterns of the insects supposed to represent the original model protective mimicry. A common example of this near home is the resemblance between the milkweed butterfly and the viceroy (see Fig. 173). 358 ELEMENTARY BIOLOGY The explanation that is sometimes given of this resemblance is as follows: The milkweed butterfly has a bitter or disagreeable taste, and therefore birds commonly avoid eating the insect. The viceroy belongs to a family that is commonly eaten by the birds, being suffi- ciently attractive to them. The resemblance between the viceroy and the monarch protects the former from the at- tacks of the birds. Of course it is not sup- posed by anyone that the viceroy butterflies have purposely mim- icked the monarch. It is only supposed that the resemblance, how- ever it may have come about, is of advantage to the insects. We do not understand how these resemblances, or others like them (see 3 4 FIG. 175. Supposed cases of mimicry /, Bombus pennsylvanicus, a bumblebee, mimicked by 2, Laphria thoracica ; j, Vespa maculata, a wasp, mimicked by 4, Spilomyia fuscia. In these cases the models and the mimics belong to entirely different orders of insects, — the former are hymenoptera, or bee order ; the latter are diptera, or fly order FigS. 174, 175), have come about. Some of the theories offered to explain them are discussed in Chapter LXXXIV. We are in doubt not only as to how such protective mimicry may have arisen ; we are also in doubt as to whether mimicry is in all cases protective. Professor Punnett, an English biologist, made a special study of this subject in Ceylon, where examples of mimicry are unusually abundant. He found, in regard to certain cases, that the model and its supposed mimic never occupied precisely the same area ; at most, the two areas overlap more or less. In the second place, the com- mon birds, against which the mimicry is supposed to be protective, do not molest either the model or the mimic ; but the lizards eat the mimic as well as the other members of the family, which are supposed to be defenseless. The only other serious enemy of these butterflies was a certain large fly that pierces the thorax of the insect and sucks the juices. But this fly, like the lizard, attacks the mimic and his defenseless cousins without discrimination. In other PROTECTIVE PIGMENTS AND APPEARANCES 359 words, the resemblance to the model does not protect. Moreover, in a part of the island where monkeys are supposed to be the chief enemies of the butterflies, the most abundant forms are those that are supposed to be defenseless forms, whereas the mimics are scarce. FIG. 176. Desert plants Cholla cactus on the western deserts. The thickened leaves and short stems, or the entire absence of leaves, may be considered as a more or less direct adaptation to the high temperature and the dry soil, which together make up the danger of excessive loss of water. (From photograph by the United States Reclamation Service) 405. Reduction of surface. Some organisms may derive a kind of protection from a reduction of surface. This is espe- cially common among plants that are exposed to the danger of drought. In desert plants we observe a comparatively small surface in proportion to their bulk (Fig. 176). CHAPTER LXVIII PROTECTIVE MOVEMENTS 406. Contractions. Contraction under stimulation is a com- mon thing among living beings. When the ameba is disturbed FIG. 177. Contraction of sea anemone When disturbed, the surface of this animal becomes greatly reduced by repeated contractions, until it resembles a wart on a rock in any one of several ways, it immediately contracts. The effect of this contraction may be protective in several ways : FIG. 178. The pill bug When suddenly disturbed, this animal curls up, thus reducing its exposed surface and concealing its most delicate and sensitive parts 1 . It reduces the total amount of surface exposed to danger. 2. It hardens (condenses) the exposed surface. 3. It withdraws the animal from the point of attack. 360 PROTECTIVE MOVEMENTS 361 Here are three results of this simple reaction that may presumably be of use to the animal under various conditions. The sea anemone shows a remarkable amount of contrac- tion when disturbed. In fact, all the animals of this branch (ccelenterates) are extremely contractile (see Fig. 177). FIG. 179. Sensitive plant (Mimosa pudica) a, leaves in normal position ; 6, leaves reduced after disturbance. It is not necessary for us to assume that this movement is of any real value to the plant. It is true that in the new position the leaf exposes less surface and sheds the water better. But hundreds of plants with similar leaves have no difficulty in shedding rain without being so sensitive. Many plants (clover, oxalis, and others) drop their leaves in the dark in a few minutes. It is possible that in the clover and others the drooping of the leaf is the direct result of reduced transpiration. But that does not give the plant any advantage It is very likely that the sensitive plant is simply more sensitive than any of its relatives (the bean family), many of which are sensitive in the same way but not in the same degree In clams and oysters, contraction of special muscles results in closing the shell. In snails, contractions withdraw the body into the shell. The turtle withdraws head and legs into his "shields," and the box turtle closes the shell up even more completely. We do not usually think of plants as moving, either to get food or to escape danger. Some plants, however, can do a great deal of moving in connection with the capture of insects 362 ELEMENTARY BIOLOGY and other animals. Several other plants are capable of moving their leaves when disturbed, as the sensitive plant (Fig. 179). 407. Color changes. To be able to elude the vision of the enemy must be of real advantage to any animal. It is therefore reasonable to assume that the color changes of the chameleon FIG. 1 80. The true chameleon African monitor (Varanus niloticus). (From photograph by American Museum of Natural History) must be of protective value to him, and that they are brought about by the color of the surroundings. The true chameleon, a native of Africa (Fig. 180), and the American chameleon (Fig. 181), or green lizard, quickly change their color through a wide range of shades, from bright green to rather dull brown. These changes are brought about by the contraction or expan- sion of various parts of the skin, containing different pigments. Careful experiments show that the color changes are produced by a response to temperature changes or by the intensity of the illumination rather than by the color of the background. PROTECTIVE MOVEMENTS 363 In many situations, however, these color changes may be protective, even though they are not necessarily protective adaptations in all cases. 408. Concealment. Another way in which an animal becomes invis- ible to its enemies is illustrated by the cuttlefish, which ejects a dark fluid into the water when it is pursued. This " ink-bag " trick clouds the water and thus enables the animal to escape from its pursuer. FIG. 181. The American chameleon The green lizard (Anolis carolinensis) . (From photograph by American Museum of Natural History) The instinct for finding shelter is very marked in many animals of nearly all classes. In many worms we may observe a strong tendency to crawl into cracks or angles. There are certain worms that are so persistent in this trait that if two of them are placed in opposite ends of a glass tube, they will approach each other and keep on driving forwards until they have worn their heads off. The contact of the body against the hard walls stimulates them to move forward, and they don't know enough to stop when they have gone far enough. A more remarkable home-finding instinct is that shown by the hermit crab, which makes itself at home in the discarded shells of snails. As the animal grows larger it abandons one shell and finds another (Fig. 182). With this instinct we may compare that of the higher animals that dwell in caves or other ready-made openings that they find. 364 ELEMENTARY BIOLOGY 409. Flight. Beginning with the ameba, that withdraws its " false feet" from a point of disturbance, and reaching to man himself, all animals that are not confined or attached protect themselves by some form of flight or escape. With this fact is associated a wonderful series of organs of locomotion, from the false feet and cilia of the protozoa, the water feet of the starfish, the flapping shell movements of the scallop, the wriggling of FIG. 182. The hermit crab These crabs make themselves at home in the cast-off shells of whelks and snails. (From photograph by New York Zoological Society) worms, and the legs and wings of insects, up to the various kinds of legs and 'wings and fins of the backboned animals. It is impossible to say that organs of locomotion are pri- marily related to protection or that they are primarily related to food-getting. At the very lowest levels of life, among the protozoa, we find the same structures and activities serving organisms in both relations. Thus, the paramecium, moving about by means of cilia, also gets food particles into the inte- rior of the protoplasm by means of cilia. And farther up we find feeding organs and locomotive organs differentiated from the same structures (see Fig. 183). PROTECTIVE MOVEMENTS 365 Even among the mammals we find the primates (monkeys, apes, man) using their front limbs in food-getting quite as much as in locomotion, or even more. 410. Migration. A very interesting prob- lem in connection with the protective move- ments of animals is that of migration. The mi- grations of the common birds are more or less familiar to all of us. Those of us who live in the northern latitudes are likely to look upon bird migration as " go- ing south in the winter to get away from the cold," or as "going south to get food." If we live in the south we may well ask why the birds ever go north ; and we can think of no advantage to their migration except that of finding a breeding place for the young in a region free from the usual enemies or other obstacles (see Fig. 184). FIG. 183. The appendages of the lobster In the Crustacea all the appendages are built on the same plan, but each segment of the body (repre- sented by Roman numerals) has a distinctive organ, /and //are sensory; ///-F combine sensory func- tions with food-getting; VI-VIII are chiefly food- getters, but are also related to breathing ; IX is the nipper ; X and XI are both grasping and locomotor organs ; XII and XIII are walking legs. The ab- dominal appendages XIV-XVIII are called swim- merets and probably assist in slow swimming. XIV and X Fare also related to reproduction in the male, and in the female all the swimmerets carry the hatch- ing eggs and larvae. XIX and XX spread out into a flat tail-paddle, used in swimming backward suddenly It is possible that some species migrate originally with relation to food and weather, and that other species migrate primarily in relation to possible enemies. Whatever the advantage to the species, it is PROTECTIVE MOVEMENTS 367 curious that year after year the birds will follow the same routes, even coming out of their way many miles to go with the flock. It is probable that the older birds lead the migrations, and that the paths are kept by force of imitation. The young follow, and the older ones FIG. 185. Migrating fish Humpbacked salmon jumping low falls, Litnick Stream, Alaska, on way to breeding grounds. (From photograph by United States Bureau of Fisheries) continue to do as they have always done. As a result, customs are established that persist even when they cease to be of greatest advantage or economy. Migrations of fishes have also been recorded, and these seem to be related chiefly to finding safe breeding places. Some, like the eel, will go out into the ocean to breed ; others, like the salmon, will spend most of their time in the ocean and will come up into the rivers to breed (see Fig. 185). CHAPTER LXIX PROTECTIVE ACTIVITIES 411. Home-making. When the earthworm burrows into the ground, it thus escapes the birds and other enemies ; but the burrowing is essentially a process of food getting, for the animal feeds by swallowing dirt as it digs along, and absorbing from it organic material left by decaying plant and animal matter. In the same way, the larvae of various insects and many adult beetles es- cape their ene- mies by boring into trees. In the simplest of animals, where all the activities of life center in FIG. 186.' The piddock This mollusk grinds its way into the rock, growing larger as it digs deeper, so that in the end it is completely imprisoned the protoplasm of a single cell, the movements related to protection or escape from injury are hardly to be distinguished from the activities related to the getting of food. The simple life will cover all of its necessities by a few acts. But with higher animals it is often difficult to draw a 368 PROTECTIVE ACTIVITIES 369 sharp line be- tween the proc- esses that are related primarily to the getting of food and those that are related to protection. In the intestines of a child a tape- worm absorbs its food from the host and it is at the same time protected from all possible ene- mies. It would be absurd to say that the parasite makes its home and its living in the intestines of a vertebrate "be- cause " that is a safe place, al- though it may be true this habitat is indeed safe enough. In the same way, we must be on our guard against explaining the activities and peculiarities of ani- mals as though they resulted from some purpose that the animals had in mind. We may be sure only that, to continue to live, an organism must be sufficiently adapted to its surroundings. FIG. 187. Nest of the paper wasp, or black hornet The queen wasp survives the winter alone. In the spring she builds a small nest of wood pulp, or wasp paper, and lays a few eggs in it. While these are hatching she fetches various grubs and caterpillars, which serve as food for the young. On becoming mature the workers proceed to enlarge the nest and to bring supplies of food. The queen continues to lay eggs throughout the summer, and most of these develop into workers, though some of the eggs hatch into perfect males and some into perfect females. After fertilization the males and the workers die, leaving the queens to live through the winter and to start new colonies in the spring 370 ELEMENTARY BIOLOGY FIG. 1 88. Finding a home for the young Nest of a bluebird in natural hollow of a tree (From photograph by L. W. Brownell) With many animals the boring or burrowing is re- lated altogether to protec- tion, as with animals that bore into rocks (Fig. 186). From our own point of view the safety obtained by an animal boring into a rock would seem to be purchased at the price of liberty. But we may be sure that from the pid- dock's point of view this trick of boring into the rock causes no ill feeling. There is safety, and there is the possibility of getting food as well, for the long siphon projects into the FIG. 189. Prairie dogs These animals dig burrows underground and live in large colonies. (From photograph by Elwin R. Sanborn) PROTECTIVE ACTIVITIES 371 ocean and a constant current of water brings oxygen and food, and carries off wastes and reproductive cells (see Fig. 44). When we come to the highest animals (the insects) of the branch arthropods and the highest animals of the backboned branch (birds and mammals), we find very complex activities related to the making of homes. The solitary wasp goes no farther than burying a few insects that later serve as food for the young. The social wasps and hor- nets, like the related bees and ants, build very elaborate homes out of "paper" (which they make from wood pulp and other materials) and out of wax and earth (see Fig. 187). Nest-building among the birds involves complex instincts, and pos- sibly in some cases a degree of real intelligence. From the crude whisps of the grouse, or the simple mud heap of the flamingo, to the deli- cate and skillful work of the tailor bird, we find a long series of nests of many degrees of complexity in structure. But with the exception FIG. 190. Nettling cell of jellyfish This specialized skin cell, A, con- tains a fine coiled thread suspended in a capsule of acid fluid. When the surface is disturbed at the trigger, /, the coil suddenly straightens out, shooting the sharp needle into the surrounding space, and at the same time the acid fluid from the cell passes through the hair. The sting- ing sensation is probably produced by this fluid. B, the discharged cell of homes made in hollows, like that of the woodpecker, whatever shelter nests may furnish serves almost exclusively for the protection of the young (Fig. 188). Indeed, we may say that the making of shelter among the higher animals is closely related to the protection of the young, 372 ELEMENTARY BIOLOGY or of a group, rather than to the protection of the individual. This is seen in the constructions of such animals as the B FIG. 191. Praying mantis This animal lies in wait for its prey with the front legs raised in a manner suggesting the attitude of prayer. It catches small insects with its strong front legs. Large species living in the tropics have been known to kill small birds beaver or the prairie dog (Fig. 189), in the hutch of the rabbit, in the diggings of the mole, and in the nest of the mouse. FIG. 192. The ant lion a, adult ; b, larva ; < larva covered with dirt ; c, incased pupa ; d, pit in sand. The larva buries itself in loose sand at the bottom of a small pit. Ants and other small crawling insects tumble into the pit and are seized by the strong jaws of the ferocious " lion" 412. Fighting. -Nothing would seem to be more helpless and less offensive than the soft-bodied jellyfish ; the very name PROTECTIVE ACTIVITIES 373 suggests something even milder than a clam. But if you have ever picked up a live jellyfish, you may have thought that a million needles had been shot into your hand. The skin of the jellyfish con- tains a large number of special cells in which there are fine hollow threads that shoot out when the animal is dis- turbed (see Fig. 190). These " nettling cells " are found in many spe- cies of ccelenterates, such as the hydra, sea anem- one, coral polyps, and sea walnuts. But some animals, like the ray, do give their enemies a real electric shock when they are disturbed, and this is no doubt of value in protecting them. The animal that has been shocked quickly lets go and learns to let other shockers alone. FIG. 193. Fighting ants Three forms of the Central American ant Chelio- myrmex nortoni: a, soldier; l>, medium worker; £, small worker FIG. 194. The sting of the bee In this order of animals the weapon is the egg-laying organ. When the bee stings some- one, the point is likely to remain in the flesh ; and as the animal flies away, some of its internal organs are mutilated and the insect soon dies. The value of this weapon is not so much for the protection of the individual as for that of the colony or species. The individual is sacrificed to protect the group or to educate the enemies of the species Another way in which the organism can make itself disa- greeable to an enemy, without really producing serious injury, 374 ELEMENTARY BIOLOGY is illustrated by the skunk. This animal, as everyone knows, is capable of ejecting a foul-smelling liquid from a gland at the rear of the body when it is greatly agitated. Real fighting appears among animals that have mouths and appendages that are capable of grasping. These organs are at the same time food- getting organs. Lob- sters and crabs are very pugnacious ani- mals, or at least that is the impression they make upon the ob- server. Most of the mollusca (clams, oys- ters, scallops, etc.) de- pend on their armors for defense against possible aggressors ; FIG. 195. The fall of a leaf A, leaf dropping off ; s, self-healing scar remaining on some of them how- twig ; B, microscopic view of section through base of leafstalk ; a, angle between base of stalk and twig. In CVCr, as the OCtOpUS, plants that regularly drop their leaves in the autumn there is formed a special layer of cells in the stalk of each leaf, and sometimes of each leaflet of a compound (^l£v 95)- very ffOod fighters Among the insects leaf. These cells, s /; are thin-walled and turgid. Their contents break down into a mucilaginous mass, which dries up. A slight movement is now sufficient to break many are predatory, the fibrovascular bundle at this point, and as the leaf is • th • aoDen(1 removed the exposed surface becomes a self-healing scar '" & ages (Fig. 191) or their mouths (Fig. 192) in catching prey. But very few use these organs in fighting their enemies. The colonial insects, especially the ants, furnish the best examples of this mode of protection (Fig. 193). The bees, wasps, and hornets fight when they are disturbed or when the colony is disturbed, but in fighting they use the sting (see Fig. 194), which has nothing to do with food-getting or with locomotion. The horns of mammals are associated with the instinct to defend or fight, and are quite independent of the organs or PROTECTIVE ACTIVITIES 375 instincts that have to do with the getting of food. Thus, while the ferocity of the tiger or the dog finds expression through organs that are related to food-getting, the strictly vegetarian rhinoceros or mountain sheep will fight fiercely and coura- geously with horns or hoofs. The branching horns of the deer or elk seem never to be used aggressively except against mem- bers of their own spe- cies, as when two males are in combat. 413. Shedding of leaves. The dropping of leaves in the autumn, while it does not in- volve movements like those of muscles, may properly be considered a protective act. The shedding of leaves seems to be related to the water factor as well as to the temperature factor, which we usually associate with the change of seasons. As the au- tumn advances and the water in the soil becomes FIG. 196. Insect galls It is probable that by the formation of such galls many plants are really protected against serious in- jury, although many of the galls may simply represent the behavior of protoplasm when injured in a certain way, rather than a useful way of behaving. It is in- teresting to note that the galls are always specific. Thus, both of these galls are on the same species — the white oak (Quercus alba) — but are produced by different species of insects : /, by Biorhiza forticornis ; 2, by Halcaspis globulus scarcer, transpiration is interfered with. Evaporation from the leaves, however, continues so long as there is water in the cells. If the loss of water cannot be compensated by the absorption of the roots, the live cells of the plant must suffer injury. The leaf cells are the first to be affected. The loss of the leaves prevents the complete drying up of the plant, and it also prevents the freezing of live cells (see Fig. 195). The relation of water to the fall of the leaf has been determined experimentally. 376 ELEMENTARY BIOLOGY 414. Insect galls. The response of plants to special mechani- cal and chemical disturbances is illustrated by the formation of the so-called insect galls, some of which are shown in Fig. 196. Many insects sting plants and suck juices from them for food. The common mosquito is an example, and the plant lice are sometimes parasitic to a degree that is very harmful. But many insects sting plants with their egg-laying organs and deposit the eggs in the tissues of the plant. Associated with the process of egg-laying there is often a secretion of some juice from the insect's body. The mechanical or chemical injury thus produced is probably very slight ; but the young that hatch from the eggs deposited in the tissues of the plant begin to feed, and the injury that they do is likely to be of a more serious nature. We find that -many plants begin to grow rapidly about a point at which insects have laid eggs, forming casings of various shapes and structures about the mass of eggs, and eventually about the young insects. Within these galls the insect larvae find a limited amount of food, and they are cut off from the rest of the plant. CHAPTER LXX THE FOREST IN RELATION TO MAN 415. Forest products. Man depends in many ways upon masses of trees growing together as forests. It is from the trees that we get one of the most useful of materials — wood. This is utilized in hundreds of ways, from the making of tooth- picks and tool handles to the timbering of mines or the making of stock for newspapers. All human habitations have some wood in their composition, and probably most people live in houses built almost entirely of wood. Every home has furniture made at least in part of wood ; and in every industry, and in every office, furniture and appliances made of wood are used. In the railroad business millions of dollars are spent every year for the ties upon which the rails are laid. Similar amounts are spent upon telegraph poles and fence posts, although these are coming to be replaced by reenforced concrete and other materials. In shipping goods of all kinds from place to place millions of feet of lumber are used up, in the form of packing cases and boxes and trunks. In addition to the wood obtained from the trees these plants furnish us with charcoal, turpentine, pitch, wood alcohol, and various gums and resins. From tropical trees we obtain rubber and quinin. To some extent the dye logwood is holding its own against the ani- lin blacks, and since the outbreak of the Great War dyewoods have taken on a renewed importance, because of the changes in the chemical industries. Bark is taken from certain trees, especially the hemlock, to be used, for the tannin it contains, in the tanning of leather. The use of wood as fuel is coming to be restricted more and more, as we find it more profitable to burn coal, gas, oil, etc., and to use the wood for other purposes. But every forest and every wood lot produces annually large quantities of wood that cannot be used in the making of paper or of other useful things, and this may well be burned. 377 378 ELEMENTARY BIOLOGY 416. The forest and the air. Another use of the forest is found in the fact that through photosynthesis fresh supplies of oxygen are thrown into the air, replacing the carbon dioxid. In addition to this, the transpiration may be considered a help in that it keeps down the temperature of the plants and so of the sur- rounding air. The shade value of trees is highly appreciated in the sum- mer time even by city dwellers, and the effect of trees in breaking the wind is appreciated in the winter time, especially by those living in the country. 417. The forest and water. The most impor- tant relation of the forest to man, aside from the direct utility of the forest products, is in its effects upon water. When we compare the action of rain water and snow on a hill- side covered with trees with the action upon a similar hillside devoid of vegetation, we can realize the practical importance of the forest in relation to our water supply. On the bare hillside the water soaks down into the soil almost as fast as it falls, or it runs off, carrying particles of earth along in its course. On the covered hillside the force of the falling raindrops is broken by the leaves of the trees, from which the water slides down to the ground along the twigs and larger stems. The rain Covered Bare April 2-9 Covered Bare April 14-17 Covered Bare April 22-24 FIG. 197. The relation of the forest to water flow In experiments made by government agents a comparison of a covered area with one devoid of trees showed (i) that in a given period the covered area accumulated more snow than the bare area (this is shown by the relative heights of the two columns in each pair), and (2) that in a given period the bare area lost more water than the covered area (this is shown by the rela- tive heights of the shaded portions in each pair) THE FOREST IN RELATION TO MAN 379 that strikes the mulch 1 soaks through slowly ; then, in the en- tangled soil beneath, it steadily works down to form the under- ground streams and the springs. Snow in the forest melts slowly and is gradually absorbed in the spongy bed beneath ; from this the water slowly escapes into the springs and under- ground currents. Snow upon the bare ground runs off as fast as it melts. Actual proof of the difference was furnished a few years ago by an extensive experiment conducted by the United States Geological Survey in the White Mountains. Two similar areas were selected, each covering about five square miles. One of the regions had been entirely cut down and burned over; the other retained the virgin forest (Fig. 197). The practical bearing of these facts is not hard to under- stand. Every year, as the snows on the hills begin to melt, the water rushes down the hillsides in the deforested regions, causing the streams to overflow their banks and the torrents to tear down and destroy everything in their path. The annual damage done by floods in this country is estimated to be equal to one hundred million dollars. This does not include the destruction of human life that is often involved in the floods. Streams depending upon deforested areas for their water will be too full in the spring and will run too low in the summer. Water used for agricultural purposes must be had in abun- dance throughout the summer, and the destruction of forests in one region has often resulted in the ruin of agriculture and the migration of peoples in a distant valley. Navigation on the larger streams is influenced by the forest in two ways : the steady flow of water is maintained by a proper condition of the forest, and the filling up of the stream by soil is at the same time prevented. 1 The mulch forms a soft, absorbent carpet, consisting largely of decaying leaves and other organic matter. 380 ELEMENTARY BIOLOGY 418. Water power. As our industrial civilization depends more and more upon the use of machinery, we are pressed to find sources of energy for driving the machines. The con- sumption of coal has increased so rapidly that the exhaustion FIG. 198. An eroded slope in western North Carolina On slopes from which the vegetation has been removed the rains and melting snows produce destructive effects of great practical importance. (From photograph by United States Bureau of Forestry) of the earth's supply is threatened. Water power seems to be the only source of energy that is constantly renewing itself at a sufficiently rapid rate. But to maintain the service of waterfalls we must be sure of the steadiness of the water supply, and this in turn depends upon the forest.1 1 When we burn coal as fuel we are of course again dependent upon the forest (though not the forest of our own times), since all coal consists of the modified remains of ancient vegetations. THE FOREST IN RELATION TO MAN 419. Soil and forests. The relation of the forest to the soil is also, of great practical importance. Every year the streams and rivers carry down to the sea a quantity of earth estimated to be worth over a billion dollars. This is not only a direct loss of agri- cultural resource ; it also interferes with the navigation of streams and with the condi- tions of harbors. Mil- lions of dollars are spent every year dredg- ing harbors in this country, to remove the soil deposited by the streams coming from deforested regions. And, finally, the mil- lions of dollars spent in reclaiming desert land would all be wasted but for sup- plies of water drawn FIG. 199. A good stand of trees, Lake Placid, New York Forest areas in good condition not only furnish in- valuable materials, but protect the soil and insure a steady supply of water. (From photograph by United States Bureau of Forestry) from regions covered with forest. 420. Forest control. Because of our de- pendence upon the products of the forest, as well as upon the water and the soil that are so much influenced by the living trees, the proper con- trol of the forest becomes a matter of national concern. We cannot depend upon the private owners of forests to handle these in such a way as to secure to the general population the full benefits and protection that are necessary. Ordinarily the owner of a forest cares only for what he can get out of it, 382 ELEMENTARY BIOLOGY and he cannot be expected to take into account effects a hundred miles away or fifty years away. The Forest Service of the United States Department of Agriculture has made many careful, scientific studies of forest conditions and has thus been able to give sound advice on the care and management of forests and wood lots from every point of view. From these investigations we learn, first, the importance of avoiding certain injuries to the forests, and, second, the methods of increasing their value. For many years, toward the end of the nineteenth century, the people of this country were using up trees about three times as fast as they could grow. This meant that before very long we should have destroyed all the usable trees and been practically without a suitable wood supply. A scientific study of the growth of trees in the forest showed that it is possible to get all the wood we really need without destroying our forest, if only certain principles are followed (Fig. 200). It is to be noted that the ordinary virgin forest is practically at a standstill so far as growth is concerned. While new growth is con- stantly taking place, this is only enough to offset the death and destruction among old trees. 421. Increasing forest area. To meet the growing need for more wood, it is possible to extend the forest area of the country. Areas that have been cut and burned over may be reforested, and this process is under way in many parts of the country. There is a great deal of worn-out agricultural land and sand-dune land that would be well suited to forests ; in many cases all that is needed is to protect the 'young growth against fires. Another method of extending the growth area is by fuller stocking of existing forest lands. Thus, some trees are found growing so close together that they never become thick enough to be of great value for timber ; but in other forests the trees are so far apart that valuable space is allowed to go to waste. By selecting trees suitable for a given region, and starting the young plants rather close together, and then thinning out carefully, the amount of timber grown on a given area can be greatly increased. 422. Increasing wood yield. Another method for increasing the wood supply is by the selection of varieties that will give a maximum of growth in each forest area. It is likely that not more than seventy THE FOREST IN RELATION TO MAN 383 of the 500 native species in this country are worth growing from the economic point of view. The red cedar grows very slowly ; the white pine or the red oak could be grown in the same soil to great advan- tage. We could replace the red spruce in New England by the Norway spruce, just as many areas of France denuded by the Great 1 FIG. 200. Cutting trees to preserve forests The preservation of the forest does not mean simply to avoid cutting timber. By cut- ting trees in zones at intervals of a number of years, and by thinning out the trees where they are too crowded, it is possible to make a given area yield continuous crops of wood. The zone a was cut first, then zone l>, and so on. By the time the last strip has been cut, the trees on the first strip are well along, and thus a succession of cuttings may be continued indefinitely War, and other European regions, are being restocked with Douglas fir imported from this country. We shall no doubt find foreign trees better suited to our purposes in many localities than the native trees. In the course of a number of years the rapid varieties will yield much more timber than the others. But rapid growth is not of itself a de- ciding factor, for it is necessary to consider the toughness of wood and other qualities. The whitewood, or tulip tree, grows much faster than the oak, but it can never be used as a substitute for the oak. 423. Improving wood quality. Another improvement is being brought about by the selection of varieties for quality. Without 3^4 ELEMENTARY BIOLOGY increasing the actual amount of growth, it is plain that the value of the growth can be increased if the trees do not have curved or twisted trunks or branches. By selecting straight-growing varieties and by concentrating the growth in the best trees (by thinning out the least desirable ones) it is possible to increase the yield of a forest area. FIG. 201. United States forest reserves The economy of national control of forests, as well as the protection of public interests thereby, has been strikingly demonstrated since our entry upon the Great War 424. Avoiding wood waste. In the national forests the lum- bermen are given a practical demonstration of the value of scientific cutting, seeding, reforesting, etc., and also of the economical handling of growth. Damage to trees often results from careless lumbering. The tree that is being cut down is sometimes damaged, and it is sometimes allowed to injure trees that are left standing. When wood was cheap, a great deal from each tree was left to rot on the ground. Now everything that can possibly be used is saved, and the remaining brush- wood is carefully burned, instead of being left under the trees as -a constant fire risk. THE FOREST IN RELATION TO MAN 385 425. Advantages of public control. The extent of the national forests is shown in the map on page 384. In these forests are conserved and protected the water supplies for more than a thousand cities and towns, for over twelve hundred irrigation projects, and for over three hundred water-power plants. In these forests nearly ten million head of sheep, horses, and cattle graze every year, and in them nearly half a million people find recreation. The forest service sells timber to private users and gives away firewood to settlers in agricultural lands included within the forest areas. 426. Forest dangers. The forest is exposed to four serious dangers : 1. The person who cuts recklessly and destroys for imme- diate profit what ought to last practically forever. This enemy can be regulated either by enforcing very strict rules as to the uses of private forests or by making it impossible for individuals or corporations to profit at all through the exploitation of forests. 2. Fire. Since this is probably always of artificial origin, it can be controlled through suitable regulation or supervision. In the national forests there are well-organized fire patrols. They have succeeded in preventing many fires and in keep- ing the total fire damage in the national forest down to a small fraction of what it is in privately owned forests. The rules for fire prevention in forests are posted on trees, and every person who has occasion to go into the woods should heed these regulations. 3. Various species of insects. 4. Various species of fungi. These classes of organisms (insects and fungi) destroy every year trees and timber worth millions of dollars, and there is no one way to fight them all. 427. Other forest relations. The forest is related to human affairs as the home of many animals and of many plants other than the trees. It is in the forest that valuable game and fur animals find their food and shelter, and the destruction of the forest means the extermination of many of these animals. CHAPTER LXXI BACTERIA AND HEALTH 428. Bacteria and specific diseases. Before germs can cause disease it is necessary that they enter the body of the host. Ordinarily they cannot get through the skin. The infection, or entrance into the body, therefore, takes place through either (i) a cut in the skin or (2) one of the regular openings to the interior of the body, as the mouth or the nose.1 Fortunately for us, most bacteria do not cause disease. We may therefore carry about with us, in our mouths and air pas- sages and food tubes, millions of bacteria without being made ill. After the middle of the last century the improvements in the microscope and the development of experimental methods made possible the discovery that certain diseases are caused by mi- crobes, and that they can be caused in no other way. Since the time of Pasteur, the French chemist who first demonstrated this idea, many physicians and biologists have succeeded in finding the particular species of bacteria connected with some of the most important human diseases, such as tuberculosis, diphtheria, pneumonia, typhoid fever, tetanus (lockjaw), cerebrospinal men- ingitis, and others. The methods developed in the course of these studies have been successfully used in the treatment and prevention of several other diseases, although the specific or- ganisms that cause these are not known in all cases. For, in addition to discovering that a given species of bacteria is the specific cause of a disease (for example, the typhoid bacillus in the case of typhoid fever), we have found (i) that the bacteria 1 In some cases the bacteria may act upon tissues without penetrating into the interior of the host, as the diphtheria germ, which lives on the mucous surface of the throat. 386 BACTERIA AND HEALTH 387 leave the host in special ways ; (2) that they are commonly transferred to other hosts in special ways ; and (3) that they then enter the bodies of the new hosts in special ways. TRANSMISSION OF COMMUNICABLE DISEASES DISEASE How GERMS COME OUT How GERMS ARE CARRIED How GERMS ENTER Chicken pox . . Mouth and nose Air Nose and mouth spray Diphtheria . . . Mouth and nose Air, objects ex- Nose and mouth spray ; saliva posed to spray or saliva German measles . Mouth and nose Air Nose and mouth spray Measles .... Mouth and nose Air Nose and mouth spray Mumps .... Mouth and nose Air, objects ex- Nose and mouth spray; saliva posed to spray or saliva Scarlet fever . . Mouth and nose Air, objects ex- Nose and mouth spray ; saliva posed to spray or saliva Septic sore throat Mouth and nose Air, objects ex- Nose and mouth spray; saliva posed to spray or saliva Smallpox. . . . Mouth and nose Air Nose and mouth spray Tetanus (lockjaw) Contact Hands or objects Breaks in skin Trachoma . . . Contact Hands, towels, etc. Contact with eyes Tuberculosis . . Mouth and nose Hands, objects, Nose and mouth, spray etc. food Typhoid fever . . Mouth, excretions, Hands, various ob- Food intestinal waste, jects, flies and occasionally through skin Whooping cough . Mouth and nose Air Nose and mouth spray 429. Infection. The table above tells how the germs of a number of common diseases are thrown off, how they are carried about, and how they enter the bodies of the new hosts. 388 ELEMENTARY BIOLOGY In all essentials the methods of infection and transmission of disease are the same for the domestic animals as they are for man. When we consider that disease is preventable just in proportion as we understand the causes and the modes of infection, we may well believe with certain special- ists that the study of bacteriology is among the most im- portant contributions of the nineteenth century to the welfare of the human race. In Fig. 202 are given the annual losses due to various diseases, and an indication of the extent to which these may be prevented. 430. Protection against infection. The chief means of pre- venting infection consists of preventing the contamination of our food by bacteria. This means that pains must be taken as to the exposure of fresh food to dust, to the mouth-spray of people or other animals, and to contact with unclean hands or with containers of all kinds. Many cities now require that all food exposed for sale, such as meat, pastry, confectionery, and the like, be covered against dust as well as against the visits of insects ; but fruit and vegetables are still commonly exposed, at least to dust. In the case of fresh fruits or vege- tables the peel is usually a sufficient protection against bacteria. But the peel of many vegetables contains very desirable food material, which should not be thrown away. Fruits and vege- tables that are cooked are generally safe, since the cooking itself kills the bacteria (see p. 112). But lettuce, celery, and other vegetables that are eaten without cooking have frequently been the means of infecting people with disease, since bacteria in the soil may cling to the plants, and some of the disease-causing bacteria may get into the soil of gardens. Such plants should be thoroughly washed before being used as food. 431. Care of food. The fact that food rots so readily when left to itself shows that it contains the materials necessary to maintain the life of bacteria. We should therefore keep it under conditions that are not favorable to the growth of these organisms. We have the practical choice between keeping our SJ8 99 00 01 02 C|3 (J4 05 C|6 07 Q8 09 1,0 1,1 12 Iff 14 15 1,6 17 FIG. 202. Mortality rates for various diseases In the early part of the period the fluctuations are irregular; in the latter part the infectious diseases show a steady decline. There is a steady decrease in the general death rate, and a steady increase in the death rate from cancer. The figures on the left indicate the number of deaths per 10,000 of population in New York City; the figures at the bottom are for the years. The general death rate and the cancer line are drawn on a different scale from the other lines 390 ELEMENTARY BIOLOGY food very hot or very cold. We are not usually ready to cook our food immediately ; meats and vegetables have to be kept for longer or shorter periods before cooking as well as after cooking. We therefore turn to the low temperature as an aid in preserving food against the destructive action of bacteria. Refrigeration has been the means of preventing great loss through the decomposition of food, since at low temperatures bacteria cannot multiply. We must be careful, however, not to assume that well-preserved food from the refrigerator is neces- sarily free from injurious microbes, since any organisms that may have been present before the food was placed in the refrigerator are still there and are still capable of growing and multiplying when the suitable temperature is reached. It is also necessary to keep refrigerators perfectly clean and free from neglected food particles that may retain bacteria. This principle applies, of course, to all cupboards, pantries, lunch boxes, or other places in which food is kept temporarily or permanently. Milk, soups, jellies, fruit juices, preserves, and similar food preparations containing a great deal of water are exceptionally favorable to the growth and multiplication of bacteria. They are therefore especially subject to the decaying action of bac- teria, and require special care in their handling and storing. In making preserves of fruits or vegetables the chief precau- tions are concerned with the destruction of the bacteria already present in the materials used, and with the prevention of the entrance of other bacteria. The first end is attained by cooking the materials until the heat kills the germs. The second is attained by placing the cooked material in perfectly clean vessels that can be covered so as to exclude absolutely all bacteria. 432. Milk regulation. The regulations prepared by the health authorities of cities and states for those who have to handle milk take into consideration the importance of milk for human beings, especially for children, and the ease with BACTERIA AND HEALTH 391 which milk becomes contaminated. The conditions under which the cows live make it almost impossible to prevent the hairs and skin of the animal from becoming the bearers of bacteria of many kinds. While the milk in the udder of the cow may be quite free of any contamination, by the time the milk has been poured from the pail to the can it is sure to have many bacteria floating in it. The high temperature makes the multiplication of the organisms proceed very rapidly. By the time the milk is ready for delivery in the city, it con- tains a large number of bacteria in every drop. On page 127 are given the rules for the care of milk in- tended for city markets. There is a biological reason for every rule given, and this should be clear to every student of the sub- ject. It has been found practically impossible to obtain milk in large quantities without excessive numbers of bacteria. For this reason the practice of pasteurization has come into more and more general use. This consists of raising the temperature of the milk to about 140°-! 5 5° F., and keeping it there for from ten to twenty minutes. Pasteurization does not, of course, remove the bacteria ; it only kills them. 433. Water supply. Next to milk, the water supply is perhaps the source of greatest danger to the community. In towns and cities that still depend upon separate wells or springs for water the amount of sickness and the proportion of deaths is likely to be much higher than in such places as have a central water supply. To be sure, if the central water supply becomes contaminated, more people are likely to be injured in a short time. But it is easier to control the sani- tary condition of one large reservoir than that of hundreds of wells. The bacilli of typhoid fever will remain alive in water for two or three weeks, and are the most frequent disease germs transmitted by water. But other diseases may also be transmitted in this way. The diagram in Fig. 203 shows the reduction in disease and death that was brought about by improving the water supply in the state of New York. 392 ELEMENTARY BIOLOGY When we consider that the contamination of wells, rivers, and lakes with the germs of disease can be brought about only by discharges from diseased persons (or at least of persons carrying the germs), we see how closely connected are the problems of sewage and health. 30 1895 1900 1905 1910 FIG. 203. Typhoid and water supply 1915 For twenty years the deaths from typhoid fever fluctuated between 18 and 31 per 100,000 of the population. Since 1907, when the state authorities (New York) took charge of water regulation, the death rate from this disease has steadily declined Wherever there is a sewage system, the law should require that every house be properly connected with the sewer. There is unmistakable evidence that the general health is better among people who use modern water closets than among those who do not. It is also certain that the latter are too frequently sources of danger to others in that the contaminations work their way through the ground into the water supplies upon which others are dependent. Thus again we BACTERIA AND HEALTH 393 see the interdependence of people, living it may be at considerable distances from each other or in different states. Because there is not yet any adequate control over the habits of those who dwell in the country, in the matter of disposing of house soil, garbage, etc., it is important for those who dwell in cities that their water supply be properly guarded, if not at the source, then through suitable filtration or sterilization. All these activities, and many others, suggest how human life is constantly influenced and modified by the activities of these minute yet significant organisms. 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 FIG. 204. The reduction of infant mortality in New York City This diagram shows, month by month, for a period of thirteen years, the proportion of infants (under one year old) who died to the infants born. There is a variation from month to month, with a very striking increase in the number of deaths during the summer months. When the records showed this big jump in 1905, physicians and nurses and sanitary experts at once took steps to discover the causes and to devise preventive measures. Year by year we can see a steady improvement. So much effort has been made to protect the children for the bad month of July, that in recent years this month has showed off rather better than the others, and August and September have become the bad months. With increased knowledge, and especially with wider application of the knowledge we already have, the high points on these black spots will be cut down, and the general level of the spots will be considerably lowered. This is but another way of showing that applied biology saves hundreds of thousands of lives CHAPTER LXXII CONTROL AND USE OF BACTERIA 434. Health and the public. As fast as we realize that our health depends upon the control of outside conditions, we extend public regulation to many matters that were formerly considered purely private and individual. A bare list of the points in regard to which public and official action has been taken will indicate how widespread are the influences of bac- teria, and how far progressive communities have gone in the attempt to control public health. Not every town has adopted regulations in regard to each of the matters mentioned in the lists below, but on each point several towns and cities (or states) have adopted definite regulations calculated to protect the public health. In regard to food in general, the methods used in preparing, handling, and exposing for sale have been regulated. In addi- tion there are special regulations regarding the following : 1 . Bakeries : conditions of work, ventilation, lighting, clean- liness, etc. ; the wrapping of each loaf of bread before removal from the factory. 2. Slaughterhouses. 3. Ice cream. 4. Soft drinks : conditions of sale, cleaning of glasses. 5 . Restaurants : condition of kitchens, cleaning of dishes, etc. 6. Drinking cups in public places. 7. Milk, butter, and other dairy products. 8. Public water supply. 9. Compulsory vaccination of school children. 10. Marriage of persons suffering from certain diseases. 394 CONTROL AND USE OF BACTERIA 395 All of these matters are related to health because they have to do with special kinds of disease microbes. For the purpose of enabling the public to measure from time to time the progress (or the reverse) in matters of health, population, etc. many states and cities require the registration of all births as well as of all deaths, and the notification of the health authorities in every case of conta- gious or infectious disease. By means of records thus obtained the public is helped in protecting itself. Many diseases are subjected to quarantine and placarding. There are provisions for supplying vaccines, serums, etc. through public laboratories, and for supervising the manufacture and sale of such prepara- tions for profit. There are laboratories for making accurate examinations of blood and other specimens obtained from patients for the purpose of diagnosis. Provision is made for disinfection of discharges from the bodies of sick people ; vac- cination where needed ; inspection of schools and factories to determine sanitary conditions ; exclusion of sick persons from schools etc. In some places there are visiting nurses, ambu- lance service, and hospital service, all helping to keep down the amount and the intensity of sickness. The activities of various classes of workers are regulated in the interests of public health. Licenses are required of physi- cians, dentists, druggists, nurses, and midwives. Rules are pro- vided to guard against the transmission of bacteria in barber shops and through manicurists and masseurs. The keeping of animals within city limits — dogs and cats, as well as horses, cows, and poultry — is regulated for the purpose of preventing the multiplication and spread of bacteria. In many cities dogs have to be muzzled ; this device must eventually eliminate all rabies from towns, since this disease is transmitted by the bites of dogs. The burial or other disposal of dead animals is also regulated. Lodging houses, tenements, and workshops must provide suit- able conditions of light, ventilation, and plumbing. Plumbing 396 ELEMENTARY BIOLOGY and drainage are subject to regulation, as well as the disposal of garbage, household and industrial refuse, ashes, etc. In many towns the scattering of ashes or other dust and the pollution of the air with smoke are treated as public nuisances. The prohibition of spitting in public places has come to be a matter of course in all wide-awake communities, and the same is true of the use of public towels. The public is also coming to insist that street cars, boats, and other public conveyances be kept thoroughly clean and sanitary. In many towns the public is making provision for baths that are either entirely free for all to use or open for a nominal fee. 435. Uses of bacteria. We have studied the changes that bacteria produce in dead organic matter, making the elements of the latter again available for the living plants, and so for animals. We have also noted the importance of certain bac- teria in making the atmospheric nitrogen available for our growing crops. There is still another way in which bacteria make dead organic matter in the soil and in waters available as food. The bacteria themselves, feeding upon the dead remains, are in turn eaten by various protozoa and other minute animals. These are then eaten by larger animals, and so on until we get to forms that are large enough to serve as food for man, as shrimps, clams, fish, etc. 436. Bacteria in industry. The decay caused by various bac- teria is utilized directly in the preparation of sponges for commerce. The sponges are allowed to lie in tanks of water until the dead cells are completely destroyed by bacteria. They are then washed clean, leaving the horny skeletons with which we are familiar. A similar process, involving the activity of different kinds of bacteria, is employed in the " retting " (really " rotting ") of the soft portions of flax and hemp stalks, to facilitate the separation of the fibers. The action of bacteria enzymes is used in the making of vinegar out of cider, wine, or other liquids containing alcohol. CONTROL AND USE OF BACTERIA 397 In these liquids the oxidation of alcohol is due to the action of bacteria. In the making of sauerkraut and other kinds of pickles, as well as in the curing of silage, bacterial fermenta- tion is used. In the work of the dairy, from the souring of the milk and cream to the curing of cheese, bacteria are used at several points. Cheeses of the Cheddar type and various cream cheeses, as well as butter, depend for their flavors upon the particular species of bacteria present during the souring. It is very likely that bacteria play a part in the curing of tobacco and in the making of hay, although the problems connected with these processes have not been thoroughly worked out as yet. In the preparation of indigo dye from the extracts of certain plants of the bean family, it is likely that an oxidizing ferment from certain bacteria performs an essential part of the work. In the preparation of hides for tanning, certain of the changes are brought about by bacterial ferments. In the disposal of sewage in large cities, a process of converting the decaying mass of organic matter into harmless or less offensive forms through the action of bacteria has come into extensive use. The sewage is collected in large tanks. After the fermentation the " sludge " may be used as a fertilizer. CHAPTER LXXIII INSECTS AS SPREADERS OF DISEASE 437. Insects eat. About one half of the different kinds of animals known to man belong in the class insects. This class of animals is spread over most of the earth's surface, and many of the species live in water (although all are air breathers). In every main division of the class are to be found species that are closely related to human welfare in one way or another. Most insects are known to us chiefly as eaters ; and they eat either materials that are of use to us, or they prey upon plants or animals that are of use to us. Like other animals, man is exposed to the attack of insects in search of food. Many species of fleas and lice, of bedbug and horsefly, of midges, black flies, and mosquitoes, have made themselves obnoxious to man by sucking his blood, by causing more or less serious irritations of the skin, and, as we have discovered only in recent times, by infecting him with microbes capable of causing more serious injury. 438. Insects move about. As carriers of disease, insects are related to us in two different ways. The first is illustrated by the common house fly, which has been shown to carry various bacteria, protozoa, and the eggs of parasitic worms on its legs and proboscis, and to leave these germs where they have a good chance of entering the body of some human being. Experiments have shown that the number and kinds of bacteria clinging to the feet of flies depend altogether upon the kinds of places in which the flies live. Flies caught in dirty streets showed more than those caught in clean streets ; those caught in a pigsty showed more than those caught in the open, and flies caught while feeding in a swill barrel showed many millions of bacteria. 398 INSECTS AS SPREADERS OF DISEASE 399 The habits of the fly are such that we cannot afford to be associated* with this animal in any way whatever. The female lays her eggs in horse manure ; but where there is no horse / \ . .« X \t 1 8 15 22 29 6 JUNE 13 20 27 JULY 3 10 17 24 AUGUST 31 7 14 21 28 SEPTEMBER 5 12 19 26 OCTOBER FIG. 205. Flies and intestinal diseases In New York City a careful study was made (1907-1908) to find the relation between the prevalence of flies and the amount of typhoid fever. The height of the dotted line cor- responds to the number of flies caught in traps, week by week, from the beginning of June to the end of October. The solid line corresponds to the number of people who died from intestinal diseases during the same period in the same districts of the city. Typhoid is most frequent where and when flies are most abundant, and there is a direct connection between the insects and the distribution of the disease manure, she will use cow, sheep, pig, or chicken manure, or decaying fruit, fish, meat, or vegetables, — ordinary garbage, for example, — or any mass of decaying organic matter. The adult fly will visit, for feeding, not only such materials as have been mentioned, but all kinds of perfectly good food that may be 400 ELEMENTARY BIOLOGY exposed in groceries, meat shops, kitchens, restaurants, dining rooms, or picnic grounds. Flies will visit open wounds or sores on the bodies of animals, and they will visit the excrements of man and other animals. We may thus see what excellent opportunities this animal has not only to collect a varied assortment of bacteria but also to distribute them widely. FIG. 206. A breeding place for house flies The community that saves itself money or trouble by permitting back yards of this kind usually pays for its economy and indifference with disease and death. With the econo- mies of motor cars and traction engines must be reckoned the reduction in typhoid fever and other fly-borne diseases From a report made by an army commission as to the causes of epidemic fevers in the army camps during the Spanish- American War, we learn that " flies swarmed over infected fecal matter in the pits and fed upon the food prepared for the soldiers in the mess tents. In some instances where lime had recently been sprinkled over the contents of the pits, flies with their feet whitened with lime were seen walking over the food." We can readily understand why it was that more soldiers were killed by intestinal diseases than by Spanish bullets. FIG. 207. Food exposed to the visits of flies FIG. 208. Food protected from flies Many food dealers have gone to the expense of installing equipment to protect their customers from the danger of contaminated food. Whether the dealer can afford to do this or not, the public cannot afford to leave its food exposed 402 ELEMENTARY BIOLOGY 439. Fighting flies. Just as soon as we realize the relations of the house fly to mankind, we are likely to be seized with a hatred for the whole tribe of flies ; and perhaps we may be tempted to " swat " every fly that we see. But if we all swatted flies, and did only that, the fly pest would hardly receive a serious check ; for flies breed faster than you and I can kill them, and there is nothing to prevent the flies raised in the stable down the street from coming into our yard. We have to attack the insects before they are old enough to fly about ; that- is, we must prevent their breeding by either removing or destroying, screening thoroughly, or poisoning, all materials that may serve as food for the maggots. The struggle between man and the fly is not a single- handed one, — that of a particular person against a particular fly. It is a struggle of one species against another, and we must carry on our end of the fight through community or group action. Better than swatting the fly is the com- plete elimination of the insect from all places inhabited by human beings. Many towns have undertaken to exterminate the fly. It has been found that the most effective method is to provide for the systematic removal of garbage and stable manure at least once a week,1 and to keep streets, back yards, markets, and kitchens perfectly clean. On the farm or in a village, stable manure can be profitably spread out upon the ground, in field or garden, every day or two. The manure spread out will dry quickly and be inca- pable of breeding flies. Exposure to sunlight will kill eggs and maggots. In larger towns and cities there should be no difficulty in organizing the work of removing manure and garbage frequently at a comparatively low cost, since the manure is worth gathering for fertilizer and the garbage has a definite commercial value. Where the amount of gar- bage or manure accumulated is so small that its removal is 1 The life history of the fly covers a period of ten days. INSECTS AS SPREADERS OF DISEASE 403 relatively expensive, arrangements should be made to screen it so that no flies can reach it ; but screening is very expensive and seldom entirely satisfactory. Lime, crude oil, copper sulfate, formaldehyde, and other poison- ous substances have been used in the treatment of garbage and manure to prevent the breeding of flies. But such treatment is in general undesirable, because it makes the manure and garbage worth- less for use as fertilizer, since it prevents also the fermentative action of bacteria, which is necessary to make available the elements of the organic compounds for plant growth. Borax and hellebore can be used so as not to injure the manure. Until a community succeeds in eliminating the flies, it is well for every household to protect its own food supply by suit- able screening of the house and by special care in regard to the exposure of food. Every purchaser of food can help by sys- tematically refusing to patronize dealers whose premises harbor flies. And we can all help by keeping our own premises clean and free from these insects. CHAPTER LXXIV INSECTS AS INTERMEDIATE HOSTS 440. Malaria. Of all the diseases from which man suffers, malaria is said to be the most widespread, occurring all around the earth as far north and as far south of the equator as mos- quitoes may be found.1 The disease is caused by any one of three or four species of protozoa related to the ameba and known as the plasmodium of jnalaria. The animal feeds upon the red corpuscles of the blood of its host, and then sporulates (see p. 294). The spores enter new corpuscles, and the process is repeated indefinitely, greatly weakening the victim and some- times ending fatally. The parasites were seen in the blood of patients by the French scientist Alphonse Laveran, working in Algeria. He succeeded in in- fecting subjects with the blood of sick people, but he could not find out how the infection takes place naturally. It took twenty years more of careful research and experimentation to establish the fact that the mosquitoes of the genus Anopheles are -the agents of infection. Two English physicians, Sir Patrick Manson and Dr. Ronald Ross, helped in the establishment of this important fact by tracing the behavior of the parasite in the bodies of mosquitoes. Finally, in 1900, an elaborate experiment was conducted by scientists cooperat- ing in England and Italy. In this experiment a number of volunteers lived in the Roman Campagna, a region that had long been notorious for being full of malaria. But the volunteers lived in houses that were carefully screened against the entrance of mosquitoes. They were also careful not to go out in the evening (when the Anopheles is about) 1 It has been estimated that in the United States the money cost of malaria has been as much as one hundred million dollars a year. This takes the form of time lost from work, the cost of drugs, nursing, and medical service, the unavailability of much fertile land, and so on. In India this disease kills over a million human beings a year, besides causing untold misery to millions of others. 404 INSECTS AS INTERMEDIATE HOSTS 405 without wearing veils and gloves. Thus they lived through the most dangerous part of the year, from early in July until late in October, and not one became sick, although many of their neighbors became FIG. 209. The malaria parasite The parasite attacks the red blood corpuscle of a human being, a, and when it has destroyed the corpuscle, d, it breaks up into a large number of spores, e, which may enter other corpuscles and start a new cycle. When blood containing the malaria organism,/, gets into the stomach of a mosquito (Anopheles), the protoplasm undergoes various changes, g, h, resulting in two sexual forms, i,j, which conjugate and produce a fertilized egg, k. This works its way into the wall of the insect's stomach, /, and breaks up into a large number of tiny bodies, m, which finally lodge in the insect's salivary glands, n When the insect again stings a person, some of these bodies, o, get into the victim's blood and find their way into the red corpuscles, a, and the cycle begins again. /, stomach of infected mosquito, showing swellings produced by the parasite infected with malaria during the summer. At the same time some mosquitoes were caught and allowed to suck blood from persons suf- fering from the disease. These mosquitoes were placed in little cages and shipped to England. Here two young men — one of them the 406 ELEMENTARY BIOLOGY Larvce, son of Dr. Manson — who had never suffered from the disease, and who lived in a region where there were no cases of malaria, allowed themselves to be stung by the suspected mosquitoes. In the course of a few days both developed the characteristic symptoms of the disease. This experi- ment showed that the night air and the vapors from the swamps of the Campagna were harmless, and that the sting of a mosquito that had once bitten a person with malaria was dangerous. Mos- quitoes raised from the eggs and allowed to sting a per- son do not cause the dis- ease to appear. Drinking the water in which the mosqui- toes developed does not cause the disease to appear. These conclusions were later con- firmed by further experi- ments, so that to-day there can be no doubt as to the relation between the mos- quito and the transmission of the disease (see Fig. 209). The most common species of mosquito found in various parts of this country belong Culex Anopheles FIG. 210. Mosquito life histories The mosquitoes of the genus Anopheles, which transmit malarial parasites, differ from the com- mon Culex in every stage. We can readily dis- tinguish the adults of the two genera by the fact that when at rest the Culex holds its body parallel to the resting surface, whereas in Anopheles the hind end of the body is farther from the resting surface than the head to the genus Culex. This is a nuisance, but, so far as known, does not transmit any disease to human beings (Fig. 210). 441. Yellow fever. This disease is found only in tropical or semi- tropical regions, although there have been epidemics of yellow fever as far north as Philadelphia, New York, and Boston. It has been in the past a much more fatal disease than malaria, and turns out to be carried by certain species of mosquito. While the parasite that causes INSECTS AS INTERMEDIATE HOSTS 407 this disease is not yet known, it is certain that, like malaria, it requires two hosts for completing its life cycle. At the close of the Spanish- American War a commission of American physicians undertook to find out whether the mosquito was really the intermediary in the transmission of the disease, as had been suspected by many students of the subject. The commission consisted of Dr. Walter Reed, FIG. 211. Camp Lazear In this building was conducted that part of the yellow-fever experiments which proved that the disease is not transmitted by infected clothing etc. The cabin consisted of a room, 14 by 20 feet, with two small windows facing south, closed with wire screens. Heavy wooden shutters excluded the sunlight. Entrance was through a small vestibule on the same side as the windows, protected by a wooden door and a screen door and separated from the main room by a screen door, to make perfectly certain that no mosquitoes could get in. This house was kept closed during the daytime and had a temperature of from 92° to 95° F. It was occupied for twenty nights by three American volunteers, and the test was repeated twice Dr. James Carroll, and Dr. Jesse W. Lazear, and they were assisted by a Cuban, Aristide Agramonte, who had recovered from the disease and was therefore immune. A camp was established in which two cottages were erected. In one of these the ventilation was intentionally very poor. In the other there was a mosquito-tight screen separating the* two halves, and the ventilation was very good. Both cottages were well screened to prevent the entrance or escape of mosquitoes. In the first cottage three volunteers received cases of clothing and bed- ding from men who were suffering from yellow fever, or who had 408 ELEMENTARY BIOLOGY died with the disease. They shook out these contaminated articles and slept in the soiled garments and in the soiled bedclothes for twenty days. None became infected. This experiment was repeated two times more, with no results that would indicate the slightest con- nection between the vomits and excretions of the patients and the infection of new cases. In the other building a volunteer allowed himself to be stung by a mosquito that had drawn blood from a patient some two weeks earlier. The bedding and other utensils were thoroughly sterilized, and the volunteer had been in quarantine for two weeks, to make sure that he was not infected when he came into the building. On the fourth day he developed the symptoms of the disease. Other volunteers, on the other side of the screen, were not affected. Ten or more individuals contracted yellow fever as a result of stings from mosquitoes that had previously bitten sick persons, and not one of those who stayed on the other side of the screen. In the course of the experiments Dr. Carroll and Dr. Lazear also became sick, the latter dying as a result (Fig. 211). 442. Fighting mosquitoes. With a realization of the impor- tance of the mosquito in the transmission of these serious dis- eases arose the question of how to combat the pests. Of course each one of us can keep on killing mosquitoes on sight and feel that he is doing his duty. But the mosquitoes do not recognize city limits or state lines, and gayly fly from one man's land to another's. So far the only effective campaign against mosquitoes has been waged on a comprehensive scale by a whole community at a time. It seems that the best way to prevent malaria and yellow fever is by means of ditches to drain off marshy land, by means of cartloads of dirt to fill in low- lying spots, by means of oil on such puddles as cannot be filled or drained, and by means of lids or screens to cover up such cisterns, tanks, or buckets as have to be kept with water stand- ing in them, while all old cans and broken crockery and other possible containers for water are scrupulously placed where the female mosquitoes cannot reach them. For the life history of the mosquito requires quiet water for the laying of the eggs and INSECTS AS INTERMEDIATE HOSTS 409 the growth of the larva and the pupa. Without such breeding places, one year would see the end of all mos- quitoes in all civilized communities. In larger bodies of water, where fish may be kept, these will usually de- stroy the larvae and thus prevent the multiplication of mosquitoes ; but in the shallow margins, where the fishes cannot reach them, the mosquitoes have things their own way. Here it is necessary to keep the borders of the ponds clear of weeds, sedges, etc. The practical effect of extermi- nating the mosquito is shown by the decrease of malaria (or yellow fever). Fig. 212 shows the results for the island of Cuba. A similar record stands to the credit of our national government in connection with the work of digging the Panama Canal. The region through which the canal runs was a veritable plague spot. During the various attempts of the French engineers to construct the canal, disease made the completion of the work practically impossible. When the United States took over the enterprise, the first step was the establishment of sanitary con- ditions ; and the largest part of the problem was the extermination of the mosquito through draining and filling in, and the inspection of inhabited regions to prevent the maintenance of breeding places for the insects. 1870 1880 1890 189.5 1897 1899 1901 1896 1898 1900 FIG. 212. The reduction of yellow fever in Cuba The mortality from this disease had always been very high, but much worse in some years than in others. The year 1896 was unusually bad, and 1897 not much better. Imme- diately after the American army of occupation began to clean up in Havana, in 1898, the sanitary condi- tions showed marked improvement. By eliminating the breeding places of mosquitoes, yellow fever has been completely banished from the island 410 ELEMENTARY BIOLOGY 443. Other disease-bearing insects. Fleas have been implicated in a very serious combination injurious to man. The bubonic plague, which has in past times been the most dreaded of diseases, especially in Asia, was found (in 1894) to be caused by a specific bacillus. But the mode of infection was not known until quite recently. The Chinese had observed, centuries ago, that there was some connection between the dying of rats in large numbers and the appearance of the plague. Modern scientists set to work to find out whether the rat plague was in any way related to the human plague, and they found that the same bacillus is the cause of both. Then it was found out that the plague spreads from rat to rat not by contact of the animals but through fleas that suck the blood of sick rats and later bite others, thus transferring the infection. Further observation showed that the plague is primarily a disease of rats, and gets into human beings when the fleas abandon dead rats and infect men and women. The plague has spread from the Orient, and cases have appeared at several ports in the United States within a few years. The methods developed for dealing with this danger are directed not toward killing the bacteria but toward killing the rats and fleas. A ship coming from an affected port is thoroughly fumigated to kill the fleas and rats, special devices are attached to ropes and chains to keep rats from getting ashore (or aboard, for that matter), and a search is made for hiding places in which rats may be concealed. In California it was found that the ground squirrels have become infected with the plague bacillus, and systematic patrols are established to catch rats and ground squirrels, which are regularly examined for possible infection. Since the United States joined the Great War, many, important medical problems have been solved by our investigators. One of these had to do with the transmission of trench fever, a disease that caused a great deal of suffering and incapacity, although it was seldom fatal. Volunteers from the ambulance and field-hospital units allowed the blood of patients to be injected into their veins. The development of the disease in the men so treated showed that the sickness is due to a germ (too small to be seen with the microscopes) present in the blood. Other volunteers allowed themselves to be bitten by lice taken from the bodies of sick men, and many of these developed. the disease, while others, bitten by lice from healthy men, remained unaffected while living under exactly the same conditions. This showed that the germs are carried over by means of the louse- INSECTS AS INTERMEDIATE HOSTS 411 Measures were then taken to exterminate the " cooties," as the lice were called by the soldiers. The service that these sixty-six volun- teers rendered in this experiment was quite as important as anything else they could do, even from a military point of view, for it estab- lished facts that made it possible to save the equivalent of thousands of soldiers for the fighting line. 444. Intermediate hosts. It is not uncommon to find among para- sites the dependence upon two hosts for a complete life cycle. This is found among plant parasites as well as among animal parasites ; and when there is any doubt about the life history of any such organ- ism, the investigators at once suspect the existence of an alternate host. Through the discovery of the complete life history of many parasites it has been possible to eliminate diseases by the attack upon the intermediate host. For example, in the case of a certain variety of " wheat rust " the alternate host was found to be the barberry plant. By cutting down the barberry plants the farmers saved many a crop of wheat from destruction by the fungus. In the case of cer- tain kinds of tapeworm it was found that the intermediate host is the pig. Here safety lies not in killing off all the pigs, but in cooking the flesh of the animal so thoroughly that the resting stage of the worm cannot reach the inside of the human digestive system alive. CHAPTER LXXV INSECTS AND HUMAN WEALTH 445. Insects as food. In certain parts of Africa and Asia, as well as in South America, Mexico, and Central America, the natives are said to use various species of locust and caterpillar FIG. 213. The clothes moth (Tinea pellionella) It is the larva of this animal that eats woolen and fur material. The eggs are laid on the material and hatch out when the temperature is sufficiently warm. It is for this reason that we rarely find the animals in the winter, and it is for this reason that furs and woolen rugs etc. are placed in cold storage during the summer months as food. Ants and termites, cicadas, the grubs of beetles, and the eggs of water beetles are also consumed. The Chinese sometimes eat the pupa of the silk moth, after the silk has been removed from the cocoon. In so-called civilized countries the 412 INSECTS AND HUMAN WEALTH 413 only insect that supplies food to man is the honeybee, whose honey has been used by man for many centuries. 446. Insect products. The wax obtained from bees is of great practical value, but it is coming to be replaced more and more by paraffin, which is obtained from petroleum. Another insect product of growing importance, and one for which no satisfactory sub- stitute has yet been found, is lac. The lac is used as a dressing for wood and other materials, as shellac, as a stif- fening for felt in the making of hats, as an insulating var- nish in electrical work, in the making of lithographer's ink and of sealing wax, and in increasing quantities in the manufacture of phonograph records. The cochineal, another member of the scale-insect FIG. 214. Destruction wrought by ants family, furnishes a beautiful Part of a post completely ruined by the excavations of carpenter ants. There are red dye, which Was formerly several species of Ctimponotus and of other i • i ... T-, . genera which are known to bore into wood. used in large quantities. This fFrom photograph by New York Botanical Source of Supply is of de- Garden) clining economic importance because of the rapid development of the anilin-dye industry. The whole silk industry rests upon the fiber obtained from the cocoon covering of the silk moth. Although the chemists have devised ingenious processes for making artificial silk out of cotton and out of wood, we shall probably continue to cultivate the silk moth for a long time to come. Many of the beetles may be considered as useful, since they destroy large amounts of dead animal remains, as do 414 ELEMENTARY BIOLOGY also some of the ants. In this way they may be looked upon as scavengers. And a few insects, in the course of their predacious activities, devour forms that happen to be injuri- ous to us. This is illustrated by some of the beetles like the ladybug, which eat plant lice and thus keep them in check. We make direct use of very few insects. Many species are nevertheless of indirect value to us as important links in that chain of life extending from decomposing organic matter at one end to the larger useful animals at the other. It is cer- tain also that very many species of insects are essential to the propagation of various species of plants, since they are the sole agents in the distribution of pollen (see pp. 309 ff.). 447. Destructive insects. In this country alone insects of va- rious kinds destroy every year materials and goods estimated to be worth more than two hundred million dollars. This includes stored food, clothing, furniture, carpets and hangings, and furs. The clothes moth is one of the most familiar of these destruc- tive insects, for it is found nearly everywhere that human beings are (Fig. 213). Thorough airing and exposure to sunlight for a few hours will be likely to kill any of the eggs. Naphthalin moth balls do not kill the animals, but repel them and thus prevent destruction. Infested material should be treated with gasoline and then thoroughly aired before being used. The cockroaches, of which there are several species, will eat almost any organic matter, but are seldom destructive to FIG. 215. Meal-worm Adult and larva of the miller beetle (Tenebrio molitor) INSECTS AND HUMAN WEALTH 415 valuable materials. Yet their presence in a house is an indica- tion that there are crumbs and other scraps of food about, and it is perhaps as well for the cockroaches to eat these as for a b c FIG. 216. The flour moth (Ephestia kuehnietla) a, larva ; b, pupa ; c, adult some more objectionable animals to do so. On the other hand, they may become a serious menace, in the course of their wanderings, since they may carry disease germs to the food. a b c d FIG. 217. The buffalo moth This insect (Anthrenus scrophulariae) is a beetle, but is commonly called a moth because it injures furs and rugs in a manner resembling that of the clothes moth, a, larva ; £, pupa in larval skin ; c, pupa ; d, adult Ants have been very destructive not only to food materials but also to furniture, clothing, wooden utensils, and even to wooden houses (Fig. 214). The Argentine ant was introduced into this country in the early nineties at New Orleans, and has been more destructive in southern parts of the country than any of our native species. 416 ELEMENTARY BIOLOGY Ants and cockroaches are always a nuisance about a house and can be exterminated, once they get in, only through care- ful attention to their nests, cracks in the walls, etc., and to the availability of material that will attract them. Corrosive sub- limate is used to poison them, solutions being sprinkled into the cracks. The cockroaches can be driven out by systemati- cally sprinkling borax about the kitchen. Weevils, small beetles with beaks, are very destructive to stored grains, beans and peas, etc. There are many different species, and they are all destructive. Infested granaries and warehouses need to be thoroughly fumigated with carbon bisulphid, which kills the eggs and the larvae as well as the adults (see Fig. 219, e). Flour is often spoiled by other beetles, as the rpeal-worm (the larva of a black beetle, Tenebrio (Fig. 215)), and by a species of moth, Ephestia knehniella (Fig. 216). Another destructive beetle is the so-called buffalo motk, shown in Fig. 217. The larva of this carpet beetle is very destructive to rugs and other woolen material. CHAPTER LXXVI INSECTS AND OTHER ORGANISMS 448. Insects and useful plants. From the time when men first began to cultivate plants for their own use, insects of one kind or another have caused parts of each year's work to be wasted. There are early records of the destruction caused by locusts, and this name has come to be applied to many varieties of insects that move in hordes. One of the plagues of Egypt was a swarm of locusts, insects which are referred to in the Bible repeatedly. The first effort in this country to aid agriculture by means of an official investigation of in- sect ravages was made in the seven- ties, when an outbreak of this pest did damage over an area of about two million square miles west of the Mississippi River. Since then the federal government and the various states have kept up systematic work through experiment stations or special 417 FIG. 218. The potato beetle (Leptinotarsa decemlineatd] There are two or three broods a year. The full-grown larva crawls into the ground, where pupation takes place. The winter is passed underground in the adult stage. The tachina fly, shown in the center, is one of the most impor- tant enemies of the potato beetle. The fly lays eggs in the larva of the beetle, and the maggots destroy their host 41 8 ELEMENTARY BIOLOGY agents and commissions, designed to counteract the injuries done by insects to valuable plants. It is estimated that the damage done to our crops by the activities of insects amounts to from six hundred million to seven hundred million dollars every year. To this must be added the injury to forests and forest products, and the injury to animals. The locusts and many other species of insects will eat almost every kind of plant ; but many insects confine their attentions 6 FIG. 219. Cotton-boll weevil (Anthonomus grandis] This animal feeds only upon the cotton plant and could probably be completely exter- minated if the planting of cotton were suspended for a year or two. This was the advice of the government experts to the growers of Texas about twenty years ago ; but it was unheeded, with the result that millions of dollars' worth of cotton have been destroyed each year since. Rotation of crops was finally forced upon many of the farmers, with beneficial results, a, larva ; l>, larva in mature boll ; c, pupa ; d, pupa in boll ; , larva, the " worm " of the apple ; f, cocoon ; d, pupa ; e, adults The ox warble lays its eggs on the cow. It is not certain whether the larvae work their way through the skin or from the FIG. 223. The horse botfly (Gastrophihis equi) The egg, a, is laid on the hair of the horse and is swallowed, together with the larva, 6, in the saliva. In the stomach the larvae attach themselves, often causing serious irri- tations and incapacitating the animal for work. The larvae escape from the host with the excrement, and then pupate in the ground. <;, pupa ; d, adult alimentary canal. They finally lodge under the skin and thus ruin millions of dollars' worth of hides, besides making the animals sick and reducing their milk and beef values. 422 ELEMENTARY BIOLOGY 450. Fighting insects. One of the first suggestions that insects could be controlled by encouraging other insects was made about a hundred years ago by two English entomologists, who declared that an increase in the number of ladybirds in greenhouses and fields would clean out the aphids, or plant lice, and insure the hops against destruction (see Fig. 224). In this country various species of native ladybirds serve as effec- tive checks upon plant lice of many kinds. It has been possible to control the destructive Hessian fly by means of the parasite Polygnotus. FIG. 224. The calosoma beetle (Calosoma sycophanta) This beautiful green animal was used by a French scientist in a campaign against the gypsy moth in 1840. In recent years this method of fighting undesirable insects by encouraging the spread of an enemy insect has been rapidly extended, especially in the United States, which leads the world in applied entomology, a, adult ; b, larva feeding on pupa of gypsy moth ; c, adult feeding on larva (caterpillar) of gypsy moth Shipments of such parasitic insects from one part .of the country to another are frequently made to meet outbreaks of injurious insects. A further step was taken when specialists were sent abroad by the government to look for natural enemies of injurious insects in the regions from which these insects originally came. The United States probably suffers more from injurious insects than any other country, because there have come here with the mi- grations of peoples a large number of foreign insects, without their natural enemies. This country has also done more than any other in the scientific and practical study of insects and of methods of control. INSECTS AND OTHER ORGANISMS 423 Ladybird beetles have been imported from China to keep down the San Jose scale. Calosoma beetles and several parasitic, wasplike insects have been imported to fight the gypsy moth and the brown- tail moth. This kind of work is growing very rapidly. There are now several stations in this country where insects are cultivated on a large scale, to be sent where needed in controlling injurious insects. Spraying of orchard trees, shade trees, and crop plants with various kinds of poisonous mixtures is one of the common means used by farmers in controlling the damage resulting from insect depredations. The rotation of crops is used for the purpose of starving out one generation of injurious insects. This can be used against insects that confine themselves to special kinds of food plants. In recent years it has been found that insects are subject to fatal diseases caused by species of fungi. Cultures of such fungi have accordingly been used to fight insects. A number of the insects are caught alive and infected with the parasitic fungus, and then set free again. The escaped insects infect -their fellows, and millions are thus killed off. This method has been successfully employed against locusts in South Africa and in Yucatan. The other natural enemies of insects are toads, frogs, snakes, and, most important of all, nearly all kinds of birds. A given amount of money spent in protecting and encouraging the native birds of a region is likely to produce more beneficial results than any other method of fighting insects. 451. The balance of nature. The number of individuals in a species fluctuates from year to year, partly on account of climatic conditions, as when an early frost destroys plants before the seeds are ripened, or when the frost kills off certain enemies. But much of thg fluctuation is due directly to the variation in other species. A good year for ladybirds will mean a poor year for scale lice ; but the following year the shortage in scale lice will reduce the number of ladybirds. The living beings are related to each other in hundreds of ways, so that the elimination or unusual increase in one species is likely to affect the well-being or even the existence of another. Every species has its friends in the living world, as well as its enemies, and when man undertakes to change the face of the earth with 424 ELEMENTARY BIOLOGY respect to any species, he must proceed cautiously and on the basis of a thorough knowledge of all the relations of the species concerned, and not merely on the basis of a superficial answer to the question, Is that species useful or harmful to me ? It is practically impossible to reduce the numbers of one species without producing far-reaching effects upon other species. It is not a matter of interfering with nature's plans, as some sup- pose. It is a matter of disturbing a certain balance that it has taken a long time to establish, with possibilities of unknown consequences. When rabbits were introduced into Australia, they multiplied so rapidly that before many years they became a real pest, and the government offered bounties for their extermination. Here was a region admirably fitted for the life of these animals, and until man interfered there had been no such animals there. The same kind of thing happened with the introduction of the water cress into New Zealand, and with the introduction of the English sparrow into the United States. Probably these organisms throve better in the new surroundings because they did not here meet their old enemies. These facts should help us to realize how closely dependent upon one another the various species of plants and animals are. CHAPTER LXXVII BIRDS IN RELATION TO MAN 452. The food of birds. As in the case of most animals, birds are important to us chiefly because of the food they eat. But unlike most insects, the feeding of birds usually turns out to be of advantage to mankind. Observation has convicted many birds of eating fruit in the orchards, and the sharp- shinned hawk has been caught carrying off hens from the barnyard. But the systematic study of the contents of birds! stomachs has shown that most of the food of practically all the common wild birds consists of insects, the seeds of various undesirable weeds, and field mice, shrews, mice, and other un- desirable animals. In other words, with a very few exceptions, the common birds are worth more alive than dead. The value of most wild birds as destroyers of insects, vermin, and weeds is vastly greater than their value as sources of feathers, as food, or as objects of sport. 453. Destruction of birds. Many birds are destroyed wantonly by ignorant boys and men, others are killed to supply feathers, and still others are exterminated in the destruction of eggs and nests out of idle curiosity or in the interests of untrained col- lecting. In rural and suburban districts the domestic cat is a serious menace to the native birds, and does damage that is far from compensated for by the mice or rats killed by the cats. During their migrations many birds are killed by flying against telephone and telegraph wires, and against plate-glass windows. Along the shores, migrating birds frequently hover about the lighthouses at night until they are exhausted. The extension of cities, the clearing of forests, and the improvement of farms are all tending to exterminate various species of birds. 425 426 ELEMENTARY BIOLOGY The destruction of dead limbs and dead trees in forests and wood lots will mean the disappearance of the downy and the red-headed woodpeckers, but it is worth while to keep the wood-lot clear. The spraying of orchard trees with poisons intended to destroy caterpillars has led to the death of thousands of birds that eat the poisoned insects. It is probable that in the long run it would be more economical to encourage the birds to nest in our orchards and let them keep the insects in check. 454. Protection and encouragement of birds. Many of the destructive agencies that affect birds are directly under our control. When once we are convinced that it is worth while to do so, it is possible to place electric wires underground, as is now being done in the cities, for example. The Royal Society for the Protection of Birds, of England, has had gratings placed upon a number of lighthouses on the coast, to serve as bird-rests. Here the migrating birds rest until morning and then continue their flight. Thousands of birds are thus saved from destruction ; and when we realize the value of the birds, we shall no doubt plan to build all of our lighthouses with some consideration for the safety of these animals. Men and boys will have to be educated to enjoy life without destroying useful animals, and to find sport in opera glasses or the camera ; and girls will have to be educated to be happy without birds' plumage, or to be content with the dyed feathers of domestic fowl. Those who have tried it seem to get as much fun out of building nest boxes and shelters for birds as others can get out of shooting or trapping them. And the birds that have been encouraged to make their homes in our immediate neigh- borhood will continue to furnish us with interesting sights and sounds long after dead birds would have been forgotten. In addition to providing suitable boxes for bird nests, we may do a great deal to protect them against starvation after heavy snowfalls. At such times there is practically no food BIRDS IN RELATION TO MAN 427 available in the fields or woods. The scattering of grain or bread crumbs will enable many birds to survive until the ground is clear and they are again able to find food for themselves. Cooper's hawk (A cdpiter cooper t) Bronzed grackle (Quiscalus quiscula) Blue jay (Cyanocitta cristata) Yellow-bellied sapsucker (Sphyrapicus varitis) FIG. 225. Some undesirable bird neighbors Cooper's hawk preys upon poultry and insectivorous birds. The blue jay and the bronzed grackle destroy the eggs of other birds, and the grackle also eats a great deal of grain. The yellow-bellied sapsucker injures standing trees The cat and all its wild relatives are so destructive to birds that it is doubtful whether we should not all be better off with the domestic cat completely eliminated from our lives. There are some good things to be said in favor of the cat; but the other things more than offset them. The red squirrel 428 ELEMENTARY BIOLOGY often destroys the eggs and sometimes even the young of birds, and does nothing to compensate for this damage. These animals should therefore be killed, to give the birds a Cedar waxwing (A mpelis cedrorttm) Crow (Corvus americanus) Red-headed woodpecker (Melanerpes erythrocephaitis) Great blue heron, or crane (A rdea herodias) FIG. 226. A bird rogues' gallery The cedar waxwing destroys fruit and disperses weed seeds. The crow destroys grain, fruit, useful insects, and the eggs of useful birds. The red-headed woodpecker destroys cultivated fruit. The blue heron eats fish and frogs better chance. The weasel, the skunk, the fox, the raccoon, and other mammals sometimes kill birds or eat their eggs ; but as they do not feed exclusively or largely upon birds, they are not to be considered serious enemies. BIRDS IN RELATION TO MAN 429 455. Undesirable birds. It is impossible to class every spe- cies of bird as altogether useful or altogether injurious. A bird may be very useful in one region and injurious in another. The red-tailed hawk feeds on field mice in one region and dis- covers that chickens are good to eat in another. The bobolink is a serious menace to the rice fields in the South, but is a valuable insect destroyer in the North. The red-winged black- bird ate so much grain in Nebraska a number of years ago that the farmers just took up arms and killed the bird off. The following year, however, the absence of the blackbirds enabled the locusts to multiply so rapidly that many of the grain crops were ruined. 456. Direct economic value of birds. The poultry and eggs pro- duced in this country and sold for food every year are valued at over five hundred million dollars. To this must be added game birds used as food, the value of which it is practically impossible to estimate, and importations of bird products. Imported feathers and downs come to about eight million dollars a year. The most valuable organic fertilizer consists of guano, which is the refuse of millions of birds, accumulated through many years upon various islands off the coast of South America (see p. 66). The satisfaction yielded to the observer by the song and chatter and by the appearance of birds would seem to be enough to pay for the maintenance of many of these interesting animals; but we can have these returns without paying for bird feed, and get in addition the very considerable contribution that they make to the suppression of undesirable insects, rodents, and weeds. CHAPTER LXXVIII SOCIAL LIFE OF ORGANISMS 457. Self-sufficient individuals. Airtong the lowest forms of plants and animals each individual is quite independent of its neighbors, as we may see in the case of the ameba, the para- mecium, the green slime, the various bacteria, and so on. FIG. 227. Diagram of sponge structure A sponge is a colony of cells arranged about hollow spaces, «, which are connected with the surrounding water by means of hollow channels, b, carrying currents inward, and by means of other channels, c, carrying currents outward through larger tubes, or " sewers," d. The currents are produced by the constant vibration of cilia projecting into the spaces, and they bring to the cells fresh supplies of food and oxygen, and carry away waste Among many of the algae the cells generally remain attached to form long filaments, but there is apparently no physiological connection, and a break in the filament does not affect the activities of the severed portions. Among the more complex algae, such as the bladder wracks, detached portions may continue to grow, since each portion de- pends upon materials absorbed from its immediate surroundings 430 SOCIAL LIFE OF ORGANISMS 431 for its sustenance. But among these higher algae the growth of the mass of cells assumes a rather definite form, and certain of the groups of cells become specialized as anchorage organs, others become specialized as reproductive organs, and so on ; and it takes all of these together to make up a complete life. FIG. 228. Colony of Hydractinia In this colonial animal (related to the jellyfish and to corals), as in many others, there are distinct kinds of individuate, called hydranths. a, vegetative, or food-getting, hydranths, which take in and digest food for the whole colony ; £, vegetative hydranths in various stages of contraction ; c, protective, or fighting, hydranths, which bear large numbers of nettling cells ; d, reproductive hydranths, male and female, which throw off sperm cells and egg cells respectively ; e, buds, or undeveloped hydranths. (Photograph from model in American Museum of Natural History) If we consider the whole plant as an individual, we see that it is quite possible for a single plant to continue its life without relation to any others of the species. In general this is true of all the plants, from the lowest to the highest. 458. Differentiated cells. When we pass from the one-celled animals to the sponges (Fig. 227), we find that while the life 432 ELEMENTARY BIOLOGY of each cell is practically independent of that of its neighbor, there is some differentiation as to cell structure, and there is a great deal of common activity. 459. Differentiated individuals. Among the Coelenterata (to which branch belong the hydra, jellyfish, sea anemones, and corals) we find species in which the individuals are quite independent, others in which there are colonies of similar individuals, and still others in which the individuals are differ- entiated in structure as well as in func- tion (Fig. 228). The 11 Portuguese man-of- war" consists of sever- al kinds of individuals, - nutritive, locomo- tive, and reproductive. In this ant (Ponera pennsylvanicd), as in so many 460. Colonial ani- others, the worker, a, is easily distinguished from the mai^ A hrvir^ tVi^c^ female, 6, and the male, c. (After Wheeler) simple colonial ani- mals we find forms in which the interdependence within the species is chiefly confined to reproduction and the care of the young (see pp. 331 ff.), but toward the upper end of each of two very important branches — the Arthropoda and the Vertebrata — there appears a form of colonial life which is very significant to us, from a practical point of view as well as from a theoretical one. In an ant colony there may be one or several queens or female ants, thousands of workers, and many soldiers. After the colony is started, the queen may lay eggs continually. The workers extend the nest and keep the structures in repair. They also go forth to forage for food, look after the eggs, larvae, and pupae, and clean out all foreign matter that cannot be used (Fig. 229). In some species the soldiers are quite FIG. 229. Ant individuals SOCIAL LIFE OF ORGANISMS 433 distinct from the workers, and engage in fights with other ants that may invade or approach the entrance of the nest, whether of the same species or of a foreign species (see Fig. 193). In some species of ants the workers are of two distinct sizes and show marked differences in behavior as well as in structure. FIG. 230. Honey-pot ants In this species of ants, found in California, some of the individuals become reservoirs for honey gathered by the forage workers. These living honey-pots cling to the roof of a chamber by their feet, and receive into their crops the food gathered by the workers. From time to time one of the nurse ants comes to the honey-pot and receives into her crop a quantity of the stored fluid, which she then transfers to the larvae In a hive of honeybees the individuals are engaged in several different occupations. Some are building comb, others hang up by their feet and secrete wax plates or scales, others pack pollen into cells, some are feeding the young, and so on. You could probably distinguish the queen bee from the others by her size, but all the workers would look alike to you. Or you might notice that the nurses were more fuzzy than the foragers. This does not mean that there are two distinct kinds of workers. Apparently the workers behave 434 ELEMENTARY BIOLOGY differently at different ages ; thus, when first out of the pupal stage the young bee is a nurse. She does not leave the hive to forage for a week or more, and she is more fuzzy now than those in the foraging stage, because she has not had the time and the trials to break or rub off her hairs. The high degree to which the division of labor is carried among the social insects, and the complexity of their various activities, have led to a great deal of speculation as to how the animals are directed in their cooperations. Do the insects remember and plan and fit their actions to purposes in the same sense as human beings do ? There is a great deal of evidence to show that what has been called recog- nition by a bee or ant of others from the same colony is nothing at all like our recognition of those with whom we are familiar. It is more like what happens when a blind person smells a rose that arouses agreeable feelings in him. And so with many other peculiari- ties of these animals, it is possible to understand much of what they do without assuming intelligence. 461. Coordination. In the division of labor there is involved a great deal of coordination. That is, so long as it is impos- sible for each individual to carry on a complete life, it is neces- sary that there be some way of exchanging materials or services. This situation has frequently been compared to the physio- logical division of labor that we find in our own bodies and in other many-celled organisms. We have seen that there is a great deal of coordination through the blood system, through the lymph, and through the nervous system. In a colony of disconnected individuals the coordination must be brought about by what the animals do to each other directly, or by means of the materials with which they all deal. But it is not necessary to assume that there is a conscious cooperation that involves common aims and intercommunication. There can be conscious cooperation only where there is also a certain degree of general intelligence. In every case there is a great deal of interdependence, so that the individual separated from the group becomes almost helpless. SOCIAL LIFE OF ORGANISMS 435 Among many species of birds and mammals there is a degree of social life that extends somewhat beyond mere gregariousness. The Russian explorer Peter Kropotkin has brought together in his very interesting book, " Mutual Aid a Factor in Evolution," hundreds of examples of animals that hunt in groups, or post sentinels to guard against danger, or fight off attacks of enemies through con- certed action, or in other ways show mutual interrelations within the species. 462. Interdependence. Every one of us is dependent upon others of the same species in hundreds of ways. As members of the community we are dependent upon each other for vari- ous kinds of personal and specialized services. We depend upon community action for our safety from various plants and animals, as well as from antisocial individuals who would prey upon the rest, and from inanimate dangers, such as fire and flood and storm. We depend upon each other as members of the state or nation in the exchange of materials that are found in some regions but not in others, and upon this is founded all of our commerce. As members of the state or nation we depend upon joint action for the regu- lation of those things that affect the very conditions of exist- ence, — as the resources of the soil and the waters, the safety of highways and waterways, the protection of food sup- plies, the prevention of infection of man or domesticated plants and animals, and so on. As members of the human race we are dependent upon those of other countries, not only for materials that are restricted in their distribution, but even more for ideas arising out of different experiences. Modern science, upon which rests so much of our present- day advance in general welfare, is altogether an international product. Every discovery and every invention rests upon hun- dreds of other discoveries and inventions made by men and women of many nations and of several generations. What we call civilization is an accumulation of the most valuable thoughts of all peoples. 23 108 167 106 33 FIG. 231. Variation in size of similar units On measuring the length of each of several hundred beans taken at random from a large lot, it was found that the largest was about twice as long as the smallest. The number of seeds of each size is shown by the relative height of the corresponding column above the horizontal line. Studies of this kind, repeated by many workers on many kinds of material, show not only that individuals vary, but that they vary in a certain way PART V HEREDITY AND EVOLUTION CHAPTER LXXIX VARIATION 463. No two alike. In some respects all the members of a species are alike ; that is why we classify them as of the same species. But in some re- spects every individual is unique. If a person should get the tips of his fingers inky and place them on a sheet of paper, he would make a mark that could not be duplicated by anyone else. Ordinarily we have no difficulty in distinguishing from each other human beings that look very much alike, although occasionally there is difficulty in identifying a person beyond every doubt. All species present this fact of variation. And variation is found with respect to every char- acter. There is variation in size and in proportions (Fig. 231), in 437 10 11 12 13 14 15 16 17 18 19 20 21 22 FIG. 232. Individual variation in the number of repeated parts The principal veins on each side of the midrib on a beech leaf vary from 10 to 22. The number most frequently found is 16. The vertical columns correspond in height to the frequencies with which the various numbers of veins occur. Broken line a, a tree in which the number of veins varied from 13 to 17, leaves with 15 veins being most frequent. Dotted line b, another tree, in which the veins varied from 15 to 20, 1 8 veins being most frequent. Each tree has its individuality, and each leaf has its individuality 438 ELEMENTARY BIOLOGY coloring and in shading, in the numbers of duplicated parts (Fig. 232), and in physiological properties (Fig. 233). Examples of all these kinds of variation are easily found. Examples of physiological variation are the yield of milk, the proportions of sugar or of some other component (Fig. 234), the amount of sleep people need, relative immunity to infection, and so on. 464. Causes of vari- ation. We know that ZZZIZI^^ZIZ^^Z^IIZZZZZI^~ when a cow is under- nourished, she will not yield as much milk as she does when she is ^ZZZZ^ZHZZZ^ properly fed and cared for. This accounts for FIG. 277. Variation in physiological , . , ,.„ characteristics much of the difference , between one farmer's Each line represents the relative amount of milk given by 16 cows in one month. The poorest yield COWS and his neighbor's (represented by the shortest line) averaged 20 pounds Q ,, other a day ; the best cow averaged 30 pounds a day. Not only did one cow differ from another, but for each hand, in a given herd cow the yield varied from day to day. In like man- f ,, f , . , ner, the percentage of fat in the milk varied from 'OWS' ail cow to cow ; and for every cow, from day to day have received the Same care and feeding from the time they were born, there will still be great variations in the ability to produce milk. In the first case we say that the yield of the cow has been 'modified by the treatment she has received. In the second case we say that the cows are of different breeds •, or strains. All around us we see examples of modifications resulting from differences in the conditions of development. Differ- ences of feeding affect plants as well as animals. In any season we may see fields of stunted, backward crops and fields of luxuriant growths. In every city we may see well-fed, vigorous, and alert men and women, as well as shriveled, miserable, and timid men and women. It is important to know whether, and how far, these differences can be controlled. VARIATION 439 It is quite impossible to say offhand, in any given rase, how much is due to variation in breed and how much is due to modifications produced by surroundings. But every farmer knows that, in addi- tion to controlling the con- ditions under which his plants and animals develop, he must also be careful to select the right kinds of seed or stock. The best of care will not make an ordinary white bean develop into a plant bearing lima beans, nor will extra feed- ing make a scrub cow give the kind of milk that may be obtained from a good Jersey cow. 465. Improvement by selection. All domestic animals and plants have 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19 FIG. 234. Variation in physiological properties Forty thousand sugar beets, tested individually, been carefully watched for Centuries for the purpose showed from 12 per cent to 19 per cent of of selecting the most de- sugan Beets containing '5'5 per cent of sugar sirable individuals as the parents of the succeeding parts more from the typical I5 5 per cent; the Crops Or generations. The number of individuals with a given sugar content best heads of wheat were selected for seeding the following year ; the best beans and the best potatoes have been set aside as the progenitors of the crops to come. And the same principle has been applied in the raising of ani- mals. The best milk cows were selected to be the mothers of the calves, the swiftest mares were the mothers of the colts, and so on. were the most frequent, but there were almost as many beets with 15 per cent or with 1 6 per cent. As the percentage of sugar de- diminishes, so that the extremely poor and the extremely rich beets are also fewest in number 440 ELEMENTARY BIOLOGY Careful observers familiar with farm life and practice have noted that cultivated races of plants and animals tend to deteriorate markedly in a very few generations unless the selections are made every year. The explanation for this was unknown, but the fact was clear, and its application was found in steadily selecting the best or most desirable individuals or FIG. 235. Improved varieties of domestic birds seeds for further propagation. For one thing was certain : it was possible to improve the stocks to a considerable degree by constant selection (see Figs. 235, 236). 466. Mixed types. In the diagrams used to illustrate the measurement of variations (Figs. 231-234) we see that for every series there is one measurement that represents the condition of a comparatively large number of individuals. This appears in the diagram as the peak of the curve. In many groups of individuals careful measurements will show two such peaks. For example, the Dutch botanist Hugo de Vries counted the number of ray-florets in a species of daisy (Chrysanthemum segetum) and plotted the results of his count- ing, which are given graphically in Fig. 237. FIG. 236. Improved varieties of domestic animals The contrast between the modern high-bred Jersey and the ancient banting, or between the high-bred Russian wolfhound and an ordinary yellow dog, is typical of the changes brought about in the course of many generations by the steady work of selecting the most desirable kinds of individuals to be the parents of the following generations 442 ELEMENTARY BIOLOGY This diagram led to the suspicion that what had been known under a single name was in reality a mixture of two different plants. In order to try out this idea, de Vries selected seeds from heads having thirteen ray-flowers, and planted them separately. The rays on the flowers of the new plants derived from these seeds were counted, and the distribution of the variations found to cluster more closely around thirteen rays, as shown by the dotted line in the figure. This would show that the wild daisies represented prob- ably two distinct races, or strains, that looked enough alike to be considered as a single species. Another illustration is furnished by some beans studied by a Danish scientist, Dr. Wilhelm Johannsen. The height of the line above the base From the appearance of the beans, and from fa appearance of the L L CUFVC showing the distribution of variations no person would FIG. 237. Two-peaked curve of variation corresponds to the number of daisy heads (Lnrysantnemum segetum) hav- ing the number of ray-florets indi- cated at the bottom. The solid line snows that there were many heads with 13 florets, and very many with SUSpCCt that they Were not all of 21, and that the intermediate num- fcj d g fc fa Dr. Johannsen bers were represented by fewer indi- iduals, as were also the numbers planted single seeds of different of i3-ray heads and kep.1 the p'ants carefu»y tion of the following year's crop, guarded against possible cross- pollenation, he obtained groups of seeds that clustered around the respective parent seeds in size. This process was continued for several generations, until the experimenter succeeded in separating nineteen pure lines of plants that remained distinct in following generations. CHAPTER LXXX HEREDITY 467. The problem of heredity. How is it that the characters of the parents are transmitted so regularly to the offspring ? How is it that, in spite of the close resemblances between par- ents and offspring, these are never exactly alike in every point ? These questions have to do with the problem of heredity. 468. Analysis of the problem. Certain facts, or laws, of heredity were first discovered in part by an Austrian monk named Gregor Mendel (1822-1884). Mendel had long puzzled about the great variations among his garden peas. There were tall plants and short ones, plants with white flowers and plants with colored flowers. In some plants the seeds were yellow, in others they were green ; some seeds were smooth, others were wrinkled. All in all, he studied seven different pairs of con- trasting characters in regard to which pea plants differ. He noticed further that a given plant might have any combination of single members of these pairs. A hairy plant might be tall or it might be short, it might have yellow seeds or it might have green seeds, it might have full pods or shrunken pods, and so on. 469. Mendel's experiments. Fixing his attention on a single character at a time, instead of trying to think of the variety as a whole, Mendel crossed garden pea plants that were different. Thus, he crossed green-seeded plants with yellow-seeded ones, tall ones with short ones, hairy ones with smooth ones, and so on for all the pairs of differences. When green-seeded plants were crossed with yellow-seeded plants, the seeds in the next generation were all yellow (see Frontispiece). 443 444 ELEMENTARY BIOLOGY Most people have the impression that where individuals with differing characters are mated, the offspring will show characters somewhere between the characters of the parents. The reason for this common belief lies in the fact that in our everyday experience we notice that children resemble both parents', but most of us have not taken the trouble to notice further that this resemblance to both parents does not consist of having every character halfway between the corresponding characters of the parents, but in having some characters just like those of the mother and other characters just like those of the father. 470. The Law of Dominance. Mendel found that this com- plete resemblance of the offspring to one of the parents (in regard to a particular character) was quite the rule with each of the other pairs of characters. Thus, the offspring of a tall- and-short cross were all tall. The offspring of a smooth-and- wrinkled-seeded cross were all smooth, and so on. This fact Mendel called the Law of Dominance. His idea was that where two characters of a pair meet in an individual, one of them dominates over the other. Of a pair of characters, that one which did not show in the offspring is called the reces- sive. It is not destroyed, as we shall see. Of course, he could not tell which of the two characters in a pair would reappear in the offspring before trying them out. In the tables on page 445 are given the dominants and their alternatives for a num- ber of characters selected from among plants and animals. . 471. The Law of Segregation. The yellow seeds of the hybrid J pea plant cannot be distinguished from the pure yellow seeds of one parent. With plants grown from the hybrid yellow seeds Mendel brought about three classes of cross-pollenation. 1 The word hybrid was formerly applied to the offspring of two parents of different species or races, as, for example, the mule, or a mulatto, or the off- spring of a Caucasian and an Indian. It is now used quite generally among biologists, horticulturists, animal breeders, etc. to mean the offspring of two parents that differ with respect to any particular character. For example, a seedsman might speak of a hybrid tomato, meaning a plant resulting from a cross between two varieties of tomatoes, and so on. HEREDITY 445 HEREDITY IN PLANTS NAME OF PLANT DOMINANT CHARACTER RECESSIVE CHARACTER Wheat Late ripening Early ripening Wheat Susceptibility to rust Immunity to rust Barley \ Beardless Bearded Wheat/ Maize Round starchy kernel Wrinkled, sugary kernel Maize Yellow grain White grain Garden pea Yellow seed Green seed Garden pea Tallness Dwarf Garden pea Tomato . Smooth seed Two-celled fruit Wrinkled seed Many-celled fruit Cotton Stock I Sweet pea I .... Jimson weed J Sunflower Colored lint Colored flower Branched stem White lint White flower Unbranched stem Nettle Saw-edge leaves Smooth-margin leaves HEREDITY IN ANIMALS NAME OF ANIMAL DOMINANT CHARACTER RECESSIVE CHARACTER Cattle Horse Hornlessness Trotting Horns Pacing Silkworm Rabbits ^ Guinea pig/ Mice . . Yellow cocoon Short fur Normal movements White cocoon Angora fur Waltzing habit Mice ^| Rabbits ^ Guinea pigj Leghorn poultry .... Salamander Canary Pigmented coat White plumage Dark color Crested head White coat Pigmented plumage Light color Plain head Poultry Rose comb Single comb Poultry Short rump Long tail Land snail Pomace flies Plain shell Red eyes Banded shell White eyes 446 ELEMENTARY BIOLOGY 2D: 2R 3D: IR W FIG. 238. Mendel's Law of Segregation When two individuals with a pair of alternative characters are mated, the offspring will all have the character of one of the parents; this char- acter is called the dominant one, and the alterna- tive character is called the recessive. The hybrid offspring of such a mating is represented in the diagram by /\. Offspring of this kind resemble the dominant parent Z>, but experiments show that there is a real difference. If such a hybrid is mated with one of the pure dominant type, /, the next generation will all be dominant. If such a hybrid is mated with an individual of the recessive type, 2, the offspring will consist of dominants and recessives, in about equal num- bers. If two such hybrids are mated, j, the offspring will show both dominants and reces- sives, in the proportion of three to one. This reappearance of recessives in the offspring of hybrids that seem to be dominants is called segregation, or splitting up 1. He crossed hybrids with plants of the yellow- seeded parent variety. 2. He crossed hybrids with plants of the green- seeded parent variety. 3. He crossed hybrids with hybrids. The results of these crosses are indicated in Fig. 238. This fact of splitting up into the two ancestral types has been found to be quite general among all plants and animals that have been tested, and it is called the Law of Segre- gation. The idea is that the hybrid plant, no mat- ter how much it may re- semble one of the parents (with respect to one or more particular charac- ters), does not constitute a pure kind of organism, inasmuch as it cannot reproduce itself in off- spring all having the same character. The plants resulting from the mating of hybrids (that is, the segregated yellow-seeded and green-seeded individ- uals) were experimented with HEREDITY 447 further, and this remarkable discovery was made : the green-seeded individuals, whether mated with one another or with green-seeded individuals of the original stock, always produced green- seeded offspring. In other words, al- though they had hybrid parents with yellow seeds, they themselves were pure in the sense that they reproduced or transmitted the green-seeded char- acter to their offspring in exactly the same way as their pure green-seeded ancestors. This principle has been found to hold true in all cases where experiments with plants and animals showing alternative pairs of characters have been carried far enough. An in- dividual of hybrid parentage having a recessive character is called an extracted recessive, or the character in question may be spoken of as the extracted re- cessive. It is just as pure with respect to that character as an organism can be. On the other hand, the yellow- seeded offspring of the yellow-seeded hybrids turned out to be of two kinds: (i) those that produced only yellow seeds in subsequent generations, pure like the yellow-seeded ancestor; (2) those that behaved like their hybrid and both showing the dominant character, are mated, they may give rise to three kinds of offspring. The germ cells given off by A are of two kinds, «3 and rz4 having the factor for dominance, while a^ and (1% are lacking in this factor. In the same way, the germ cells given off by B are of two kinds. Now there are four possible combinations of two kinds of eggs with two kinds of sperms : (i) a recessive egg combines with a recessive sperm ; (2) a recessive egg combines with a dominant sperm ; (3) a dominant egg combines with a recessive sperm ; (4) a dominant egg combines with a dominant sperm. The result is that half the offspring are again hybrid and the other half pure, and the pure are likely to be dominants and recessives in equal numbers. Note that the hybrids resemble the dominant grandparent, giving the appearance of one recessive to three dominants FIG. 239. The Law of Segregation A hybrid individual produces germ cells of two kinds with respect to the character in question, — one bearing the elements needed to bring about the development of the dominant character, and the other kind lacking this element. If two individuals, A and B, both hybrid 448 ELEMENTARY BIOLOGY parents, that is, split up again into dominant-appearing and recessive- appearing individuals in the proportion of three to one. The two classes are produced in the ratio of one pure dominant to two mixed or hybrid, although the two classes may have the same appearance. The simplest explanations for these mathematical relations between the char- acters is to be seen from the diagram in Fig. 239. 472. Law of Unit Characters. In the meantime we must not overlook the fact that every organism is made up of many characters. After finding out that his peas behaved according to the law of dominance and according to the law of segregation with respect to several of the pairs of characters, Mendel went farther and studied the results of his crossings to find out the behavior of combinations of characters. For example, suppose that two plants used in an experiment differed with respect to height as well as with respect to seed color, what would fe faQ results ? Experiments showed that the tall, green-Seeded parent trans- mitted the tallness as a dominant and the greenness as a recessive. In other words, if a tall green was crossed with a short yellow, the next generation appeared to be all tall yellow, resembling one parent altogether in one character, and the other parent altogether in the other character (see Fig. 240). The conclusion from this experiment, and many others like it, is that each pair of dominant-recessive characters behaves according to the two laws described above, without regard to what other characters may be present. This general fact has been found to hold for many species of plants and animals FIG. 240. Inheritance of two or more characters The offspring of two parents, A and B, resembles both par- ents, but it does not, as a rule, stand midway between the parents with respect to the several characters. Instead, the offspring will be like one parent in some characters and like the other parent in other characters HEREDITY 449 that have been used in experiments, and it is known as the Law of Unit Char- acters (see Fig. 241). This principle will help us to understand how there can be such great diversity among the individ- uals of any given species of plants or animals, or even among the brothers and sisters of any family. The greater the number of char- acters, the greater is the possible number of combinations; and the smaller is the chance of any given combination occur- ring again. These three laws of heredity — domi- nance, segregation, and unit character — are known as the Mendelian laws, or principles, because they were first dis- covered by Gregor Mendel. They have been found sufficiently reliable to serve as a basis for practical work of great impor- tance in connection with the breeding of plants and animals. FIG. 241, The law of unit characters illustrated by guinea pigs Pigmentation in these animals is dominant over albi- nism. Short hair is dominant over long hair. Rough coat is dominant over smooth coat. When two pure individuals like those shown are mated, the offspring will be short-haired, dark, and rough-coated. On mating the hybrids together in sufficient numbers, the segrega- tion will result in producing every combination of these three sets of characters : dark-short-rough ; dark-short- smooth ; dark -long-rough ; dark-long-smooth ; white- short-rough ; white-short-smooth ; white-long-rough ; white-long-smooth. The proportions will be such that for each pair of contrasted characters there will be one recessive to every three dominants. (From photographs lent by Professor W. E. Castle) CHAPTER LXXXI APPLICATIONS OF THE PRINCIPLES OF HEREDITY 473. Applied Mendelism. In the region about Pullman, Washington, which is one of the best wheat-growing countries in the world, the farmers had for years tried out many varieties of wheat in order to decide which was the most profitable to grow. They found only one variety that was at all satisfactory, and that had serious faults. This variety was known as the " Little Club " and had the advantage over others that the straw was strong enough to withstand the summer storms and that the head remained closed after the grain was ripened, thus preventing loss before harvesting. The one great draw- back of the Little Club wheat was the fact that when planted in the fall, it would sometimes be frozen during the severe winters (once in about every three or four years) ; and although the farmers could get a better crop by planting in the fall, they could not afford to lose every third or fourth planting. The prob- lem was, therefore, to combine the good stem and head qualities of the Little Club with the frost-resisting qualities of some other variety. Mr. W. J. Spillman, of the United States Depart- ment of Agriculture, at that time agriculturist of the experiment station at Pullman, began a series of experiments in crossing, or hybridizing, the Little Club wheat with other varieties. He found that whichever plant (variety) was used as the pollen parent, the next generation always showed the same kinds of combinations of qualities. This is in accordance with what we have learned as Mendel's Law of Dominance. He also found that in the offspring of the hybrids every possible combination of characters shown by the grandparents occurred. This is in accordance with the Law of Segregation. 450 APPLICATIONS OF PRINCIPLES OF HEREDITY 451 By selecting individuals in this third generation and grow- ing from them, and by keeping records of their behavior, he succeeded in establishing strains that transmitted the desired combination of characters. This is in accordance with the Law of Unit Characters (see p. 448). In this way it was possible to combine in one variety of wheat the strong stem, the closed head, and the winter-resisting qualities needed for successful wheat farming in this region. By similar methods it has been possible to combine three or more characters desired in a plant or an animal from as many different varieties of ancestors. 474. Breeding for immunity. The chief problem of those who have to do with plants and animals is to get organisms that combine desirable qualities and show none of the unde- sirable qualities. Thus, there is the American brand of cattle, raised for beef on the large prairie ranches ; this has good beef qualities and is very easily handled in large herds. But, unfortunately, most of our cattle are very susceptible to the destructive Texas fever, which has caused the loss of millions of dollars' worth of cattle. It had been observed that the so-called Brahmin cattle of India were immune to the Texas fever. On mating an immune animal of this breed with one of the susceptible varieties, it is found that the immunity is dominant. A number of years ago a herd of the Brahmin cattle was imported into this country for the purpose of cross- ing with our native cattle, in order to establish a variety that would have the beef qualities of the American cattle and would at the same time be immune to the Texas fever. This undertaking seems to produce successful results.1 1 In the meantime it has been found out that the Texas fever is transmitted by a little animal known as the tick, which sucks the blood from the diseased cattle. By suitable quarantine it has been possible to restrict the Texas fever ; and by applying to the bodies of the cattle something that will either kill the ticks or prevent their biting the cattle, it may be possible to eradicate this costly disease. But if we could replace our present herds of cattle with a type that is quite immune, the added cost would no doubt be made up in a very short time. 452 ELEMENTARY BIOLOGY Immunity to disease is not always dominant. In the case of wheat, immunity to the rust is recessive. It is nevertheless possible to establish strains of wheat that combine immunity with other desirable qualities, since, as we have seen, it is only necessary to breed the hybrids into the next generation in order to get a complete segregation of the various characters in all possible combinations, and then select in a third genera- tion the offspring that have the desired character in a pure dominant or pure recessive condition. 475. Breeding for special points. Those who have to handle cows or sheep often find the presence of horns in these animals a nuisance. Many farmers therefore take steps to prevent the development of the horns by destroying the " button " in the young animal by means of alkali or other chemicals. Occasion- ally, however, animals have appeared that failed to develop horns at all. When such a naturally hornless animal is mated with one that has horns, the offspring is found to be without horns. In other words, the polled, or hornless, condition is dominant. It is therefore possible to establish breeds of cattle that never produce horns, and this has actually been accom- plished by breeders. In raising sheep certain kinds of fleece are found to be more profitable than others. In order to combine the merino wool with hornlessness, for example, it would be necessary to find out by means of breeding experiments which characters were dominant and which recessive, and then establish in three generations new breeds possessing the desired characters. 476. Practical breeding. The failure of their hybrids to breed true has been the despair of agriculturists and breeders in the past. Only those who were patient enough to try out large numbers and be content with a small percentage of successes, or only those who, like Luther Burbank, were keen enough to detect the rare individuals that would breed true, were suc- cessful. With the discoveries of the biologists, it is possible for every intelligent fancier of plants or animals to produce APPLICATIONS OF PRINCIPLES OF HEREDITY 453 new varieties of organisms without limit, having almost any combinations of useful or fancy characters that he may desire. This does not mean that new characters are produced by these methods. When Burbank produced a "white black- berry " he did not get a plant with a new character, in the FIG. 242. Spineless cactus (Opuntia) This variety was established by Luther Burbank through experimentation. It grows in arid soil that is otherwise useless, and promises to become a valuable fodder for horses and cattle. (From photograph lent by Mr. Burbank) biological sense. He combined a plant -having pale yellow berries, of no value as fruit, with one having large, black berries, — the Lawton blackberry. From the hybrids he was able to select for segregation and for ultimate fixation the indi- viduals that combined pure lack of pigment with some of the other desirable qualities. Another new creation of Burbank's is the spineless cactus (see Fig. 242). Many of the new varieties of plants are hybrids that are incapable of breeding true. These plants are propagated by means of cuttings or 454 ELEMENTARY BIOLOGY grafts, or by means of bulbs or tubers. The Burbank potato, for ex- ample, which is the best potato grown in this country, is propagated by means of the tuber. Seedless varieties of grapes, apples, oranges, plums, and so on would be propagated by grafts or cuttings ; and hybrids of various kinds of fruits, although they may have seeds, Dr€) CrrO' BjO died of heart both affected disorder FIG. 243. Heredity of human traits (Squares represent males ; circles represent females.) /, pedigree of a family showing artistic (dark upper portion), literary (right section), and musical (left section) ability; 2, family with digestive troubles ; j, family with heart disorders ; 4, family with feeble- mindedness (/, died in infancy; ?, uncertain mental condition). (/, 2, and 3, after Davenport ; 4, after Goddard) would be propagated in the same way. From what we know of the fact of segregation, we can understand how unreliable the seeds of most common fruits are likely to be, when we consider that most seeds are probably hybrids. 477. Heredity in man. So far as reliable facts are available, heredity seems to follow the same course among human beings as among other organisms. Human beings take a comparatively long time to mature, so that in order to get complete records for many generations APPLICATIONS OF PRINCIPLES OF HEREDITY 455 it would be necessary to go back several centuries ; and such records were not kept in so remote a period. With annual plants and many animals it is possible to get a generation each year, and with some organisms several generations in a year. The number of offspring in human matings is comparatively small, so that it is never possible to get even an approxi- mation of all the character combinations in any one family. Experiments are, of course, out of the question. Finally, what we call the human race is really a mixture of many distinct types or combinations of characters, and these are so thoroughly mixed up that we never find a pure race of human beings at the present time. Nevertheless, by comparing such family records as are avail- able with the behavior of various characters in the pedigrees of plants and animals, it has been possible to show that many human characters follow the same hereditary principles of dominance, segregation, and recombination. The diagrams in Fig. 243 show the course of certain characters in several carefully studied cases. The following table gives a list of human characters that are known to be dominant or recessive : HEREDITY IN MAN DOMINANT CHARACTER RECESSIVE CHARACTER Curly hair Dark hair Beaded hair Brown eyes Normal pigmentation Hapsburg lip Normal muscular tone Nervous temperament Fused fingers or toes Supernumerary digits Broad fingers (lacking one joint) Limb dwarfing Normal growth Straight hair Light ; red Even hair Blue eyes Albinism Normal lip Low muscular tone Phlegmatic temperament Normal digits Normal number Normal length Normal proportion General dwarfing 456 ELEMENTARY BIOLOGY The application of our knowledge of heredity to human affairs will probably be along the line of showing us what types of marriages are likely to produce offspring that are undesirable in one way or another. We already know that certain abnormalities of physical structure or of mentality are transmitted in a definite way, and we are therefore warranted in counseling men and women who belong to families that show these characters not to marry others of similar stock. In the course of time we shall no doubt develop certain standards of fit- ness for marriage which will be enforced largely by the same kind of public opinion and tradition as now distinguishes the customs of different peoples. CHAPTER LXXXII HEREDITY AND PROTOPLASM 478. What is inherited? It is common to speak of the inheritance of characters as though something passed on from parents to offspring. But a moment's thought will show that nothing is transmitted in the ordinary literal sense. What is really meant by saying that a plant or animal has inherited certain characters from his parents is that there is something in the fertilized egg that makes possible the development of those characters, and whatever is in the egg must have come from the gametes, and so, presumably, from the parents. 479. Nuclear division. A study of cells shows that the nucleus contains a tangle of substance which behaves in a very definite way before cell division takes place. This substance is called chromatin because it readily absorbs various anilin dyes and thus appears highly colored, in contrast to other portions of the cell, when looked at under the microscope after treat- ment with the dyes (Fig. 244). Just before the cell divides, the chromatin breaks up into separate bits of chromatin called chromosomes, which means " color-bodies." Each chromosome splits lengthwise into two pieces, and one of these pieces goes into one of the two new cells (see Fig. 244). In this way each cell as it divides distributes one half of its chromosomes to each of the two daughter cells. Thus it comes about that each cell has exactly the same number of chromosomes as the other cells in the body. 480. Formation of germ cells and zygotes. This process of nuclear division goes on as the developing organism continues to grow by the production of new cells. In the formation of 457 458 ELEMENTARY BIOLOGY germ cells, however, the chromosomes divide in a different way. In a cell that is to form an egg cell half of the chromo- somes separate out ; the remaining chromosomes then split 4 5' 67 FIG. 244. The nucleus in cell division /, Diagram of a cell with the chromatin in a tangle, or network ; before cell division the chromatin assumes the form of a continuous thread. The thread breaks up into a definite number of pieces, or chromosomes, 2 ; the number of these is constant for any given species of plant or animal. The chromosomes arrange themselves in a central ring, j ; the membrane inclosing the nucleus disappears ; fine threads appear to connect the chromosomes with tiny bodies at opposite ends of the cell. Each chromosome splits in two lengthwise, 4. The members of each pair move from each other to opposite ends, 5. The half-chromosomes form two new tangles, 6, and gradually lose their definition. The new masses of chromatin become the nuclei of two new cells, 7 lengthwise (see Fig. 245). As a result of these two divisions, which usually follow each other in rapid succession,1 the egg cell contains one fourth of the chromatin present in the cell from which it was formed, and only one half of the usual number of chromosomes. The division which separates the 1 It is believed that in some cases the splitting of the chromosomes takes place before the separation into the reduced numbers. HEREDITY AND PROTOPLASM 459 chromosomes into two groups is called the reduction division, since it reduces the number of chromosomes. In the formation of sperm cells also a reduction division occurs. But instead of producing polar bodies the sperm mother cell forms four sperm cells. The polar bodies formed by the maturing of the egg cell die and disappear. When a sperm cell unites with an egg cell in fertilization, the resulting zygote contains the full number of chromosomes, P PS FIG. 245. The formation of an egg cell The chromatin material of the nucleus network, /, arranges itself into a definite number of chromosomes, 2, which divide up into two equal groups, j. Half of the chromosomes are pushed out of the cell, 4, and form the first polar body,/. The chromosomes of the polar body, as well as the chromosomes remaining in the mother cell, split lengthwise, and half of each chromosome is pushed out, j. The first polar body thus becomes two bodies, p^ and /2; and the mother cell puts out a third polar body, /3, retaining half the original number of chromosomes. This cell is now the egg cell half derived from the male parent and half from the female parent. From all the evidence that is now available it would seem that the chromosomes are the features of the germ cell which bear whatever it is that determines the development of the characters that distinguish the individual from others of the same species, and at the same time those characters that identify it with others of the same species. 481. The germ plasm and acquired characters. According to August Weismann (1834-1914) each organism is what it is because it developed from a certain germ plasm. When this organism produces new germ cells, it merely transmits 460 ELEMENTARY BIOLOGY to these cells some of the germ plasm from which it had itself developed. In other words, the eggs produce the organism, not the organism the eggs (see Fig. 246). According to this notion it would be impossible for the ex- perience of an individual to influence the germ cells in such a way as to make the offspring show the effects. For example, the result of exercise or of mutilations or of sickness should not appear in the following generation. As a matter of fact we have no evidence whatever that modifications produced in FIG. 246. The idea of the germ plasm When a fertilized egg, g, develops into a new individual, 3, part of the protoplasm becomes the body, or soma, and part remains germ, within the body, where it is nurtured. The germ is not a product of the body in any sense. Each body, b-±, b^, b%, is a branch, or development, of the germ, but the stream of germ material is continuous. The nature of the germ determines the kind of individuals or persons that will develop ; the body does not influence the germ an individual in the course of his lifetime are ever reproduced in the offspring, although you will find many people who firmly believe that such modifications are actually transmitted. 482. Sports. The appearance from time to time of an un- usual kind of individual that the breeders and horticulturists call a sport would suggest that germ plasm may undergo important changes. There appeared on a farm in Massachu- setts, in 1791, a queer sheep with a long body and very short, crooked legs. This " ancon " ram was kept for many years, and had many offspring with normal sheep. All the hybrids showed the same curious character. This "turnspit" type of rubri- lamarcki- scintil- gigas nlbida oblonga. nervis ana nanella, . lata lans 176 8 14,000 60 73 i 10,000 3 4th Generation 1895-1896 1 3d Generation 1890-1891 2d Generation 1888-1889 1st Generation 1886-1887 FIG. 247. Mutation in evening primrose In 1886 Hugo de Vries gathered the seeds from a number of evening primroses (CEnotliera lamarckiana) which he found growing in a vacant lot. These plants are natives of North America, and probably escaped from some Dutch garden. From seeds thus obtained about 15,000 ordinary lamarckiana plants were grown, and in addition five peculiar dwarfed individuals (nanella) and five with broad leaves (lata). These two sets were different from the parents as well as from any other known varieties of CEnothera. Later these new forms appeared again in very small numbers from very large sowings of lamarckiana seeds (third and fourth generations). In the third sowing (1890) another new form appeared, having reddish veins (rubrinervis) . In the same manner new types have appeared in each succeeding generation, and some of these types appeared anew several times. By means of careful breeding it was possible to preserve the new quali- ties, and to recombine them with one another and with the contrasting characters of the parent type according to the Mendelian principles 462 ELEMENTARY BIOLOGY animal is sometimes found among dogs. Peacock fanciers sometimes find a single bird with plain black plumage. Several times whole flocks of such birds have been established from a single freak mated with the normal type. 483. Mutations. In more recent years special attention has been given to these sports by many biologists, and the Dutch botanist Hugo de Vries has developed a theory to account for the origin of new kinds of plants and animals, based on the fact that such freak individuals are often able to establish dis- tinct lines of descendants. De Vries has himself cultivated many lines of new plants that originated in this sudden manner from various wild and cultivated species. Such suddenly arising de- partures from the ancestral type are called mutations, and the individuals bearing these characters for the first time are called mutants. De Vries's mutation theory does not attempt to ex- plain how it is that such plants or animals originate ; it only tries to show how such mutations may lead to the establishment of new races of organisms (see Fig. 247). 484. Origin of new characters. There are a number of experiments that throw some light on the origin of mutations. One of the best known is that by W. L. Tower, an American biologist, who subjected the eggs and larvae of potato beetles to various unusual conditions of temperature, moisture, and nutrition. The individuals that developed under these extreme conditions showed no evidence of having been mistreated. But some of the offspring of these beetles departed in a marked way from the usual appearance of their ancestors. In the Frontis- piece are shown some of the new types of beetles that appeared in Tower's experiments. It is supposed that the conditions to which the developing beetles were subjected had no direct effect upon the bodies of the animals, but did have an effect on the chromosomes of the cells that later became the germ cells. On mating some of these new types with the normal ones, it was found that there really were new characters, for it was then possible to establish lines that would breed true. CHAPTER LXXXIII EVOLUTION 485. All things change. We understand that we live in a world of change, that in fact all of our experience, all of our life, has to do with these changes. All theoretical studies are concerned with changes ; and all practical studies — agriculture and medicine, engineering and statesmanship — are concerned with three sets of problems. These are 1 . How can we cause desirable changes to take place ? 2. How can we prevent undesirable changes from taking place ? 3. How can we best meet the unavoidable changes? 486. Cyclic changes. Many of the changes that go on about us are of a cyclic nature ; that is, they keep on repeating them- selves. For example, the day gives way to night, but the night gives way to day again, and so on indefinitely. Our seasons illustrate cyclic changes. To a mosquito (an individual adult) weather may mean a continuous, progressive change from warm to cold, resulting in death and ending everything. The seasonal changes show us not only cyclic variations in temperature, moisture, etc. but cyclic changes in the organic world. Eggs hatch, individuals develop to maturity, reproduce themselves, and die. But the following year we see a repetition of the same life histories, and so on, generation after genera- tion. With some species of animals the cycle extends over many years, but the point is that, however far-reaching the indi- vidual development may be, it does not go on forever ; it comes to a close and is replaced by others that go through exactly the same kinds of stages. In other words, life forms or life stages 463 464 ELEMENTARY BIOLOGY repeat themselves, generation after generation, in the same sense as seasonal conditions of weather repeat themselves. 487. Progressive changes. While much of the happening in the universe is of a cyclic nature, there may nevertheless be, for the world as a whole, a certain continuity of change that has been called evolution. We need not feel called upon to prove that all changes are altogether cyclic, or that all changes are altogether evolutionary. It is quite reasonable and consistent to recognize that both kinds of changes do actually take place. 488. Fossil evidences of evolution. How can we tell whether the plants and animals of past times were different from those of to-day ? The most direct evidence is furnished by the fossilized remains of ancient plants and animals. Some two hundred years ago people became interested in hard coal as a fuel ; and in the digging of the coal, and in the digging into the earth alongside of the coal seams, they came across structures that in many ways suggested plant forms. Later they also found stony structures that very decidedly suggested animal forms. A study of these structures naturally led to an attempt to classify them and to compare them with existing plants and animals. These classifications lead to finding many resem- blances between the organisms of the past and the organisms of the present, but they also brought out marked differences. Moreover, by arranging the series of fossils according to their relative ages (which can be judged by their relative positions in the layers of rocks) it was found possible in many cases to show that the forms which were intermediate in age were also intermediate in structure between the most ancient and the most recent (Fig. 248). One of the best examples of this is presented by the horse and his probable ancestors (see Fig. 249). Similar series of fossils have been worked out for the elephant in Africa, for various fishes in England and else- where, and for many lines of birds and reptiles in all parts of EPOCH SYSTEM Pre- Archaic Cambrian FIG. 248. Geology and evolution Man Mastodon Univalves Conifers (Angiosperms) (Tapir) Nummulite (Birds) Univalve (Palms) Pearl mussel Bivalves Ammonites Crustacea Cycads Winged reptiles Crinoid Gryphaea Crocodile Bivalves Lamp shells Insects Amphibia Tree ferns Ganoid fish Starfish Trilobites Sponge spicules Graptolites of jellyfish In the oldest rocks no remains of plant or animal life are to be found. In each succeed- ing age of the earth more and more highly developed plant and animal forms lived, as shown by the remains of the organisms preserved in the rocks 466 ELEMENTARY BIOLOGY the world. In Germany there has been found a remarkably complete series showing successively different types of snails leading down to the present-day forms. Without regard to the question as to how it comes about that descendants do differ from their ancestors, there can be but one reasonable explana- tion of the facts, — namely, that there has been modification of plants and animals in the course of their descent. Protorohippus Eocene Mesohippus Oligocene Protohippus Miocene Equus Modern FIG. 249. Evolution of the horse Our ideas of the probable ancestors of modern animals are based on fossil remains. These represent real organisms that lived thousands and thousands of years ago. In the diagram the oldest type does not resemble the familiar horse very strikingly, but with each succeeding age we find animals having a closer resemblance to the horse of to-day. (After Osborn) 489. Evidence of evolution from structures of organisms. We have observed over and over again that individual plants and animals differ from each other, and that at the same time they resemble each other in groups. The members of a group that are sufficiently alike to be recognized by the casual observer we speak of as being of the " same kind." Thus, while no two oak trees are exactly alike, they are all sufficiently alike to be recognized as of the same kind. Now we expect, from our observations, that all the oak trees, however different they may be from each other, will give rise to new plants that will also be enough like the parents to be classed as oaks. And for the same reasons we take it for granted that all the oaks of to-day are related, in the same sense in which we speak of our cousins and second cousins as related. That is to say, EVOLUTION 467 we believe that they are descended from common ancestors. An attempt to classify existing plants and animals leads to an arrangement in which similar plants (or animals) are grouped together into species, or kinds, and these species are grouped together into large assemblages, and these into still larger, and so on, on the basis of resemblances. Now, since we assume relationship or common ancestors in proportion to similarities of structure,1 the classification suggests that if we go back far enough, we shall find that all birds are related (that is, descended from the same ancestors), and that if we go back still farther, we may find that birds and reptiles are all descended from common ancestors ; or, if we go back still farther, we may find that all backboned animals are descended from the same ancestors. We can find no reasonable explanation for this " branching- tree arrangement " of the different kinds of living beings, except the supposition that they have descended from common ancestors and have become modified in the course of time. 490. Evidence from development. In our study of develop- ment (p. 277) we saw that in the course of each individual's lifetime he passes through a series of more or less distinct stages ; and the farther back we go to the one-celled stage, the more and more are these stages like the corresponding stages of other species of organisms. Moreover, it has been pointed out that the similarities found between different species in the various early stages of development are in a measure parallel to the similarities of the adults or the groups. For example, the larvae of different kinds of mosquitoes are more alike than are the larvae of mosquitoes and beetles ; the larvae of insects in general are more alike than the larvae of insects and crabs ; and so on. And in the life history of a mammal there are 1 Most of our classification is necessarily based upon structure. A com- parative study of the structure of organisms — the branch of science known as morphology — shows us similarities in detail of structure that are even more remarkable than the superficial resemblances that are obvious to the casual observer. 468 ELEMENTARY BIOLOGY suggestions of structures found in the life history of birds and of fishes. Now the only explanation of these facts that appears at all reasonable is that there is a common (or, similar) develop- ment just to the extent that organisms are related through descent from common ancestors. 491. Vestigial structures. Another line of evidence is found in the presence, among plants and animals, of certain organs that FIG. 250. Vestigial structures The vermiform appendix, a, in some mammals is reduced to an insignificant trifle, as in man, / ; whereas in other mammals, as in some of the rat family, 2, it is capable of holding a considerable amount of food in the process of digestion. The horse walks on his third toe, j, the others being entirely absent or represented in part by the reduced " splints," b are quite useless from the point of view of adaptation, but which are nevertheless persistent through whole groups. For example, the whale develops legs that are never used, and the same is true of certain snakes. The skeleton of many a bird shows dis- tinct signs of fingers, or claws, among the wing bones.1 Other examples can be readily understood if we suppose that all plants and all animals are related through having had common ances- tors ; but they cannot be understood on any other supposition. 1 The vermiform appendix (see /, Fig. 28) in man is the lingering reminder of an organ that developed and functioned in other backboned animals, but that has no practical meaning in the life of man to-day — except to make trouble sometimes. EVOLUTION 469 492. Geographic evidence. We expect every group of organisms to expand its range just as far as conditions permit, and we rather expect a given kind of situation to maintain one kind of population and a different kind of situation to maintain a different kind of popu- lation. Yet when we examine the distribution of species over the surface of the earth, certain curious facts appear. In the first place, we find regions in every way similar, so far as climate, soil, etc. are concerned, inhabited by totally different plants and animals. Thus, the climate of Australia is not very different from that of most of Europe and large parts of Africa, Asia, North America, and South America ; but when Europeans first came there, they found plants and animals that are not found living naturally in other parts of the world. The same kinds of facts are found in abundance on comparing many regions with one another. In the second place, we find regions that are very different occupied by forms of plants and animals that are sufficiently similar to be con- sidered as of the same families. Thus, goats and sheep, obviously related to each other genetically, are found in the tropical zones as well as in the temperate, and well up to the arctic and antarctic circles, living in many varied kinds of surroundings.1 Darwin pointed out that where we have similar regions occupied by different flora and fauna, these regions are always separated from each other by impassable barriers, as oceans, mountain ranges, deserts, etc. On the other hand, where we find similar plants and animals inhabiting regions that are markedly different in their climate, soil, etc., these regions are connected directly or show evidence of having been connected in the past. For example, the plants and animals found in oceanic islands are frequently quite distinct from those found elsewhere, but they are also as a rule closely related to the inhabitants of the nearest mainland. Facts of this kind can easily be explained by the assumption that all the organisms are derived from ancient forms, with modifications, and they cannot be easily explained in any other way. 493. Summary. The evidences for organic evolution or for the descent of plants and animals from common ancestors, with divergence from ancient types, are (i) palaeontological, the 1 Of course we are now speaking of the natural range of various wild species. 470 ELEMENTARY BIOLOGY evidence of fossil remains; (2) systematic and morphological, the evidence from the structure of plants and animals ; (3) em- bryological, the evidence from development ; (4) anatomical, the evidence from rudimentary, or vestigial, organs; (5) regional, the evidence from geographic distribution. The important thing to note is that all the facts in these several groups of facts can be harmonized by the idea of evo- lution and by no other explanation. There are many attempts to explain how evolution is brought about, and these may be called theories of evolution ; but as to the fact of evolution biologists are in substantial agreement. CHAPTER LXXXIV APPLICATIONS AND THEORIES OF EVOLUTION 494. Direct evidence of evolution. Within the memory of men and women now living there have appeared new varieties of potatoes, apples, plums, oranges, sheep, horses, rabbits, poultry, cats, dogs, walnut trees, wheat, and so on. And the new varieties of plants and animals are, at least in many instances, just as truly new species as any that occur in nature. It is true that very often these new species have arisen under artificial conditions ; but it is also true that there is nothing in these conditions that may not occur in nature, except the protection of the new forms from early extermination. We have, then, not only indirect evidence that evolution has taken place, but direct evidence that plants and animals can behave in agreement with the assumption that evolution did take place. In recent times — within a century and a half — there have been many attempts to formulate a theory to explain how evolution takes place. These theories may be grouped accord- ing to their family resemblances, but there are only three or four types of theories that are at present worth considering. 495. Lamarckian theory. The French zoologist Lamarck (1744-1829) laid emphasis on the fact that the development of many organs is influenced by their activity, and on the fact that many organisms (particularly animals) adjust them- selves to their surroundings in the course of their lives. He came to the conclusion that "all that has been acquired, begun, or changed in the structure of an individual in the course of its life is preserved in reproduction and transmitted to the new individuals which spring from that which has experienced the change." 471 472 ELEMENTARY BIOLOGY With our present knowledge of physiology and heredity, most of Lamarck's theory is seen to be unsound. Notwith- standing all the evidence to the contrary, however, many people still believe that evolution (and human progress) takes place through the accumulation of the results of experience in the course of generations. 496. Selection theory. The theory of natural selection is associated with the name of Charles Darwin (1809-1882), but it was also formulated independently by Alfred Russel Wallace (1823-1913) and Herbert Spencer (1820-1903). This theory is that animal and plant species evolve through the selective action of the environment, which kills off those individuals in each generation that are least adapted to the conditions of life, — a process resulting in the "survival of the fittest." The theory rests on the fact of variation (see pp. 437 ff.) and of overpopulation — that is, the fact that more individuals (eggs, seeds, spores, etc.) are born than can possibly reach maturity and reproduce themselves. It assumes that individual differences may be inherited, whereas the evidence shows that only certain kinds of differences are transmitted to offspring. The theory assumes that the agencies which kill off so large a proportion of each generation are selective, that is, really act upon important individual differences, whereas we know that at every stage of development plants and animals are killed off by agencies and forces and accidents that make no discrimination whatever between the fit and the unfit. Many studies and experiments in recent years have thrown doubts upon the theory of natural selection. It does not account for the origin of new characters, and we know that there is a limit to the improvement that may be brought about by artificial selection. There is at least this much truth in the doctrine, however : plants and animals that are unsuited to the conditions about them, for whatever causes, are not likely to leave similar progeny. Fitness is a requisite for all life. APPLICATIONS AND THEORIES OF EVOLUTION 473 497. Mutation theory. Charles Darwin had collected many examples of sports that occurred in various crops or herds, but he supposed these to be so exceptional that he did not consider them seriously as the material upon which selection operates in the formation of new species. But Hugo de Vries has emphasized just this class of facts. We saw (p. 462) that he had made direct observations as to the appearance of muta- tions among plants kept in his gardens and greenhouses, and we saw also that mutations are capable of transmitting their peculiarities to their offspring. The mutation theory of evolution declares that selection can establish new species only if there first appear individuals with heritable qualities that are distinctive. It is not claimed that the mutants have advantages over their parental type, although they may have ; it is sufficient for the theory if the new types are capable of living and of establishing themselves. This theory of evolution, and all the other newer theories, are closely connected with the study of heredity and are supported by the results of experiments. 498. Applications. It should certainly make a practical dif- ference to us which of the many theories of evolution is proved to be true. Suppose we were convinced — as many people are — that the gains and losses of the individual organism affect the constitution of the offspring. Would that not make a dif- ference in our handling of our crops and our domestic animals ? Would it not make a difference in the way we conduct human affairs ? We might then believe that the son of a criminal must be a criminal, or that the son of a judge must be righteous. We might make our laws much more rigorous for the chil- dren of evildoers, and much more lenient for the offspring of good citizens. Or, suppose we were certain — as many people are — that the selection theory is true. Then we should follow the recom- mendations of those who tell us not to build hospitals for the sick, but to let them die — or survive. This recommendation is 4/4 ELEMENTARY BIOLOGY made on the supposition that by letting disease kill whom it will, we shall at last have a population that is immune to all diseases. The same people would do nothing to mitigate what they call " the struggle for existence," for they believe that this struggle is necessary to bring out the best qualities of the race and to prevent the multiplication of the unfit. With the discoveries of recent years regarding the facts of heredity, our whole view of evolution has undergone important changes. For one thing, " struggle for existence " no longer suggests the fierce competition between individuals of a species that it formerly suggested. In the second place, the survival of the fittest can be seen to add nothing new to the composi- tion of a line of plants or animals. In the third place, the characters that distinguish one race or variety from another need have absolutely nothing to do with being better fitted to live. And, finally, we may think of the progressive changes in species as resulting from the successive modification of the germ plasm, with the elimination of those resulting forms that are not livable. We have seen that the application of modern knowledge about the evolution of organisms has increased our wealth incalculably by establishing varieties of plants and animals that are more resistant to disease or to other unfavorable con- ditions and by establishing varieties that bear more abundantly of those materials for which we care, — for example, more wool in the case of the sheep, more sugar in the case of the sugar beet, and so on. In similar ways the solution of the problems of evolution must continue to contribute to human welfare on the economic side, and probably also on the social and moral side. PART VI MAN AND OTHER ORGANISMS CHAPTER LXXXV THE CLASSIFICATION OF ORGANISMS 499. Scientific classification. Apart from the fact that many people derive satisfaction from collecting and sorting various classes of objects, classification is of value because it facilitates the work of reference. Just as the classification of the books in the library makes it possible to find a particular book, or a particular kind of book with the least effort, so classifying plants and animals furnishes a convenient scheme for placing each specimen where it belongs. In recent times the study of classification has acquired new mean- ing because of the light it throws on problems of evolution and because of its aid to the study of heredity. Every scheme for sorting things must provide a way of bring- ing together plants or animals that are truly related to each other, and it must at the same time avoid bringing together, because of superficial resemblances, plants and animals that are not related. 500. The basis of classification. If we should sort our books according to size or color of binding, we should often bring two books on Mexican history together ; but we should be just as likely to bring together a book on Mexican history and one on astronomy, and we should be sure to separate books that really belong together. In the sorting of plants and animals it is necessary to find a basis that will secure the desired results. 475 4/6 ELEMENTARY BIOLOGY The structure of organisms furnishes the basis for modern classifications, but the word structtire has a wide significance. According to outward appearance we might place certain small snakes with certain large worms ; but a study of the internal structure at once separates them very widely. Again, the appearance of certain caterpillars is much like that of certain worms, but a study of the structure at various stages — that is, the development — at once separates these two groups. Modern classification of organisms accordingly considers all that can be known about the living things, and not merely their appearance or their uses. CHAPTER LXXXVI KINDS OF PLANTS 501. Higher and lower plants. We often speak of a given kind of plant or animal as being higher or lower than another kind. What we usually have in mind in making this distinction is the fact that some organisms are simpler and others more complex in structure. A dandelion is higher than the taller willow, and the willow is higher than the pine, for the same reason that we consider all three of these plants higher than a fern or a seaweed. Complexity of structure has to do with the number of dif- ferent parts or organs. Physiologically this corresponds to a greater division of labor. We may compare three plants to see the general differences as to structure and specialization of functions. On the vegetative side the Spirogyra cell may be considered a complete individual. It is capable of getting from the sur- rounding water all that it needs to keep it alive, at any portion of the surface. In a moss plant we may already see a division into rootlike part, stemlike part, and leaflike part. The photo- synthesis is carried on in one part of the plant ; the absorption of water and salts is carried on in another part. The stem is a connecting organ that holds up the leaves and also transports (or, rather, transmits) materials between the two other organs. In the bean plant we see a more complex root, with several kinds of cells (tissues), a more complex stem and a more com- plex leaf, both having several kinds of cells. The protective layers of cells are different from the supporting (mechanical) tissues ; these in turn are different from the conductive tissues and from the photosynthetic tissues. 477 4/8 ELEMENTARY BIOLOGY On the reproductive side we may see a similar advance in complexity from the lowest to the highest of these three plants. In the Spirogyra every cell may act as a gamete, after behaving for some time as a vegetative cell. In the moss certain special cells are borne, in special organs, in a special region of the vegetative plant. And the two gametes (male and female) are borne on two different individuals. In the bean the gametes are still more highly specialized, each being produced in a very simple plant that is parasitic upon the parent. But these two simple plants grow from very highly specialized structures (embryo sac and pollen grain) that are in turn borne on very highly specialized organs (pistils and stamens) which together form a structure (the flower) that is almost completely devoid of any vegetative behavior. In the flower we have an organ that is specialized for producing bodies which have to do ex- clusively with reproduction and the protection and distribution of the next generation. 502. The basis of classification. While it is possible to say in a general way that a given plant is higher than another, it is quite impossible to place all the known plants in a series from the lowest to the highest. This would be as absurd as trying to arrange all people in a series from the " worst " to the " best." We find that there are several main divisions, some of which we should place higher and some lower. But we find in each division so many degrees of complexity that there is consider- able overlapping when it comes to arranging all the plants. The first separation that we can make is one between plants that bear seeds and those that do not. The seed plants can be further divided into those that have closed carpels (pistils), like all the flowering plants, and those that have open or exposed ovules, like the cone-bearing plants. Among the non-seed-bearing plants there is a large group in which the egg cell is borne in a special organ, the arche- gonium (see pp. 320-321); this includes the ferns and their allies, the mosses and the liverworts. FIG. 251. Genealogical tree of plant life This diagram is intended to suggest the common origin of all plant forms, with the constant progressive departure from ancestral types, now in one direction and now in another, like the branching of a tree. Lower and higher mean nearer to or farther from the original types. The closer together two forms are on a given branch, the more closely related they are considered (cf. Fig. 252) 480 ELEMENTARY BIOLOGY Below the archegonium plants are all those that lack special- ized vegetative organs. Here are included all the seaweeds, the bacteria, and the fungi. 503. The main groups of plants. The chief groups of plants are indicated in the following outline : DIVISION I — THALLOPHYTES. Plants showing little or no differentiation into stem and leaf. A. Schizophytes ("splitting plants"). Each cell splits into two ; no other reproduction. 1. Cyanophyceae. Splitting plants with chlorophyl, — the blue algae. (Examples. Oscillatoria, Rivularia, Nostoc.) 2. Schizomycetes. Splitting plants without chlorophyl. This group includes all the bacteria. The distinction between having chlorophyl and not having chlorophyl sep- arates all the thallophytes into two main groups, the algae and the fungi. B. Algae. The chlorophyl-bearing thallophytes. 1. The green algae. Usually yellowish green. (Examples. Pleu- rococcus, desmids, Spirogyra, Vaucheria, stonewort, sea lettuce.) 2. The brown algae. (Examples. Bladder wrack, Laminaria, Sar- gassum, diatoms, sea palm.) 3. The red algae. Mostly marine ; reddish to purple. (Examples. Nemalion, Polysiphonia, Batrachospermum.) C. Fungi. Thallophytes without chlorophyl. 1. Phycomycetes. Alga-like fungi. (Examples. Water molds [often parasitic on fishes], phytophthora [the cause of the potato rot], grape mildew and other parasitic forms, black mold.) 2. Ascomycetes. Fungi bearing spores in sacs. (Examples. Yeast, cup fungi, the edible morel, the mildews, black knot.) 3. Basidiomycetes. Fungi bearing spores on outside of structure called basidium. (Examples. Rusts, smuts, mushrooms, pore fungi, shelf fungus, puffballs.) D. Lichens. These curious structures are compound growths of fungi and algae. The hyphae in these partnerships generally belong to ascomycetes; the algal partner is a green alga related to pleurococcus or one of the blue-green algae. (Examples. Rein- deer moss, Iceland moss, Spanish moss. The common names introduce the word moss, although these plants are in no way related to the mosses.) KINDS OF PLANTS 481 DIVISION II — BRYOPHYTES. Mosses and their allies. Archegonia but no vascular system. A. Liverworts. B. Mosses. DIVISION III — PTERIDOPHYTES. Ferns and their allies. Archegonia and vascular system; no seeds. (Examples. Club mosses, quillworts, scouring rushes (or horsetails), adder's-tongue, maidenhair.) DIVISION IV — SPERMATOPHYTES. Seed-bearing plants. A. Gymnosperms. Naked-seed plants. (Examples. Sago palm, ginkgo, yews, larches, pines, cypress, sequoia.) B. Angiosperms. Inclosed-seed plants. 1. Monocotyledons. (Examples. Cat-tail, water plantain, grasses, grains and sedges, palms, Indian turnip, rushes, spiderwort, lilies, bananas, orchids.) 2. Dicotyledons. a. Archichlamydeae. Flowers having no corolla or one of distinct petals. (Examples. Catkin-bearing trees (willows, wal- nuts, oaks, beeches), smartweed, pink family, buttercup family, water lilies, rose family, bean family, parsley family.) b. Sympetalae. Flowers having corollas in which the petals are united. (Examples. Heath family, primrose family, gentian family, mint family, morning-glory family, plantain family, madders, honeysuckles, composites,— daisy, aster, sunflower, goldenrod, etc.) CHAPTER LXXXVII KINDS OF ANIMALS 504. The classification of animals. Distinctions between higher and lower among animals are based on the same con- siderations as those among plants, — namely, complexity of structure and specialization of functions. The most striking division among animals is that between the vertebrates (animals having a backbone) and the invertebrates. The latter group is made up of many diverse types that have little in common except the fact that they are animals. 505. The main groups of animals. The chief groups of animals are indicated in the following outline : DIVISION I — PROTOZOA. The simplest animals ; body of one cell. (Ex- amples. Ameba, Paramecium, Vorticella, Plasmodium of malaria.) DIVISION II — PORIFERA (" pore-bearing " animals). This includes all the sponges. DIVISION III — CCELENTERATA. Radially symmetrical animals having a single cavity in the body ; all aquatic, mostly marine. Class i — Hydrozoa. (Examples. Fresh-water hydra, certain small jellyfish.) Class 2 — Actinozoa. (Examples. Sea anemones, most corals.) Class 3 — Scyphozoa. (Examples. Most of larger jellyfish.) DIVISION IV — FLATWORMS (Platyhelminthes). (Examples. Tapeworm, liver fluke, planarians.) DIVISION V — ROUNDWORMS (Nemathelminthes). (Examples. Hookworm, trichina, thorn-headed worm.) Many of these animals are dangerous parasites on man or on domestic animals. DIVISION VI — WHEELWORMS (Trochelminthes). The Rotifera, or wheel animalcules. Mostly microscopic. 482 anemone FIG. 252. Genealogical tree of animal life This diagram is intended to suggest the common origin of all animal forms, with the constant progressive departure from ancestral types, now in one direction and now in another, like the branching of a tree. Of course only the main branches are shown. There are probably over a million species of animals living to-day (cf. Fig. 251) 484 ELEMENTARY BIOLOGY DIVISION VII — ECHINODERMATA (" spiny-skinned " animals). Radially symmetrical, all marine. Class i — Asteroidea. Starfish. Class 2 — Ophiuroidea. Brittle stars. Class 3 — Echinoidea. Sea urchins. Class 4 — Holothuroidea. Sea cucumbers. Class 5 — Crinoidea. Sea lilies. DIVISION VIII — ANNELIDA ("ringed" animals). Wormlike animals with segmented bodies. The two most important classes are represented by earthworms, sandworms, etc. and by the leeches. DIVISION IX — ARTHROPODA ("jointed-legged"). The body segmented; exoskeleton. Class i — Myriapoda (" thousand-legged "). (Examples. Myriapods, centipede.) Class 2 — Crustacea ("crusty" shells). Head and thorax fused; water- breathers; antennae. (Examples. Lobster, crayfish, crab, shrimp, barnacle, sow bug.) Class 3 — Arachnida (spider family). Four pairs of legs ; air-breathers ; no antennae. (Examples. Scorpions, spiders, daddy longlegs, taran- tula, mites, ticks.) Class 4 — Insecta. Segmented bodies; distinct head, thorax, and abdo- men ; antennae, compound eyes ; three pairs of legs ; one or two pairs of wings (a few forms wingless); air-breathers. A list of the chief orders of this important class is given on page 487). DIVISION X — MOLLUSCA (" soft " animals). Unsegmented animals, most of them bearing shells. Class i — Gastropods (" belly-footed "). Having shells of a single piece. (Examples. Snails, slugs, periwinkle, whelk.) Class 2 — Pelecypoda (" hatchet-footed "). Bivalve (having shells of two valves). (Examples. Oysters, clams, piddock, scallop, mussel, shipworm.) Class 3 — Cephalopoda (ft head-footed "). The foot partly surrounds the head and has a number of arms, or tentacles. (Examples. Octopus, cuttlefish, squid, nautilus.) DIVISION XI — CORDATA. Animals having a notocord, or internal axial basis for a skeleton. It is from this structure that the vertebral column develops. There are a number of small animals which never develop a true backbone, but which nevertheless have a structure that suggests the beginning of such a column. These are included KINDS OF ANIMALS 485 among the cordata, although they are not strictly vertebrate. (Ex- amples. Acorn worm, lancelet, sea squirt.) The five important classes of vertebrates are Class i — Pisces (fishes). The stone hag and the lamprey are sometimes called fishes, though they are distinct in having a round mouth (no jaws) and no fins or scales. They never develop bones, the skeleton remaining cartilaginous. There are four orders of true fishes : 1. Cartilaginous fishes. Gill slits not covered; "skin teeth." (Ex- amples. Skates, torpedoes, sharks.) 2. Armored fishes (Ganoidei}. Large, bony scales in the skin, espe- cially about the head. In former times this order was very numerous. (Examples. Sturgeon and gar pike.) 3. Bony fishes (Teleostei\ (Examples. Salmon, herring, perch, cod, flounder, etc.) 4. Mud fishes (Dipnoi}. Fishes with lunglike structures. Only three living representatives, all in the southern hemisphere. Class 2 — Batrachians (amphibia). Breathe by means of gills in early stages, and later develop lungs. Bony skeleton with two pairs of appendages ; no exoskeleton. (Examples. Frog, toad, newt, sala- mander, mud puppy, hellbender.) Class 3 — Reptilia. Wholly air-breathers ; plates or scales in the skin. Four orders are usually recognized : 1. Chelonia. (Examples. Turtles and tortoises.) 2. Serpents. (Examples. Snakes, adders, cobras.) 3. Lacertilia. (Examples. Lizards, chameleons, horned toad, Gila monster.) 4. Crocodilia. (Examples. Alligators, crocodiles.) Class 4 — Aves (birds). Warm-blooded ; exoskeleton of feathers ; front limbs wings; tendency for the bones to fuse; air spaces in bones;' no diaphragm ; eggs with limy shells. Living species of birds may be divided conveniently into the running birds (ostriches, the casso- wary, and the emu) and the flying birds. The latter include two groups of orders, — the water birds and the land birds. Some of the important orders are 1 . Anseres. (Examples. Swans, ducks, geese.) 2. Longipennes. (Examples. Gulls, petrels, terns.) 3. Pygopodes. (Examples. Loons, grebes, auks.) 4. Heron order. (Examples. Storks, ibis, bittern.) 5. Plover order. (Examples. Snipe, curlew, rail, sandpiper.) 6. Gallinae. (Examples. Hen, turkey, guinea fowl, peacock, pheas- ant, partridge, ptarmigan.) 7. Columbae. (Examples. Pigeons, doves.) 486 ELEMENTARY BIOLOGY 8. Passeres. Perching birds ; includes about one half of our native birds. (Examples. Sparrows and finches, swallows, robins, thrushes, crows, etc.) 9. Raptores. Predatory birds. (Examples. Eagle, hawk, owl.) 10. Pici. (Examples. Woodpeckers, sapsuckers.) 1 1 . Cuckoo family (including kingfishers). 1 2. Whip-poor-will order (including humming birds). Class 5 — Mammalia (mammals). Warm-blooded ; hairy exoskeleton ; diaphragm ; suckle young. 1. Monotremata. Egg-laying mammals. (Examples. Duckbill, spiny anteater.) (With the exception of these two, all mammals develop the young within the body of the mother.) 2. Marsupials. Carry their immature young in a special abdominal pouch. (Examples. Kangaroos, wombats, opossums.) The rest of the mammals are divided into the following orders : 3. Edentata (" toothless " mammals). (Examples. Sloths, armadillos, hairy anteaters.) 4. Cetaceans. (Examples. Whales, dolphins, porpoises.) 5. Sirenia. (Examples. Sea cow, manatee, dugong.) 6. Ungulata (" hoofed " animals). a. Odd-toe. (Examples. Horses, zebras, rhinoceros.) b. Even-toe. (Examples. Ox, sheep, antelope, camel, giraffe, deer, pig, hippopotamus.) c. Proboscidea (elephants). 7. Rodentia (" gnawers "). The largest order. (Examples. Rabbits and hares, squirrels, chipmunks, porcupine, gopher, muskrat, rats, mice.) 8. Insectivora (" insect-eaters "). (Examples. Moles, shrews, hedge- hog.) 9. Chiroptera (" hand-wings "). (Examples. Bats, vampire.) i o. Carnivora (" flesh-eaters "). (Examples. Cat family, dog family, bears, weasel, seal, walrus, otter, mink, skunk, badger, raccoon, etc.) n. Primates ("the first," or leading, order of animals, including man). The families contained in this order are given in the list on page 488. 506. The orders of insects. The insects constitute the most numerous class of animals. The many species show adapta- tions to all sorts of conditions, and from a human point of view furnish us many friends as well as many enemies. KINDS OF ANIMALS 487 1. Aptera (" without wings "). The most primitive insects now living. (Examples. Silverfish and springtail.) 2. Orthoptera (" straight-winged "). Wings lying parallel with body or folding lengthwise; incomplete metamorphosis; biting mouth. (Examples. Locusts, crickets, walking sticks, katy- dids, cockroaches, mantis.) 3. Neuroptera (" netted-veined wings"). A large group broken up into several orders by entomologists ; complete metamorphosis ; biting mouth. (Exa?nples. Mayflies, dragonflies, termites.) 4. Hemiptera (" half-wings "). Basal part of wings often thickened and without distinct veining ; incomplete metamorphosis ; suck- ing mouths. All true bugs. (Examples. Squash-bug, bed-bug, water-bug, plant lice, scales, lice, cicada.) 5. Coleoptera (" sheath-wings "). The front wing a hard protective cover ; complete metamorphosis ; mostly with biting mouth. (Examples. Beetles, weevils, fireflies, ladybird, June-bug.) 6. Lepidoptera (f f scale-wings "). Rigid membranous wings covered with minute scales ; complete metamorphosis ; sucking proboscis. (Examples. All moths and butterflies.) 7. Diptera (" two-wings "). Hind wings reduced to tiny knobs, or " balancers " ; complete metamorphosis ; sucking or piercing mouth. (Examples. Mosquitoes, gnats, midges, house flies, stable flies, botflies, warbles, fruit flies. 8. Siphonaptera (" tube-wingless "). Sucking mouth, wings reduced ; complete metamorphosis ; parasitic on birds and mammals. (Examples. Fleas of all kinds.) 9. Hymenoptera (" membrane wings "). Complete metamorphosis ; biting or sucking mouth. (Examples. Wasps, hornets, bees, ichneumons, ants.) 507. The families of primates. The order Primates is no doubt of great importance in the world, since it includes the human species. From a zoological point of view, however, it cannot be considered the highest group of animals, since in many respects the skeleton, the muscles, and the teeth of man and the apes are not so highly specialized as are the corresponding organs of such animals as the whales or the elephants, for example. Nevertheless, the remarkable develop- ment of the nervous system among the primates entitles them to a distinctive place in the animal world. 488 ELEMENTARY BIOLOGY This order consists of the following families : 1. Lemuroidea. Small, squirrel-like animals living in trees and bushes. The lemurs are found in Madagascar, the marmosets in South America. 2. Cebidae. The New World monkeys. Nearly all have long, grasping tails and flat noses. Smaller than the Old World monkeys. (Examples. Howling monkey, spider monkey, capuchin.) 3. Cercopithecidas. The Old World monkeys. Tail not grasping, or short ; nostrils pointing downward. Distinct, opposable thumb. (Examples. Baboons, mandrill, macacus, Indian ape.) 4. Simiidae. The anthropoid (manlike) apes. No distinct tail ; arms longer than legs. (Examples. Gibbons, orang-utans, chimpan- zees, and gorillas.) 5. Hominidae. The human race. 508. Varieties of the human race. The so-called races of mankind have from earliest times puzzled the thoughtful. On the one hand, the most diverse of peoples are still so much alike as to leave no doubt as to their being human beings. On the other hand, the variations in skin color, in hair characters, in eyes, in shape of head, in form of eyelids, in stature, and in other physical characters are so great as to exceed by far the ranges of " individual variation" found in most other species. In mental characters also, in the rate of development, and in chemical characters (as shown, for example, by distinctive odors of perspiration and by specific immunities to diseases) the races differ in many striking ways. Nevertheless, by every test that zoologists apply, all belong to the same species. Attempts to classify the species Homo sapiens range from three main groups proposed by Linnaeus (White, Yellow, Black) to as many as fourteen or sixteen offered by other students. The most common division recognizes five main races, as follows : i. The Caucasian, or white-skinned, or European, races and tribes : hair from pale yellow to black ; eyes from blue to brown ; wide varia- tion in stature and in form of features. This is to-day, and has been throughout historic times, the most aggressive and masterful branch of the human family. KINDS OF ANIMALS 489 2. The Mongolian, or yellow-skinned, or Asiatic, races and tribes : hair black and straight ; eyes dark, with characteristic eyelids ; gener- ally shorter than the Caucasians ; rather high cheek bones and broad skull. The Chinese and Japanese are the chief representatives. Of very ancient civilization, that seems to have stagnated for a long time ; probably likely to play a larger share in world affairs in the future. 3. The American, or red-skinned, races and tribes : in many respects the North American Indians resemble the Mongolians. The Eskimos and certain Siberian tribes show remarkable resemblances to both groups. Partly because of the treatment to which they were subjected, and partly because of their racial characteristics, these people are of diminishing importance in the world. 4 and 5. The Negroid, or black-skinned, races and tribes : hair black and woolly, or frizzled ; eyes dark ; nose broad. The African, or Ethiopian, blacks differ in many ways from the Melanesian and Australian blacks. Indeed, but for the color of the skin they should be put into as distinct groups as the Chinese and the Irish, for example. Huxley, who lumps the red-skinned and the yellow-skinned peoples into one race, nevertheless makes two races of the Australian and Melanesian blacks, on the one hand, and the African blacks, on the other. CHAPTER LXXXVIII MAN AND HIS RELATIVES 509. Man as organism. Any definition that we can make of living things must apply to man. We have seen that growth and development, irritability and contractility, in general char- acteristic of the behavior of protoplasm, are also found to be FIG. 253. Limbs of primates Hind and fore limbs (legs and arms) of man, gorilla, and lemur essentially the same in human beings. Like other living things man depends upon food and water and oxygen and minerals for his material existence and activities. Like them he is helped as well as hindered by the things about him, — by the living things and by the non-living things. Like them he must at every point adjust himself to his environment or be overcome by it. Like them he reproduces his kind. 490 MAN AND HIS RELATIVES 491 510. Man a primate. In trying to place man in the world of animal life we must follow the same methods as would be used if we should find some animal that no one had ever seen before. We have no difficulty in recognizing a newly discovered FIG. 254. Comparison of primate skeletons A comparison of the skeletons of the lemur, the gorilla, and man show many similarities in details as well as in general plan of structure animal, as, let us say, an insect, even when we see it for the first time. We may be able to tell even in what order of in- sects it naturally belongs. According to our familiarity with the group, we may be able to tell the family or even the genus. In the same way we may at once place man among the backboned animals, and, without any hesitation, in the class of 492 ELEMENTARY BIOLOGY vertebrates known as mammals, because, like the mammals in general, the human species suckles its young and has a hairy outgrowth on the skin. And, of the ten or a dozen orders into which the class Mammalia is usually divided (see p. 486), the highest, or primates, seems best to agree with what we know of our own structure. In the matter of the limbs, all primates have the thigh quite free from the body, and the hand and foot have five distinct digits, with the first one (inner) usually opposable, thus con- stituting a grasping organ. The soles of the feet (or hands) FIG. 255. Skulls of primates Compare the size of the face and jaw with the size of the brain box. Note the appear- ance in man of a distinct chin and a nose bridge. /, lemur ; 2, gibbon ; j, man come down flat on the ground. The lemurs are obviously less like man than are the monkeys, and these less than the apes. 511. Man and other primates. The important differences between man and his nearest living relatives are found in the erect walk, the differentiated appendages, a distinct chin, de- cidedly larger brain, and articulate speech. Animals other than man are able to get up on their hind legs for longer or shorter periods, but none of them ever acquire the definite, erect walk that all human beings develop (Figs. 253, 254). It has been pointed out that acquiring the habit of walking alto- gether on their hind legs gave the ancestors of the human race an opportunity to free their arms and hands for other activities, and that therefore it became possible to develop these organs to higher skill. We must be on our guard against assuming that the evolution of man (or of any other species) proceeded by the hereditary transmission from generation to generation of the effects of practice or experience. FIG. 256. Ancestors of man represented by remains of skulls /, Pithecanthropus erectus, the " erect ape-man " of Java ; 2, the Neanderthal man ; j, the negroid man of Laussel ; 4, Nebraska glacial man. These four types represent succes- sive advances in the evolution of the human races, although we must not think of them as a straight series of our ancestors. Compare the size of the brain at different stages of development: Pithecanthropus, 850 cc. ; Piltdown, 1300 cc. ; Neanderthal, 1600 cc.; modern man, 1500-1800 cc. 494 ELEMENTARY BIOLOGY The larger brain, carrying with it possibilities of learning, imitating, and planning, is perhaps the most important advance made over the simian ancestors (see Fig. 255). Fig. 256 is a diagram which shows the distinctive traits of several ancient specimens supposed to represent different stages in the evolution of the human species. Fig. 258 repre- sents restorations of some of these primitive forms ; from these we can get an idea of how some of the intermediate ancestors probably looked. FIG. 257. Fossil remains of man The pieces of skull, jawbone, and tooth found in England at Piltdown in 191 1 rep- resent a lower type of human being than any that had been previously discovered 512. Evolution and man. Fifty years ago much of the discussion among thinking people centered around the question of the validity of the evolution theory as applied to man. There were many who were prepared to believe that evolution has taken place among plants and lower animals, but who hesitated to accept the same explanation for the appearance of man upon earth. One of the strongest objections urged against the theory was the fact that it had been impossible to produce a complete record of a graded series con- necting man of to-day with his supposed non-human or prehuman ancestors. This argument of the " missing link " carried a great deal of weight with people who did not appreciate how unlikely it would be for complete series of specimens to be preserved through geologic times. Of the millions of human beings and other vertebrates that die in a given region during a century, how many skeletons are likely to remain sufficiently intact to be recognized from ten thousand to fifty thousand years later ? From a scientific point of view it would be sufficient if the scattered pieces found at widely different levels (geological ages) do actually fit in with a supposed series. The few bones found in Java by Professor Dubois in 1894 fit into such a series in a very satisfactory way. The type of animal to which these bones belong was named Pitheca?ithropus erectus, and 2 « 8 495 496 ELEMENTARY BIOLOGY probably represents a " missing link." This animal had among his contemporaries a form of elephant, rhinoceros, Indian hippopotamus, tapir, hyena, a deer, and an animal somewhere between a tiger and a lion. The climate and vegetation were similar in many ways to those we now find in southern India and the islands of that region. This form is in many ways intermediate between the apes and more recent man, but we must not expect it to be an average between the two extremes. It is more like Homo in some ways and more like the apes in others ; and in some respects it is between, as in the character of some of the teeth. A more recent discovery of ancient remains in Sussex (England) seems to point to a more closely related ancestor. The skull is larger than that of Pithecanthropus, and the teeth are more like those of modern man (Fig. 257). Large numbers of specimens have been found in various parts of France, Germany, and Belgium that belong apparently to the same races of primitive men. The first of these was found in a cave in the Neanderthal in Germany, in 1856, and the type is frequently referred to as the Neanderthal race. Although these had much larger skulls 'than the Piltdown (Sussex), — larger even than is found among races living to-day, — the characters of the jaws and teeth, the low and retreating forehead, the prominent ridges over the eyes, and other features indicate a lower stage of development. This group has been named Homo primigenius, or Homo neanderthalensis. CHAPTER LXXXIX MAN'S BRAIN 513. Hand and brain. The hand of man and the brain of man are the organs that make all the important differences Pigeon Dog Monkey Man FIG. 259. Brains of vertebrates Note the relative size of the cerebellum in the bird and mammals. In the mammals, note the great increase of cerebrum and the increasing amount of convolution, or wrinkling, of the brain surface. The greater brain area in the higher animals corresponds to greater numbers of association neurons, and thus to greater intelligence between him and the other animals. And the doings possible for the hand depend finally upon the powers of the brain. It is this organ, therefore, that may properly be considered man's supreme possession. 497 498 ELEMENTARY BIOLOGY 514. Structure of the brain. In all vertebrate animals the front end of the central nervous system is enlarged into a mass of neurons, connective tissue, and blood vessels constituting the brain (see Fig. 259). In man the brain is not only a larger part of the whole body than it is in any other animal, but it is absolutely the largest brain, excepting only that of some of the larger elephants. The cortex, or "bark," of the cerebrum consists of nerve cells. In mammals this gray layer is very much wrinkled, so that there is relatively more surface than in lower vertebrates. It seems that the extent of the convolution is related to the num- bers of cells and to the complexity of their connections. The white part of the brain consists of connecting fibers, or axons. On the ventral surface of the brain are many connecting nerves, containing efferent and afferent fibers (see pp. 220-223). The hind-brain and the mid-brain have to do with reflexes and automatic movements of various kinds. In the cerebrum, nerve action is connected with consciousness and voluntary movements. The activities of the heart, the digestive system, the breathing apparatus, etc. may go on indefinitely without being influenced in any way by what happens in the cerebrum, and without pro- ducing any effect upon the cerebrum (except to keep it sup- plied with blood). Many of our activities and movements are unrelated to the cerebrum ; but every thought, every conscious desire, and every deliberate or purposeful action depends upon impulses starting from the gray matter in the brain or leading to the gray matter. Experimental studies upon various mammals, and the experiences with the diseased or injured brains of human beings, have established the fact that each portion of the cerebral cortex is concerned with specific feelings, ideas, or movements. In the diagram in Fig. 260 are indicated some of the localizations of brain function that have been determined in these studies. The special study of the activities of the cerebrum, as they show themselves in thinking, feeling, willing, is called psychology. MAN'S BRAIN 499 515. Tools and weapons. The natives of Madagascar say that if a spear is thrown at a lemur, the animal will catch it and throw it back with deadly precision. Monkeys will crack nuts by pounding them against some hard object, and gorillas FIG. 260. Localization of functions in the cerebrum By studying human beings and other animals in which the brain had been injured, and by making experiments, it has been ascertained that certain regions of the brain cortex are related to receiving sensations from specific regions of the body, while other regions initiate movements of specific muscles. Most of the sensory and motor nerves pass through the spinal cord, SC. The thinking is carried on by the so-called association areas, A-i and A-z, The frontal association area has to do with abstract thinking, self- control, concentration, and making decisions. The hind association area has to do with knowing and understanding concrete facts and relations will fight with a stick used as a club. But probably no gorilla or monkey ever carried a club or a stone about with him for use in possible emergencies, and that is something that man has done. Even among the earliest remains of human activity there are indications that man chipped stones to fit his hand, to be used as weapons or perhaps for breaking shellfish (Fig. 261). 500 ELEMENTARY BIOLOGY 516. Building. Of course other animals have built shelters, and many species of birds build much neater nests than the apes do, and much neater, probably, than primitive man built in the tree tops. But man has finally succeeded in building shelters FIG. 261. Relics of man in the Stone Age /, hatchet ; 2, hammer-head ; j, ax ; 4, j, arrowheads ; 6, fishhooks. (/, 2, and 4, after Tyler ; j, j, and 6, original) so far beyond anything that other animals have made, that it seems ridiculous to compare them. The point is, that although the bees and the birds may build very good shelters, they build the same, generation after generation, controlled in their actions by comparatively fixed impulses or instincts ; whereas man has no natural skill or plan for doing these things, yet can learn, MAN'S BRAIN 501 because of the constitution of his brain, to make infinitely more complex structures. 517. Fire. What the use of fire has meant to man is hard for most of us to realize, because we have always had the bene- fits of fire, and grow up accepting it as a matter of course. It made possible his wandering from the tropics ; it made possible his descent from the trees to dwell in caves or even in the open, for with fire he could keep the beasts away ; it made available to him food that he could otherwise not use ; and it was probably helpful in early times in many other ways. It is not to be wondered that people of all races and in all times have not only seen in fire a great benefactor, but have been impelled to worship the mystery of it as well. Even in our own times the symbolism associated with fire still persists in festivals and religious ceremonials. 518. Speech. Many animals are capable of making several distinct sounds that actually serve as means of warning or other communication between members of the species. The hen, for example, can utter some twenty distinct sounds, and each one has a different meaning. Other animals have been observed to communicate with each other by means of calls or cries. But in human speech there is more than a set of significant ejaculations. Human language comes to be built up into words, each of which consists of definite successions of sound (which we represent by means of vowels and conso- nants), and these words are combined into sentences capable of expressing all kinds of ideas. Of course one may say that our language is more complex because our thoughts are more complex, and that is true enough. But human speech differs from the crowing and growling and snarling of other animals in what may be called its structural possibilities as well as in its actual complexity, — it is capable of constant readjustment to the needs of the thinking animal in a way that the expres- sions of other animals are not, For example, if you have a 502 ELEMENTARY BIOLOGY new idea, you are quite capable of giving expression to it so that another person can understand you, by means of the language you have acquired. It is not necessary to devise new kinds of noises, and it is not often necessary to make up new words. But the use of all these things — tools, fire, speech, etc. — is but the external indication of the fact that man has a superior brain. This we can see when we compare, in a general way, man's adjustment to his surroundings with that of other animals. 519. Man's handicaps. It does not take a very close exami- nation to show us that as a living machine man is in many ways decidedly inferior to other animals. For example, his skin is much more tender than that of any other animal of his own size, and the hairy covering is not of much help. When it comes to fighting, his nails and claws are very poor rivals for those of cats, let us say, and his teeth, which he does indeed sometimes use, are not nearly as formidable as are those of many other animals of his own size. His muscular develop- ment, too, is rather inferior, when it comes to wrestling with a non-human enemy ; and when it comes to running away, he would be easily overtaken by very many of the inhabitants of the forest. Man has a very good eye, compared to other animals, and a pretty good ear, — though not one of the best, so far as dis- covering faint sounds is concerned ; but his smelling ability is of very low rank. These three senses, which are so valuable to animals in helping them discover their enemies or their prey at a distance, are of great value to man also ; but on the whole he has no advantage in competition with the other inhabitants of the forest. In spite of these various shortcomings man has contrived to hold his own, and some branches of the species have become virtually masters of their environment through the use of the brain. Man has made up for his thin skin by borrowing the skins of other animals and by devising substitutes for skins MAN'S BRAIN 503 (fabrics) out of other material. He has strengthened his arm by means of sticks and stones, and has lengthened his legs — that is, increased his speed — by means of iron and brass. He has extended the reach of his eyesight millions of miles beyond the surface of the earth, and has seen into the world of the little, — a thing no other backboned animal has ever done. He can hear the footsteps of a fly (although he does not need it either for protection or for food), and he has caught vibrations through miles of space. In every direction man has made up for his organic insufficiency by using his thinking organ to guide his hand. CHAPTER XC MAN'S CONQUEST OF NATURE 520. Learning from experience. We have seen that one of the peculiarities of the human organism that gives it advan- tages over others is the fact that it can learn from experience. There are, indeed, other animals that also learn from experi- ence. Experiments made with turtles, cats, crabs, earthworms, starfish, even Paramecium, and many other animals show that to a certain extent these organisms can profit from experience. 521. Learning from others. When human beings gather into groups, each one learns not only from his own experience but from the experiences of others. Experiments made with many different animals showed that the monkeys were the only ones that made any attempt to imitate what others were doing ; and they were the only ones, therefore, who could possibly learn from the experiences of others. Among human beings there is the possibility of learning from others, not only through imitation but also through direct instruction. And in the fact that human beings organize for various activities (as hunting, fishing, fighting, migrating, etc.), and cooperate, there is a further possibility of learning, — one that other animals do not have in anything like the same degree. 522. Preserving experience. If a wasp should learn a new trick for catching caterpillars, and use it successfully in gather- ing food for her offspring, her acquired wisdom would die with her, for the eggs which she lays do not hatch out until after she is dead. Among human beings, however, we have an extreme example of the possibility of carrying on the results of experience from generation to generation. Although it is impossible to transmit through heredity the modifications that 504 MAN'S CONQUEST OF NATURE 505 occur in the individual organism, it is possible for man to transmit what he has learned, through tradition or ceremonial. Savages who know how to make fire teach their young to go through the ceremony of making fire ; this is too important, too sacred a thing, to tell a youngster offhand. In the history of primitive peoples we find over and over again that every good idea that they get — and many a foolish one too — is care- fully preserved by being organized into a sacred ceremonial that must be performed just so on special days. In this way these people preserve whatever wisdom they manage to gather up, as well as a great deal of what seems to us to be foolish superstition. 523. Knowledge and control. Wherever men have known the relations of forces and materials, wherever they have under- stood the behavior of plants and animals, they have been able to control nature. And wherever they have controlled they have felt secure and confident, at peace with themselves and with their gods. But wherever they have failed to control, they have been aware of their own weakness ; there we find people modest and humble to the last degree ; there we find them cringing and fearful and superstitious. This is well illustrated by the differing attitudes of men toward their industries, on the one hand, and toward their crops and their health, on the other. Men and women who know their trades — that is, who understand the materials and the forces with which they work — are confident about the outcome of their undertakings. They know that handling tools in a cer- tain way will produce certain results, and they have no fear, no hesitation, in their undertakings. But when it comes to raising crops or looking after animals, there is no such certainty. These things depend upon the weather — and who can control that ? So we find people making mystic signs and muttering magic words to appease the spirits of the wind and the rain, or we find them offering sacrifices — yes, even human sacrifices - to gain the favor of the spirit that controls bugs or mildews. And even then they are not sure of the results, but worry 506 ELEMENTARY BIOLOGY along in doubt and dread until the crop is ripe. And when the crop is in, they rejoice as those coming to the end of a danger- ous journey, and again they have their ceremonials and magic. We may observe similar attitudes toward problems of health. Before people know what causes plague or malaria or tuberculosis, before they know what kills their sheep and their potatoes, they are just as fearful, just as superstitious, as they were during the Dark Ages. In those times when witches were burned for bringing on plague by uttering wicked words, or when foreigners were tortured for poisoning wells by making wicked signs, no one knew and everyone feared and suspected. 524. The measure of control. We have seen that the use of tested knowledge to solve human problems may bring about measurable results ; that is, we can measure what difference it makes whether we use the scientific method or some other method. Thus, we can measure how much work it takes to pro- duce a given quantity of potatoes with the use of suitable fertilizer and how much it takes to produce the same without fertilizer, and so find the advantage of one method over the other. The following table shows the number of hours of human labor required to produce by machinery, under conditions that prevailed at the close of the last century, given amounts of various commodities which it took a thousand hours of hand labor to produce under the conditions that prevailed at the close of the Civil War. At the pres- ent time every process has been improved so much, especially under the pressure of the Great War, that the figures in the last column may be reduced by one half or more in most cases COST IN HUMAN HOURS OF PRODUCING BY MACHINERY THE EQUIVALENT OF 1000 HOURS OF HAND WORK UNITS OF VALUE HOURS UNITS OF VALUE HOURS Barley, 470 bu 42.4 Books (binding), 2190 vols. 263.4 Corn, 220 bu 151 .3 Shoes, 45 pr 135-° Oats, 606 bu io?-5 Newspapers, 1,750,000 pages 4.8 Potatoes, 2000 bu. . . . 345.3 Envelopes, 230,000 . . . 72.6 Wheat, 310 bu 46.0 Grani te (dressing), 6150 sq. ft. 77.9 MAN'S CONQUEST OF NATURE 507 A study of the average length of life in various countries during the nineteenth century showed that in several countries the average length of life was increased (through the applica- tion of the results of scientific study) by as much as from five to twenty-nine years. In India, where alone the people refused to adopt the modern methods, there was no improvement whatever. The table below shows the death rates (number of deaths per thousand of the population in the course of the year) for several different countries. The extremely high death rate of India, compared to that of other countries, or the fact that the average length of life in India is only about twenty-four years, compared to from forty to fifty years in the other coun- tries, shows a measurable difference ; and all the evidence that we have indicates that a large part of the difference lies in the different attitudes of the people toward life. There are differences in the theories that people have about the causes of sickness. DEATH RATES IN VARIOUS COUNTRIES Denmark (1906) 13.5 Sweden (1906) 14.4 England and Wales (1906) 15.4 United States (registration area, 1907) 16.5 Germany (1905) 19.8 France (1906) 19.9 Italy (1906) 20.8 Japan (1905) 21.9 India (males, 1901) 42.3 The diagram on page 389 shows the steady improvement of health conditions in New York City as measured by the declining death rate for a period of years. Such measurements are constantly being made and are a fair indication of the effectiveness of our ways of doing things. The study of such measurements will tell us just how far it pays to know. 525. The social nature of science. What we call knowledge (or, in its organized forms, science) is never the result of an 508 ELEMENTARY BIOLOGY individual's isolated efforts. It is always the product of human intercourse. Not only does its production involve the inter- change of thought and experience of many people ; it involves also the preservation of thoughts and experiences for genera- tions. Each one adds a little to what has gone before ; if he did not know what had been learned before him, he would have to start at the beginning, and so each one could get no farther than a child's experience. Unless we think about the matter for some time, we are not likely to realize how far-reaching is our dependence upon others for the thousands of ideas — useful or entertaining — that we absorb from our surrounding civilization, through books, through customs, through instruction, through social intercourse and various institutions, such as the church, clubs, games, election campaigns, and so on. We get our ideas not only from our immediate neighbors but from all corners of the earth, — not only from our contemporaries but from the remotest antiquity. Again, our applications are largely social. We have seen this to be the case in hygienic matters. It is impossible for me to save myself from tuberculosis infection by minding my own business or by refraining from spitting etc. My safety depends in large part upon what other people do. The same principle holds in the matter of fighting any plant or animal pest, whether it is merely a nuisance or a menace to our eco- nomic welfare. The same thing holds in utilizing most of the great discoveries or inventions, such as the telephone or the wireless, the steam engine or the electric light. The development of inventions depends not only upon the accumu- lated knowledge of the past but also on the possibility for joint use. It is only after years of experience with electric-lighting plants, for example, that we may at last contrive to establish a small plant for serving an isolated farmhouse ; at first the development is possible only where people live together in communities and exchange their services readily. CHAPTER XCI SCIENCE AND CIVILIZATION 526. Casual and purposeful science. Much of the science of the past has been a casual or even accidental product. People just happened to discover this or that. But in modern times — within the last three centuries, and especially during the past fifty years — science is being systematically studied for the purpose of solving special problems. Instead of depending upon an occasional man who is both interested in scientific study and free to devote himself to the study without needing to earn a living, we are coming more and more to provide the opportunity for those who show special aptitudes in that direction. Every large university is in a position to pay a few hundred dollars a year to several students who are willing to devote themselves to special investigations, and who have shown that they have the ability to do work that is worth while. Scholarships for such investigations are provided by the direc- tors of industries who wish to have special problems pertaining to their materials or processes investigated ; or wealthy people endow such scholarships as a means of contributing to the general betterment of society. 527. Organization of research. As we come to realize the value of such investigations to the whole nation or to the race, we depend less and less upon the casual endowment of research by people who have money to spare, and depend more and more upon public effort in this direction. Thus, every state in the Union has one or more agricultural experiment stations, in which investigations are being carried on with a view to finding out the behavior of different kinds of soil in relation to crops, the most favorable conditions for the growth 509 510 ELEMENTARY BIOLOGY of various crops, the best methods for eradicating weeds or exterminating certain insects, how to get the best results from feeding cattle or hens, the best kinds of plants or animals to raise for various purposes, and so on. The United States Department of Agriculture is not only cooperating with the various state agencies, but is directing special investigations of a kind that may be too costly for a single station to undertake, or on problems that concern the people of more than a single state. In every large city the department of health has a number of men and women whose business it is to make special inves- tigations on special phases of the local health problems. They study the water supply, the milk supply, the markets, the sewerage system, the disposal of garbage ; they examine speci- mens for the more accurate diagnosis of disease, and they con- duct experiments with a view to increasing the accuracy of diagnosis or to shortening the time of diagnosis, for in some diseases a few hours may be of great significance. They ex- periment for the purpose of improving materials and methods in the preparation of vaccines and serums, and they investigate the relative efficiency of different methods of fighting flies or of ventilating factories or schoolhouses. The departments of health in the various states are also doing more and more systematic work in extending our knowl- edge of the conditions that make for health. The United States government contributes to the solution of these problems through the work of the Public Health Service, which not only has the supervision of the marine hospitals, but conducts im- portant investigations on special diseases 1 and on methods for preventing epidemics. 1 Important investigations were conducted by the United States govern- ment scientists, leading to the discovery of the hookworm disease and to the development of methods for curing it among the victims, as well as for preventing it in the future. They made important studies on the relation of rats and fleas to the plague, on the relation of pellagra to the diet, and on other health problems. SCIENCE AND CIVILIZATION 5 1 1 In addition to the work done by the departments of the city, state, and national governments,1 a great deal of scientific investigation is carried on in a number of institutes devoted especially to scientific research. The Rockefeller Institute for Medical Research, the Wistar Institute of Anatomy, the Phipps Institute, the Carnegie Institute, and many others in this coun- try are carrying on scientific investigations in various fields, for the benefit of the public. It is well understood that science cannot be developed either by those who shut themselves up from the rest of the world or by those who seek to make some private gain through exploiting nature's secrets. Sooner or later the search for the world's truth must come in contact with human knowledge on the one side, and with human welfare on the other. If man is to continue to be master of his environment, it will be first of all because he has learned to organize his machinery for finding otit, and because he has learned to make general application of what he finds out. 1 It is impossible to list all the public agencies that are regularly engaged in scientific research, even in this country alone. Some of the important ones besides those mentioned in the text are The United States Fisheries Commission The United States National Museum The United States Bureau of Standards The United States Census Bureau The United States Coast and Geodetic The National Observatory Survey The United States Bureau of Mines The United States Bureau of Ethnology The United States Weather Bureau Under the jurisdiction of the Department of Agriculture, investigations are carried on in several different fields by special staffs of scientists. Some of the bureaus, or divisions, are The Forest Service The Bureau of Plant Industry The Biological Survey The Bureau of Animal Industry The Bureau of Entomology The Office of Experiment Stations The Bureau of Chemistry The Bureau of Soils In many cities there have been established, in connection with the education departments, staffs of scientists to investigate various problems arising in the work of education, psychological clinics, and other arrangements for finding out what it is important to know. In a number of cities investigations are being made for the purpose of determining the best way to prevent fires, — of discovering the sources of fire dangers and how best to meet them. 512 ELEMENTARY BIOLOGY 528. Man's place in the world. We have studied the con- ditions of life and have seen that in every essential respect man is like other living things. We have glanced at man's nature and have seen that in many important respects man is decidedly different from other living things. We do not know when or where or how man first came to use fire or weapons or tools ; we do not know when he took to ornamenting his body with paint and beads and feathers and nose-rings ; we do not know how he acquired the art of weaving or the art of pottery or how he came to sow seeds or to domesticate animals. We do know that he has been doing these things for hundreds of thousands of years, and that during the past four or five thousand years he has been developing what we like to call civilization. We know that the life of man — that is, civilized man, the man who has the benefit of all the experience of the race — does differ from the life of beasts and from the life of the savage in many important ways. From the condition in which all activities were concerned with obtaining the means of livelihood, we have passed to the condition in which only a relatively small part of our waking time is needed for this purpose. From a state of uncertainty and fear about the workings of nature, — animate nature and inanimate nature, — we have passed to a very satisfactory knowledge of many of these workings, and to confidence in our methods for finding out more just as rapidly as we apply ourselves to the investigation. From a condition of fear and suspicion toward everything strange, — strange people as well as strange plants and animals, — we have passed to a condition of interest and toleration. We have developed art that may be of value to others, but that interests us in the first place for itself ; that is, we have found things to do other than those absolutely necessary to keep us going. In the same way we have become interested in problems the solution of which may be helpful, but which interest us without regard to their pos- sible use ; in other words, we have found things to think SCIENCE AND CIVILIZATION 513 about that are not directly concerned with getting our food and dodging our enemies. Because civilized man has accumulated, in the course of his development, so many kinds of interests, one of his real needs, one that distinguishes him from other animals, is the need for leisure. It is not enough to have food and clothing and shelter ; the dray horse has that. Man wants time to use in his own way. He wants to play games, he wants to talk things over with people of like minds, and he wants to argue with people of unlike minds. He wants to produce music, or he wants to listen to music. He wants to let himself out in making something of his own design, or at least he wants to look at the pictures and sculptures and handicraft of other people's make. He wants time to think matters over undis- turbed, when he is not exhausted, and he wants a change of air and of scene. He would like to see how other people live, and he would like to make new acquaintances. Perhaps he wants to cultivate a garden or keep rabbits. There are thousands of things that men want to do, and the doing of which gives them at least as much satisfaction as any of the activities that are directly related to keeping alive. Indeed, these other things are on the whole far more interesting. No matter how much we like our food, no matter how much we value the comfort of a warm fireside on a stormy night, or the comfort of a good waterproof when out in the rain, it is these other things that really matter most to-day — these things without which life would still be possible, it is true, but without which our lives would not be very different from the lives of beasts. It is these things that man can do, over and above making his living and keeping his body in working condition, that distinguish him from all other animals. And it is in proportion as these other things play a larger and larger part in our lives, and in proportion as food and clothing and shelter play a smaller and smaller part, that we may consider ourselves humanized. 514 ELEMENTARY BIOLOGY So far as the race as a whole is concerned, we have already solved the urgent problems of producing the necessities of life. Without extending our science beyond what we know to-day, we are in a position to produce in abundance, and with a very small outlay of human effort, all our food material and all the material needed for our clothing and our dwellings, and for building cities and railroads etc. The Great War has demonstrated our wonderful power and resourcefulness in these respects. On the other hand, it has also made clear our shortcomings. We need to know much more in order to carry on the affairs of community life more effectively (the urgent problems are problems of education, of crime and delinquency, and of political adjustment of individuals and races), and, above all, in order to distribute leisure so that each may live as human a life as possible. INDEX Absorption, 40, 87, 1 14, 1 18, 477 ; in nutrition, 260 Acclimating, 131, 257 Accretion, 18 Acetanelid, 257 Acids, 202 Activators, 50 Adaptability, 19, 39 Adaptation, 19, 55, 56; of flowers, 310; and instincts, 244; and pur- pose, 369 Adenoids, 150 ff. Adjustment, 131 Adrenin, 189 Adulteration of food, 123, 124 Adventitious roots, 47, 48, 49 Afferent nerves, 219, 221, 222, 498 After-effect, 136 Agglutinins, 196 Agramonte, Aristide, 407 Agriculture, and bacteria, 396 ; biol- ogy and, 4, 67 ; and birds, 429 ; and food supply, 75; and forests, 379; and heredity, 450 ff. ; and insects, 417 ff . ; and science, 506 Air, circulation of, i 57 ; composition of, 12 ; excess of, 339 ; and fire, 12 ; and forests, 378; humidity of, 157 ; and life, 172-173; pollution of, 158, 396; and protoplasm, 143; tem- perature standards of, 156 Air cells of lungs, 148 Air requirements of man, 154 Albumen, 51 Alcohol, 133 ; and Committee of Five, 258; and digestion, 135; and fer- mentation, 135; fighting the evil of, 139; and infectious disease, 135; and longevity, 134; prohibi- tion of use of, 142 ; reasons for be- ginning to drink, 137 ; and recrea- tion, 140; and the senses, 253; and society, 137, 141 ; and vinegar, 396; and work, 136 Algae, 295, 298, 480 Alimentary canal, 81 ff. Alkaloid, 74, 202, 203, 257 ; in tea and coffee, 254 All life from life, 19 Alternation of generations, 320 ff. Ameba, 24, 80, 96, 229, 260, 294, 338, 363 American races, 489 Amino-acids, 109 Amoeba, see Ameba Amphibians, 350, 485 Anaerobes, 143 Analysis, 9 Ancestors of man, 493 ff. Ancon ram, 460 Anesthetics, 259 Angiosperms, 481 Animals, alternation of generations in, 326; classification of, 482 ; cold- blooded, 337; colonial, 431, 432; compared with plants, 20 ; genea- logical tree, 483 ; heredity in, 444 ; infancy among, 333 ; reproduction in, 327 ; regulations concerning domestic, 395; useful, and insects. 420 Annelids, 484 Annual plants, 44 Anopheles, 404, 406 Ant lion, 372 Antheridium, 320 Antiseptic, 192 Antiseptic mouth wash, 121 Antiseptic solution, 192 Antitoxin, 193 ff. ; for diphtheria, 195 Ants, colony of, 432 ; destruction wrought by, 413; destructive, 415; extermination of, 416; honey-pot, 433 Aorta, 185 Appendix, vermiform, 82, 468 Appetite, 104, 1 1 1 ; loss of, 1 18 ; stimu- lation of, 117, 138 Arachnids, 484 Arc, reflex, 220 Archegonium, 320, 478 Armors, protective, of organisms, 345 515 5i6 ELEMENTARY BIOLOGY Arrhenius, Svante, and the nitrogen problem, 63 Arsenic, becoming accustomed to, 132 ; insecticide and fungicide, 131 Artemia, 289 Artery, 185; pulmonary, 187 Arthropods, 280, 289, 432, 484 Arts, development of, 512 Ascent of sap, 176 Asexual reproduction, 299 Asphyxiation, 170 Assimilation, i8>, 50, 260, 265, 285 Associative neurons, 221, 499 Astigmatism, 235 Astringent, 192 Atwater, W. O., food doctrines of, 90, 93> 99 Auricle, 185 Aves, 485 Axon, 220, 498^ Bacteria, 61 ; in carbon cycle, 58 ; control and use of, 394 ff. ; and decay, 112, 1 18 ; destroyed by cook- ing, 112; digestion in, 80 ; and disease, 386; in food, 112, 120; in industry 396 ; in life cycle, 63 ; and nitrogen, 62, 67, 396 ; and soil, 396 ; on teeth, 120; and wounds, 192 Bacteriology, 388 Balance of nature, 423 Balancing organs, 242 Bark, in roots, 43, 60 ; fibers and ves- sels in, 176 Bast, 79, 176 Baths, after meals, 1 16 ; hot and cold, 206, 207 ; public, 396 Batrachians, 485 ; reproduction among, 328 Becker and Hamalainen, nutritive tables of, 101 Bee, 329; and orchard, 313 ; sting of, 373 Beetle, calosoma, 422 ; ladybird, 423 ; potato, 418, 462 Behavior, as problem of biology, 5 ; mechanical aspects of, 39, 217, 224 Bends, 340 Beriberi, 108 Biennial plants, 44 Bile, 86, 118, 184, 216 Biology, 3; and cost of living, 129; and efficient living, 4 ; and enjoy- ment of life, 512-513; and food supply, 130; and health, 3, 507; public protection, 126; applied to saving of life, 393, 507 ; of the soil, 66 ; use of, 474 ; and wealth, 4, 506 Birds, 485 ; destruction of, 425 ; diges- tive system of, 87 ; economic value of, 425, 429; eggs of, 330; food of, 425; and insects, 426; migration of, 366, 426; nests of, 371 ; protec- tion of, 426 ; undesirable, 429 ; young of, 334 Black Hole of Calcutta, 155 Black hornet, 369 Blackbird, red-winged, 429 Bladder, 206, 262 Bladder wrack, 298 Blanching of plants, 73 Blastophaga, 312 Bleeding, 192 Blood, 179; changes in, 188 ; circu- lation of, 185 ; clotting of, 18 1 ; co- ordination through, 434 ; ferments in, 1 88; a living tissue, 193 ff. ; and respiration, 146; a transportation system, 262 Blood pressure, 190 Blood system, 264 Blue jay, 427 Bluebird, nest of, 370 Blues, 118 Bobolink, 429 Bowels, see Intestines Brain, structure of, 498 Brain food, 107 Brain functions, localization of, 498 Brains of vertebrates, 497 Breathing, 144, 148, 149, 172-173; control of, 149; and digestion, 116, 118; habits of, 151 ff . ; nose and mouth, 150; in plants, 74 (see also Oxidation) ; summary of hygiene of, 172-173 Breathlessness, 190 Breeding, for immunity to disease, 450; for special points, 452 Breeds, of plants and animals, 438 ; variation in, 439 Bristles, 350 Bronchus, 148 Bronzed grackle, 427 Bryophytes, 481 Bubonic plague, 410 Buds, first, of seedling, 33 ; resting, 270 Buffalo moth, 415, 417 Burbank, Luther, 452, 453 Burbank potato, 454 INDEX 517 Burrowing, 368, 370 Butter, flavor of, 397 Butterflies, development of, 281 By-products of organisms, 201 Cactus, desert vegetation, 359 ; spine- less, 453 Caffein in coffee and tea, 254 Calcutta, Black Hole of, 155 Calorie, 91 ; measure of daily require- ments, 93, 100 Calorimeter, respiration, 92 Calosoma beetle, 422 Calyx, 303 Cambium, 145, 271, 347; in roots, 43 Camouflage, 354 Capillaries, 181, 185, 187, 188 Carbohydrates, 51, 52, 54, 55, 90, 93, 97 (see also Food); digestion of, 81, 85, 87, 112 Carbon cycle, 57, 58 Carbon dioxid, 57, 71, 143 ff. ; in food- making, 53, 54 Carbon monoxid, 158, 164 Carroll, Dr. James, 407, 408 Casein, 51 Castle, W. E. 449 Caucasian race, 488 Cedar waxwing, 428 Cells, 21 ; absorption by, 40; animal, 22; budding, 292 ; colonies of, 420 ; differentiated, 431 ; differentiation of, in development, 275; division of, 76, 265, 274, 275, 291, 457, 458; egg, 299 ; exchange of material in living, 1 74 ; fission (see Cells, division of) ; germ, 447 ; independent, 76, 430 ; nerve, 219, 267 ; nettling, 375 ; oosphere, 458; origin of, 274, 457, 459; plant, 23 ; respiration in, 144 ; sperm, 299, 459; structure of, 21 ; in tissues, 477 ; walls of, 57, 345 Cellulose, 40; digestion of, 112 Central cylinder, 43 Cerebellum, 497 Cerebrum, 497, 498 ; localization of functions in, 499 Ceremony, 505 Chameleon, 362 Chances of death, 134, 507 Change, chemical, 7 ; cyclic, 463 ; physical, 7 ; universality of, 7, 463 Chapin, Dr. Henry Dwight, 109 Cheese, curd of, 51 ; curing of, 397 Chemical changes, 7 ; in pigments of animals, 353 ; in soil, 66 Chemical composition, of air, 23 ; of carbohydrates, 54 ; of fats, 55 ; of food, 51; of human body, 16; of plants, 2 1 ; of proteins, 55 ; of soil, 30 Chemical cycle of life, 57 Chemical engine (chlorophyl), 54 Chemical fixation of nitrogen, 63 Chemical influence upon develop- ment, 289 Chemical injury to nervous system, 253 Chemical sense in lowest organisms, 24, 224 Chewing of food, 81, 115, 117, 1 19 Chitin, 346 Chloral hydrate, 257 Chlorophyl, 54, 73, 75, 184 Chloroplast, 54 Chromatin, 457, 458 Chromosomes, 457, 458 Cilia, 295 Circulation, of air, 157; of blood, 185 ff . ; closed and open, 185; de- pressed, 118; double, 187; and exercise, 116; in plants, 174 ff. Circulatory system, 262 ; hygiene of, 190 Civilization, 435, 51 2 ; and infancy, 335 Clam, breathing of, 145 Classification, basis of, 475 ff . ; of animals, 482; of organisms, 475; of plants, 478 ; uses of, 475 Cleanliness and health, 128 Climbing plants, 48, 49 Clothing and health, 152 Clotting of blood, 181 Coagulation of blood, 181 Coal and forests, 380 Coal seams and fossils, 464 Cocain, 256 Cochineal, 413 Cockroaches, 414; extermination of, 416 Codein, 257 Codling moth, 419, 421 Ccelenterata, 432 Ccelenterates, 373, 482 Coffee, 254 Cold-blooded animals, 337 Collaterals, 219 Colloids, 78 Colonial animals, 431, 432 5i8 ELEMENTARY BIOLOGY Color's, changes in, 362 ; warning, 355 (see also Pigments) Committee of Five on alcohol, 258 Communicable diseases, transmission of, 387 Community, 435; life of, 514 Competition, 343, 344, 474 Competitive relation, 342 Compound, chemical, 9 Concealment, 363 Conducting system of plant, 174 Conjugation, in Paramecium, 296, 297 ; in Spirogyra, 296 Connective tissue, 267 Consciousness, 498 Conservation, of energy, 10; of food supplies, 112, 129-130; of forest, 382 ; of manure, 60 ; of matter, 8 ; of resources, 4 ; of sewage, 60 ; of soil, 68, 380, 381 Constipation, 88, 118 Contraction, 24, 218, 360 Control, by means of knowledge, 505 ; of breathing, 149; of movements, 217; measure of, 506 Cooking, 112, 431 ; waste involved, 112 Cooperation, 434, 510 Cooper's hawk, 427 Coordination, 434 Cordata, 484 Corpuscles, 179; red, 183, 184, 262, 294; white, 181, 182, 193'ff., 273 Correlation of functions, 262 Cortex, of brain, 220, 498 ; of root, 43 Cotyledon, 33 Crab, hermit, 363, 364 ; horseshoe, 345 Crop of bird, 87 Crops, rotation of, 62, 423 Crow, 428 Crustacea, 346, 484 Crustacean, 289 Crystalloid, 78 Cuckoo, 342 Culex, 406 Cup, public drinking, 394 Cutin, 345 Cuts, treatment of, 191 Cuttings for propagation, 48, 269 Cuttlefish, 363 Cyclic changes, 463 Cyst, 295 Cytolysins, 196 Cytolysis test, 197 Dairy and bacteria, 397 Darwin, Charles, 273, 469, 472 Death rates, in various countries, 507 ; from various diseases, 389 ; of infants, reduction of, 393 ; diphtheria, 194, 195; malaria, 409; typhoid fever, 299, 392 ; yellow fever, 409 Decay of food, 80, 124 Dehiscent fruit, 316, 317 Dendrites, 220 Descent of sap, 176 Desert land, reclamation of, 68 Desert plants, 359 Development, 274, 476; and classi- fication, 476; and metamorphosis, 283 ; conditions for, 285-289 ; of plants, 284 ; stages in, 276 De Vries, see Vries, Hugo de Diaphragm, 82, 149, 150, 153 Diastase, 78 Dicotyledons, 34, 481 Diet, and age, 102 ; and appetite, in; balanced, 102; and climate, 102; standard, 99; studies of, 89, 90, 510; and work, 100-102 Dietaries, 90 Differentiation, 182, 477 ; of cells, 275' 431 Diffusion, 40, 76, 78, 83, 85, 144, 147, 174, 179. See also Absorption Digestion, 78, 80, no, 111-112; and alcohol, 135; in bacteria, 80; con- trol of, 89, 116; exercise and, 116; in gut, 84-86 ; hygiene of, 88 ; in human body, 81 ; in mouth, 81 ; organs of, 82, 83, 87 ; in stomach, 83 Dimorphic flowers, 304 Dioecious plants, 306 ' Diphtheria, 386; antitoxin of, 195 Disease, control of, 395, 398 ; and heredity, 200 ; microbes as cause of, 200 ; and parasites, 341 Diseases, alcohol and infectious, 135 ; of animals, 420; and insects, 410; mortality rates (see Death rates) ; specific, and bacteria, 386; trans- mission of communicable, 387 Division of labor, 262, 434, 477 Dogs, licensing of, 395 Dominance, law of, 444, 445, 450, 455 Dominant characters, 445 ; in man, 455 Drowning, 169 Drowsiness, 118, 155 INDEX 519 Drugs, action on protoplasm, 132; habit-forming, 132, 137, 139, 255; regulations concerning, 257 Dubois, Professor Eugene, 494 Dust, and breathing, 155; in indus- try, 159-161 ; and occupations, 158 ; as source of infection, 126, 158, 173, 192 Dusting, 162 Ear, 240 ; in man, 502 Earthworm, regeneration in, 268 ; respiration in, 146 Eating, habits of, 89, 114 ff . ; and mental states, 114; and muscular work, 116; pleasures of, 113 Echinodermata, 347, 484 Economics, and alcohol, 133, 139, 253 ff. ; of bacteria, 388, 394 ff. ; and biology, 4, 439 ff., 449, 450 ff. ; of birds, 425 ff . ; of fatigue, 21 1, 213 ff. ; of food, 122 ff., 128, 388; of forests, 377 ff. ; of heredity, 449, 450 ff . ; of insects, 311-313, 398 ff., 404 ff., 413 ff., 417 ff.; of photo- synthesis, 74, 75; of roots, 46 ff . ; of science, 124, 129, 474, 505-506, 512 ff.; of soil, 65 ff., 380, 381; of stems, 176, 202, 377, 453; of tobacco, 167 ff. Efferent nerves, 219, 221, 222, 498 Efficiency, biology and, 4 ; fatigue and, 211, 216; habits and, 251; health and, 124, 128, 172; smoking and, 1 66; temperature and, 156; ventilation and, 156 Egg, of birds, 330 ; of fishes, 327 ; of frog, 275; of insects, 279, 281 ; of mammals, 330 ; origin of, 457 ff. ; of plants, 299, 304, 320 ff. Egg cell, 299 Electric shock, 170, 373 Elements, chemical, 9 Elimination of refuse, 88, 1 18-1 19, 261 Embryo, development of, 35, 274 ff. ; in seed, 32, 276 Embryo sac, 302, 324, 478 Endosperm, 39 Energesis, 143, 149, 285; and excre- tion, 262 ; oxygen in, 143 ; and res- piration, 261 Energy, 7 ; chemical, 7,11; conser- vation of, 10 ; daily consumption by body, 93, 100; sources of, n ; transformation of, 13,70; units of, 91 Environment, 26; and development, 285 ff . ; and health, 214; life and, 337 ; man's mastery of, 502 ff. Enzyme, 78, 79, 396, 397. See also Ferments Epicotyl, 33, 358 Epidemics, prevention of, 122, 125 ff., 163 ff., 387 ff., 394 ff., 399 ff., 404 ff. Epidermis, 23, 43, 70, 345, 348 Erosion, 380 Esophagus, 82 Etiolation (blanching), 73 Euglena, 229 Eustachian tubes, 82, 240 Evolution, 463, 471 ; applications of, 473 ff. ; evidence of, 464 ff .; of horse, 466; theories of, 471 ff . ; and man, 494 Exchange, in cells, 144, 174; of gas, 73-74 Excretion, 76, 201, 262 ; in animals, 203 ; and fatigue, 208 ; hygiene of, 205 Exercise, 116, 118; and breathing, 152; and digestion, 141; and the skin, 207 Exo-skeleton, 346 Experience, learning from, 504 Experiment, problems solved by, 28, 37. 73' 93» II0' IT4, "5' "7> 121, 125, 129, 130, 132, 133, 134, 135, 143, 154, 155, 156, 162, 166, 171, 175, 176, 182, 183, 189, 193, 208, 209 ff., 222, 239, 245, 268, 270, 272, 285, 290, 352, 362, 378, 379, 386, 398, 404, 407, 410, 442, 446, 448, 452, 455, 462, 473, 498, 499' 5°4, 5IQ Experiment stations, agricultural, 417 Expiration, 149 Extracted recessive, 449 Eyelids, 232 Eyes, and light, 229 ; compound, 230; human, 232, 502 ; hygiene of, 235 ; infection of, 237 Factory regulations for health, 128, 156, 158, 162 Fall of leaf, 374 Fat, 51, 52; digestion of, 85-86; origin of, 55 Fatigue, 208 ; and efficiency, 210 ; ex- cretion and, 208 ; and eyestrain, 236; and health, 215; and rest, 215; and work, 210, 213, 215-216 Fatigue poisons, 208 520 ELEMENTARY BIOLOGY Feather, growth of, 349, 350 Female, see Sex Fermentation, 133 Ferments, 52, 78-79, 80, 85 ; in blood, 1 88 Fern, 322-323; infancy in, 331; spores of, 294 Fertilization, 299, 302, 303, 323, 327, 328» 459 Fertilizers for soil, 66, 429 Festering of wound, 192 Fever, see Diseases, Typhoid fever, Yellow fever Fibers, plant, 44, 79, 176; nerve, 219, 498 Fibrinogen, 181 Fibrovascular bundles, 44, 70, 175 Fighting, among ants, 373; against flies, 402; against insects, 422; against mosquitoes, 408 ; among organisms, 372, 374 Fire, and air, 12; forest, 385; as source of energy, u; use of, 501 First aid, in asphyxiation or drown- ing, 169; in bleeding, 191-192; for eye, 236 Fish, 485; breathing in, 147; devel- opment of, 252 ; digestive system of, 87 ; hatching of, 333 ; infancy and care of young, 334 ; migration of, 367 ; reproduction in, 327 Fisher, Professor Irving, food tables, 94, 96, 103 Fission, see Cell, division of Fitness of organisms, 19, 39, 3ioff. See also Adaptation Flatworms, 482 Fleas and plague, 510 Fleshy fruits, 317 Flicker of light and eyestrain, 236 Flies, breeding places, 400 ; elimina- tion of, 402 ; house, and food, 399, 400 ; relation to intestinal disease, 399 Floods and forests, 379 Floral envelope, 300 Flour moth, 415, 416 Flowering plants, 294, 481 Flowers, 300, 478; adaptations of, 310 ff . ; dimorphic, 304; fertiliza- tion in, 303; and insects, 310 ff. ; polymorphic, 305 Fly, Hessian, 419 Food, 31, 50, 51 ; adulteration of, 123 ; of birds, 425 ; care of, 388 ; choice of, 89, 98 ; composition of, 94, 95, 96,98; conservation of, 129; con- tamination of, 124, 125; daily needs, 93, 100; decomposition of, 125; and development, 288 ; digestibil- ity of, in; distribution of, 130; economics of, 112, 122-123, I29» 130 ; and flies, 401 ; fuel, 52 ; func- tions of, 51 ; ideal, 102-103 > insects as, 412; kinds of, 51 ; minerals in, 108, 112; need for bulk in, 118; plant, 3 1 , 50 ; for plant embryos, 33 1 ; preservation of, 124, 125; propor- tions of, needed, 70, 102-104 > Pr°- tection of, 124, 125-126; public regulation of, 122, 125; relation of light to making of, 39, 53 ff. ; in seeds, 35, 46; summary of, 102, 1 16; taste of, 1 10 ; translocation of, 79 Food habits, 89, 1 14 ff. Food-getting organs, 364, 375 Fruit, 303; dehiscent, 316; fleshy, 317; and seed, 302; seedless, 303 Fuel values, 91 Fumes and the lungs, 158 Function, biological idea of, 15-16 Fungi, 294, 341, 480; dangerous to forest, 385 ; used in fighting insects, 423 Gall, 86. See also Bile Galls, insect, 374-375 Gametes, 297, 324, 332, 478; water essential to, 328 Gametophyte, 322, 323, 324 Garbage, 3, 396, 399, 402 Gas exchange, 73-74, 144, 145 ; in blood, 188 ; in leaves, 74 Gases, injurious, 158 Gastric juice, 83-84, no Genealogical tree, of animal life, 483 ; of plant life, 479 Generations, alternation of, 320 ff. Geology and evolution, 464-466 Geotropism, 37, 38, 241 Germ carriers, 152 Germ cells, 447 ; origin of, 456, 463 Germ plasm, 459 Germination, 36 Gills in respiration, 146, 147, 183 Gizzard of bird, 87 Glands, 263 ; ductless, 188 ; of intes- tines, 84, 86, 1 02 ; milk, 335 ; saliva, 81, 83; stimulation of, no; of stomach, 83, 84 INDEX 521 Glare, 236 Glucose as adulterant, 123 Gluten, 51 Glycogen, 189 Gonococcus bacteria, 237 Grackle, bronzed, 427 Grafting, 270, 271, 272 Grains, 34, 78, 217 Gravity, responses to, 38, 241 Great blue heron, 428 Green slime, 92 Growing period of various animals, 335 Growth, 17; as cause of movement, 38 ; discontinuous, 269, 292 ; limits of, 265 ; and regeneration, 265 ; in roots, 43 ; and smoking, 165 ; in stems, 347 Guano, 60, 66, 429 Guard cells, 71, 72 Gudernatsch, Dr. J. F., 285 Gullet, 82, 87 Gut, 84, 101-104. See also Intestine Gymnosperms, 481 Gypsy moth, 419, 422 Habit, 248 ff. ; formation of, 150 ; for- mation of, by protoplasm, 131 Habit-forming drugs, 255-257 Habits, eating, 89, 114; feeling, 250; health, 114-119; kinds of, 249; selection of, 250; thinking, 249; tobacco, 163 ff. ; value of, 251 Habituation, 131, 163 Haddock, breathing of, 147 Haemocyanin, see Hemocyanin Haemoglobin, see Hemoglobin Hairs, of mammals, 349, 350; of plants, 347, 348 ; root, 42 Happiness, biology and, 4 Headache, 118 Headache powders, 118 Health, alcohol and, 133; and bacte- ria, 386; and biology, i ; fatigue and, 215; food and, 89; habits for con- trol of, 1 14-119; public, 10, 394, 510 Healing, 266 Hearing, 238 ; in fishes, 239 Heart, 185, 186; beat of, 190; care of, 190; effect of smoking upon, 164; leaky, 190; training of, 191 Heat, 9, u, 27, 54, 72, 143, 155, 156, 1 57, 206, 348, 359, 363 (see also Tem- perature) ; and living matter, 337- 338, 391 (see also Frontispiece) ; measurement of, 9 1 (see also Calorie); from organism, 92 Hemocyanin, 179 Hemoglobin, 179, 183, 184 Heredity, 443 ; in animals, 444 ; appli- cations of, 450 ; and disease, 200 ; in man, 454, 455 ; of modifications, 461 , 492, 504 (see also Frontispiece) ; in plants, 444 ; and protoplasm, 457 ; and theory of evolution, 473, 492 (see also Frontispiece) Hermit crab, 363, 364 Heroin, 257 Heron, great blue, 428 Hessian fly, 419 Heterospory, 324 Home-finding instinct, 363 Home-making, 368, 371 Homo, 488, 495, 496 Homology, 365 Honey, 313, 413, 433 Honey-pot ant, 433 Hookworm, 341, 510* Horns, 315 Horse, evolution of, 466 Horse botfly, 421 Horse power, 91 Horsefly, 398 ff. Human race, 455, 488, 501 Humanization, 513 Humus, 75 Huxley, Thomas H., 489 Hybrid, 445 Hybridizing, 450 Hydrotropism in plants, 38-39 Hypocotyl, 33 Illumination, 362. See also Light Imitation, 494, 499 ; learning by, 504 Immunity to disease, 198, 452 ; ac- quired and natural, 199; active and passive, 200; breeding for, 451 Indigestion, 115, 118, 135, 138 Individual differences, no-iii, 198, 234, 258 (see also Variation) ; in digestion of food, 106; in size of stomach, 115; in susceptibility to alcohol, 134 ; in susceptibility to tobacco, 163 Individuality, 437 Industrial dangers, 169, 171, 214, 236 Industrial hygiene, 128 Industrial poisons, 158 ff. Industry, biology and, 4 Infancy and parental care, 331 ff. 522 ELEMENTARY BIOLOGY Infant mortality, reduction of, 393 Infection, 386, 387 ; eye, 237 ; of food, 152; protection against, 388 Inhibition, 248 Ink bag in cuttlefish, 363 Inorganic, 16-17, 178 Insecta, 283, 484 Insects, and birds, 426; damage done by, 418 ; fighting against, 422 ff. ; as food, 412 ; galls produced by, 374- 375 ; as germ carriers, 126, 398, 410, 420 ; injurious, 414 ff. ; intelligence in, 434 ; as intermediate hosts, 404 ; life stages of, 279, 281, 282-283; orders of, 486 ; predatory, 374 ; relation to flowers, 309 ff., 312; reproduction in, 329 ; respiration in, 145 ; sense organs of, 239, 240, 241; social, 433-435; and useful animals, 420 ; and useful plants, 417; and wealth, 412; and young, 332 Inspiration (breathing), 149 Instincts, 244, 500 ; home-finding, 363 ; modification of, 245 ; relating to young, 334 ; for selection of food, 90, no Intelligence, 434, 502, 504 ff. Interdependence, between organisms, 5, 63, 64, 424, 434, 508 ; of flowers and insects, 310 ff. ; of people, 393, 435' 5°4. 5°7> 5°8> 5°9 ff- Intermediate hosts, 404, 411 Internal factors in development, 290 Internal secretions, 188-189 Intestines, 82, 83, 84, 86-88, 188; evacuation of, 88, 117, 118-119; length of, 1 06; of various animals compared, 105-106 Invertebrates, 327, 482 Invisibility, protection by, 352 Involuntary muscles, 219 Irritability, 18, 24, 158, 182, 220, 226, 287 Jellyfish, 372, 373 Johannsen, Dr. Wilhelm, 442 Kallima butterfly, 355 Kangaroo, 335 Katydid, 351, 354 Kidneys, 203, 262 ; effect of alcohol on, 203 ; hygiene of, 205 Knowledge and control, 505 Kropotkin, Peter, 435 Lac, insect, 413 Ladybug (ladybird) beetle, 414, 422 Lamarckian theory, 471-472 Lancelet, development of, 274, 276 Language, human, 501 Langworthy on daily food consump- tion, 101 Lateral line, 238, 239 Laveran, Alphonse, 404 Layering, 48 Lazear, Dr. Jesse W., 407, 408 Leaf, 70-72, 300; and elimination of refuse, 261 ; fall of a, 374, 375; gas exchange in, 144; and light, 73; uses of, 74 Legume family, 62 Legumin, 51 Leisure, 513, 514 Lenticels, 71, 145 Lice, 410 Lichens, 480 Life, without air, 143; and the environment, 336 ; length of, 507 ; origin of, 19; salts and, 338; and temperature, 337 ; unity of, 260 Light, eyes and, 229 ; in food-making, 39' 54' 73 ff- » influence on growth of plant, 38, 39 ; and life, 338 ; and pigmentation, 352, 362 ; and tan, 352 Light tropisms, 230 Linnaeus, 488 Liver, 85, 86 Liverworts, 481 Living bodies, characteristics of, 15 Living forms, multiplicity of, 260 Lizard, green, 363 ; regeneration in, 270 Lobster, appendages of, 365 ; breath- ing in, 146; feeding of young, 332 Localizations, of brain functions, 498 ; of functions in cerebrum, 499 Locomotion, 364 Locusts, 417 Lungs, 147, 148 ff., 183; capacity of, 151-152 ; hygiene of, 151 ff. Lymph, 180, 262 ; coordination through, 434 Machine, 14, 17 ; and organisms, 17, 20 Malaria, 294, 404, 506 Malaria parasite, 326, 341, 405 Male and female, 299 Malt, 78, 133 Mammalia, 486 INDEX 523 Mammals, 492 ; egg of, 330 ; grow- ing period of, 335 ; hairs of, 349 ; infancy among, 334 Man, ancestors of, 493; evolution and, 494 ff. ; handicaps of, 502 ; mastery of environment by, 502- 504 ; as organism, 490, 502 ; and other primates, 491, 492; place in the world, 490 ff., 512; primitive, 500 ; relation to other organisms, 6 ; relics of, in Stone Age, 500 ; senses of, 502 Manson, Sir Patrick, 404, 406 Mantle of mollusks, 346 Manure, 60, 66 ; and flies, 399, 402 Marriages, productive of undesirable offspring, 456 ; regulation of, 394 Marsupials, 335, 486 Matter, composition of, 9 ; conserva- tion of, 8 ; kinds of, 9 ; states of, 7 Meals, number and frequency, 115-116 Meal-worm, 414, 416 Meat in diet, 105-107 Medicine (see Drugs) ; as source of alcohol habit, 137, 139 Mendel, Gregor, 443, 446, 448, 449, 461 ; law of segregation, 446. See also Frontispiece Mental state and digestion, 114 Metamorphosis, 283 Metchnikoff, filie, 182 Microbes, 67 (see also Bacteria) ; de- stroyed by cooking, 112 Micropyle, 32, 303 Migration, 365 ; of birds, 366, 425, 426; of fish, 367 Milk, care of, 391 ; food value of, 97, 104; preservatives in, 125; regu- lations in regard to, 126, 390 Milk standards, 127 Milt, 327 Mimicry, 356, 357, 358 Minerals, in food, 52, 108, 112, 202; in manufacture of proteins, 56; in plants, 30 ; in soil, needed by organ- isms, 66 Missing link, 494, 496 Mixed types, 440 Modifications, by surroundings, 439 ; transmission of, 461, 492, 504 (see also Frontispiece) Moisture, and hair formation, 349 ; and ventilation, 155, 173 Mold, reproduction in green, 292 Mollusca, 346, 374, 484 Mollusks, 289 Molting in cicada, 280 Mongolian races, 489 Monocotyledons (monocotyls), 34, 481 Monoecious, plants, 306 Morphin, 199, 256 Mortality, reduction of infant, in New York City, 393 Mortality rates, in various countries, 507 ; for various diseases, 389 Morula, 276 Mosquito, extermination of, 409 ; fighting against, 408, 409 ; life his- ories of, 406 Moss, alternation of generations, 32 1 ; antheridium of, 320; archegonia of, 320; infancy in, 331; life history of a, 320 ; spore formation, 293 Moth, brown-tail, 420; buffalo, 415, 416; clothes, 412, 414; codling, 419, 421; flour, 415, 416; gypsy, 419, 420, 422; silk, 413 Mouth, digestion in, 81-82; watering of, no, 217, 227 Mouth-breathing, 150, 151, 173 Movements, in organisms, 18, 24, 217, 223 (see also Muscles) ; in plants, 361 ; voluntary, 498 Mucin, 82, 121 Mulch, 379 Muscles, 218, 228; kinds (voluntary and involuntary), 218, 219 ; reflexes dependent upon, 218 Mutants, 462 Mutation, 461, 462, 473 Mutilation, 269 Myosin, 51 Narcotic, 131, 132 National forests, 385 Natural selection, 472, 473-474 Nature, man's conquest of, 504 Navigation and forests, 379, 381 Neanderthal man, 493, 495, 496 Nectar, 309, 310 Negroid races, 489 Nerves, 218 ff., 267, 497 ; development of, 246 ; effects of smoking on, 1 64 ; kinds of, 221, 222; and the reac- tions of organisms, 217 ff. ; and re- flexes, 218 Nervous system, 487 ; chemical influ- ences, 289 ; chemical injury to, 253 ; coordination through, 434 ; correla- tion with other systems, 263 524 ELEMENTARY BIOLOGY Nest of bluebird, 370 Nests, 334, 371 Nettle, 347 Nettling cells, 373 Neurons, see Nerves Newt, development of, 282 Nicotine, 163 Nictitating membrane, 234 Nitrates, 60 Nitrogen, and bacteria, 62 ; utilization of atmosphere, 63 ; and war, 64 Nitrogen cycle, 59, 60, 61 Nitrogen problem, 61 Nitrogenous wastes, 90 Nose as sense organ, 227 Nosebleed, 192 Nose-breathing, 150 Nucleus, cell, 23, 296 ff ., 303 ; division of, 457, 458, 459 Nutrition, 260 Nutritive ratio, 97, 100, 103 Nuts, 317 Occupational dangers, 158, 235-236 Occupations, and air, 156; and diet, 101 ; and dust, 158-162 ; regulation of, 122-124, 213 ff., 216, 395 Odors, 202 Oesophagus, see Esophagus Oleomargarine as butter substitute, 123 Ommatidium, 232 One-celled animals, 24 One-celled plants, 75-77 Oosphere, 299, 458 Ophthalmia neonatorum, 237 Opossum, 335 Opsonins, 198 Orchids, 308, 312-313 Organic, 16, 51, 53, 176 Organism, 15, 16, 24; how it learns, 245; origin, 19, 291 ff. Organs, 15; origin of, 276 Osmosis, 40-41, 76, 78, 79, 80, 144 ff., 149, 174 ff., 180-181, 188, 201 ff., 260 Ovary, 301, 302, 303 Overpopulation, 472 Ovules, 301, 324 Oxid, 13 Oxidation, 13, 59; in organisms, 13,29, 51* 52> 55, 57> 58> 59» 132, '43 ff-> 20"i Oxygen, 12, 52 ; in energesis, 143, 144; in photosynthesis, 54 Oxygen cycle, 59 Palate, no Palisade cells, 23, 70 Pancreas, 82, 85, 86, 188 Paper wasp, 369 Parallelism in development, 277, 278, 467 Paramecium, 224 ff. ; conjugation in, 296 Parasites, 410; alternation of genera- tions in, 326; in food, 112 Parasitic diseases of animals, 420 Parasitic relations, 341 ff. Parasitism, 342 Parent, dependence of offspring on, 332 Pasteur, Louis, 133, 386 Pasteurization, 391 Patent medicines, 256, 257 Pawlow, Professor I. P., experiments on digestion, no Pedicel, 70 Pellagra, 109; relation to diet, 510 Pepsin, 83 Peptones, 83 Peristalsis, 85 Perspiration, 204 Petals, 303 Pfliiger, Eduard F. W., experiments on breathing, 143 Phagocytes, 182 Pharynx, 82 Phloem, 79, 174, 176 Photosynthesis, 53-56, 59, 73, 378, 477 Phototropism, 38, 230 Physic, 119 Physical changes, 7 Physiological variation, 438, 439 Physiology, 15 Pickling and bacteria, 397 Piddock, 368, 370 Pigments, 202, 203, 287 ; and food, 354; and light, 288, 351, 352; and temperature, 353; protective, 351 Piltdown skull, 494 Pisces, 485 Pistil, 300, 478 Pith, in root, 43; in stem, 175 Pithecanthropus erectus, 493, 495, 496 Plague, 506, 510 Plants, chemical composition of, 31 ; classification of, 478; compared with animals, 19; conducting sys- tem of, 174 ff. ; desert, 359 ; genea- logical tree of plant life, 479 ; hairs of, 347 ; heredity in, 444 ; infancy INDEX 525 in lower, 330; kinds of, 477, 480; movements in, 37-38; seed-bearing, 478 ; sensitive, 361 ; useful, and in- sects, 417 Plasma, 182 Plasmodium of malaria, 404 Play, 513 Plumage, color changes in, 286 Plumbing, regulation of, 395 Poisons, 118, 131 ff., 194-195, 253 ff., 355 ; fatigue, 208 ; in soil, 67 Polar bodies, 459 Pollarded trees, 271 Pollen grain, 324, 478 Pollen tube, 303, 324 Pollenation, by birds, 306, 307 ; close and cross, 305 ; cross, 442 ; of fig, 311; function of, 304; by hand, 313; by insects, 309, 311 ; obstacles to close, 305 ; self, 304 ; by water, 308 ; by wind, 308 Pollution, of air, 396; of water, 122, 392 Polycotyls, 34 Polymorphic flowers, 305-307 Population, and climate, 339 ; pres- sure of, 342, 472 Porifera, 482 Posture and breathing, 152 Potato, Burbank, 454 Practice and learning, 249 Prairie dogs, 370 Praying mantis, 372 Precipitin, 193 Precipitin test, 197 Predatory insects, 374 Predatory relations, 341 Prepotency, 307 Preservatives in food,"i25, 126 Pressure, atmospheric, 340 Primates, 486 ; families of, 487 ; limbs of, 490; and man, 491-492; skele- tons of, 491 ; skulls of, 492 Primitive man, see Man Production, cost of, 506 ; of necessi- ties, 514 Prohibition of alcohol, 142 Propagation, by roots, 48 ; vegetative, 269 (see also Regeneration) Protection, of birds, 426 ; against in- fection, 388 ; of young, 37 1 Protective activities, 368 Protective appearances, 351 Protective armors, 345 Protective coloration, 352 Protective invisibility, 352 Protective mimicry, 357 Protective movements, 360 Protective resemblance, 354 Protein, 51, 52; daily need, 90, 93; danger of excess of, 90, 104, 107 ; digestibility of various kinds, in ; digestion of, 35, 83 ; nutritive ratio, 97 ; origin of, 55, 60 ; specific effects of, 107, 108, 109 Prothallus, 322, 323 Protoplasm, 21, 24, 80, 131, 132, 181, 224, 228, 229, 265, 275, 287, 291, 295, 298, 302, 303, 337, 490 ; hered- ity and, 457 ; light destructive to, 38, 339; movements of, 21 ; needs of, 50 Protozoa, 295, 341, 404, 482 Protozoon, 294 Proventriculus of bird, 87 Psychology, 498 Ptarmigan, 286 Pteridophytes, 481 Pulmonary artery, 187 Pulse, 190 Punnett, Professor R. €.,358 Pure lines, 442 Pure races, 447, 455 Purpose, 222-223, 225> 3IO> 332> 358, 369> 433-434, 498> 5°9 Pylorus, 82 Quarantine, 395 Quills, 350 Rabbits accustomed to poison, 132 Rabies, 395 Rats and bubonic plague, 410, 510 Ray (fish), 373 Reaction of organism, general, 224 ; and nerves, 217 Recapitulation in development, 278 Recessive character, 444 ff., 450 ff. ; in man, 455 Reclamation, 68 Recombination of characters, 455 Rectum, 82 Red-headed woodpecker, 428 Reduction division, 459 Reed, Dr. Walter, 407 Reflex, 217 ff., 498; as adaptation, 244; chains, 244; without conscious- ness, 222 ; nerve connections in, 221 ; using an animal's, 217 Refrigeration, 390 526 ELEMENTARY BIOLOGY Regeneration, 268 ; in earthworm, 268 ; of the eye, 268 ; and growth, 265 ; in lizard, 270 ; in plants, 47, 269-270 ; in starfish, 269 Registration of vital statistics, 395 Regulators, 50 Reproduction, 291 ; in animals, 327 ; asexual and sexual, 299, 328 ; among batrachians, 328 ; in fern, 322 ; in fishes, 327 ; in insects, 329 ; in ver- tebrates, 330 Reptiles, egg of, 330 Reptilia, 485 Research, organization of, 509; scien- tific, 511 Resemblances, protective, 354 Respiration, 144; artificial, 170-171; blood and, 146; calorimeter, 92- and energesis, 261 Respirator, 161 Rest and work, 214 Resuscitation, I7off. Reversal of physiological effects, 136 Rexford food tables, 97, 98 Rickets (rachitis), 108 Rockweed, 298 Roots, 32, 42, 300, 477 ; adventitious, 47, 49; as binding agents, 46, 47; climbing, 48, 49 ; and elimination of refuse, 261 ; forms of, 44, 45; prop, 47, 49 ; structure of, 42, 43 ; tap-, 45 ; tubercles, 62 ; uses of, 46-49 Root hairs, 42, 44 Root pressure, 45 Ross, Sir Ronald, 404 Rotation of crops, 62-63, 423 Rotting of flax, 396 Roundworm, 482 Salamander, regeneration in, 268 Saliva, 78, 81, 82 Salivary glands, 83 Salts, and life, 339 ; relation to pro- teins, 556; relation to protoplasm, 52, 339 ; in soil, 30, 66 Sand dunes, 46 San Jose scale, 419 Sap in plants, 46, I76ff. Sapsucker, yellow-bellied, 427 Scale, San Jose, 419 Scales of fishes and reptiles, 350 Scholarship and smoking, 165 School lunches, 128 Science, and agriculture, 67 ; and cost of living, 124, 129, 506 ; and natural resources, 69 ; origin of, 509 ; and research, 511 ; social nature of, 507 Scion, 271 Screening against flies, 403 Seedless fruit, 303, 454 Seedling, 35, 36 Seeds, 32, 315; and alternation of generations, 324, 325 ; distribution of, 317 ff . ; embryo in, 33; escape of, 316; food reserve of, 34, 35, 301-302; and fruit, 301, 302, 315; origin of, 301, 303; protection of, 315, 316; size of, 36, 319 Segregation, law of, 445, 446, 447, 449, , 45°' 455 Selection, improvement through, 439, 440, 441 ff . ; natural, 472 Self-pollenation, 304 Semen, 327, 329 Sensation, sound, 238 : and stimula- tion, 228 Sensitive plant, 361 Serum, 181 Sewage, 60 ; and bacteria, 397 Sex, 296 ff., 320 ff., 331 ff. See also Reproduction Shedding of leaves, 375 Shellac, 413 Shelter, making of, 371 Shelter, primitive, of man, 500 Shoot (plant axis), 32 Sickness, control of, 194 ff., 395, 507 Sieve plates, 176 Skeletons of primates, 491 Skin, 345, 347, 351 ; hygiene of, 206; temperature regulation by, 155 Skulls of primates, 292 Skunk, 374 Sleep, 118, 215 Sleeping sickness, 341 Smell, organs of, 227 ; sense of, in man, 502 Smoke, and health, 162 ; and plants, 162 ; tobacco, 162 Smoking, and school standing, 165; effects of, 163, 164 ff. Snoring, 151 Sociability, in animals, 432 ff. ; in man, J38, 435' 5°4-5°5 Society, and air, 1 58 ff. ; alcohol and, . 137; and birds, 4256°.; and evolu- tion, 473-474; and food, 122 ff.; and the forest, 378 ff . ; and health, 388 ff., 394 ff., 399 ff. ; and insects, INDEX 527 399 ff., 408 ff., 417 ff. ; and mosqui- toes, 408 ff. ; and science, 507 ff. ; and the soil, 65 ff. Soil, 30; and bacteria, 396; biology of, 66 ; chemical changes in, 66 ; conservation of, 68, 380 ; erosion, 47, 380 ; exhaustion of, 66 ; and for- ests, 380-381; increase of, 67-68; intensive cultivation of, 67 ; relation to sprouting, 29 ; as source of organic bodies, 65 Sound, sensation of, 238 Spencer, Herbert, 472 Sperm cell, 299, 327, 459 Spermatophytes, 481 Spermatozoa, 299, 329 Spillman, Dr. W. J., 450 Spineless cactus, 453 Spiracles, 145 Spirogyra, 76, 266; conjugation in, 296 Sponges, 430, 431 ; and bacteria, 396 Spores, 291, 293, 296, 321, 324, 331, 332 ; in animals, 294 ; of fern, 294 ; of moss, 293 ; swimming, 295 Sporophyte, 322, 323, 325 Sport (mutation), 460, 462, 473 Sporulation, 326, 404 Spraying, 423 Sprouting of seeds, 25 ff., 29 Stamens, 478 Standardizing work conditions, 216 Starch, 51 ; digestion of, 94, 100-101 ; energy in, 75 ; origin of, 54 Statistics, vital, 395 Statolith, 241, 242 Stem, 32, 174 ff., 269 ff., 300, 477 Stickleback, 332 Stigma, 301, 303, 304 ff., 31 1, 313 Stimulants, 119, 131 ff., 253 ff. (see also Alcohol) Stimulation, of glands, no, 112, 116, 217 ; of intestines, 119; from meat, 106; of muscles, 217; and sensa- tion, 228 Sting of bee, 373 Stomach, 82, 83 ; glands of, 84 Stomate, 70, 71, 88, 144, 261 Stone Age, 500 Struggle, competitive, 343 ; for exis- tence, 474 Suffocation, 169 Sugar (see Carbohydrates) Sun, relation of, to life, 55, 75 Superstition, 505 Suprarenals, 188 Surface, of cell in relation to growth, 265 ; reduction of, as protection, 359 Surinam Ameiva, 270 Survival of the fittest, 472, 474 Susceptibility, 198. See also Immunity Swallowing, 82, 100 Tannin, 202, 255 Tanning by light, 352 Tapeworm, 342, 369, 411 Tap-roots, 45 Taste, organs of, 226; in selection of food, iio-i 1 1 Tea, 74, 254 Teeth, 81, 105, ii9ff., 502 Temperature, and breathing, 156; and development, 286 ; and life, 337 (see also Heat) ; color changes re- . lated to, 353, 362 ; regulation of, in body, 187; relation of, to sprouting, 27 Test, cytolysin, 197 ; precipitin, 197 Texas fever, 451 Thallophytes, 480 Thymus, 189, 285 Thyroid, 188, 289 ; influence on devel- opment, 285 Tick, 451 Tigerstedt on daily food require- ments, 10 1 Timber supply, 382, 383 Tired feeling, 118 Tissues, 24, 267, 276, 477 Toads, 329; and young, 334 Tobacco and bacteria, 347 Tobacco industry, 74, 167 Tobacco smoke, 162 ff. Tonus, 132 Tools and weapons, 499 Touch, organs of, 226, 228 Tourniquet, 191 Towels, public, 395 Tower, Professor W. L., 462. See also Frontispiece Trachea in man, 148 Tracheae in insects, 145 Transformation (metamorphosis), 280 Transmission, of communicable dis- eases, 317 ; of modifications, 460 Transparency, 352 Transpiration, 71, 72, 178 Transportation, within organism, 44, 79, 174 ff., 179 ff., 185 ff. Tree hoppers, 354 Trees, products of, 377. See also Forest 528 ELEMENTARY BIOLOGY Trench fever, 410 Trimorphic -flowers, 305 Trimorphic insects, 357 Tropism, 37, 38, 230 ; and reflexes, 218 ; and senses, 224 ff. Tubercles, bacteria in, 67 ; nitrogen, 62 Tuberculosis, 1 52, 386, 389, 506, 508 Turnspit animals, 460 Type, mixed, 440 Typhoid fever, 391 ; and flies, 399 ; and water, 392 Underfeeding, 288 Underwing moth, 352 Unit characters, law of, 448, 449, 451 Urea, 144, 201, 204 Urine in diagnosis, 205 Vaccination, compulsory, 394 ; against typhoid fever, 197 ; and vaccines, 198 Vaccines, see Vaccination Valves of heart, 186, 190 Vanessa. butterfly, 287 Vanilla, 312 Variation, 258, 437, 472; causes of, 438 ; physiological, 438, 439 ; in size of similar units, 436 Vegetarianism, 105 Veins, 185 ; of leaves, 34, 70, 175, 176 Ventilation, 154 ff. ; and efficiency of work, 185 ; and moisture, 189 ; of lungs, 148, 152 ; regulation of, 395 ; summary on, 173 Ventricles, 185, 186, 187 Vermiform appendix, 82, 468 Vertebrata, 432 Vertebrates, 482, 492 ; brains of, 497 ; reproduction in, 330 Vessels, blood, 185 ; in plants, 44, 174, 176 Vestigial structures, 468 Vibrations, perception of, 238 Villus, 86, 87 Vinegar and bacteria, 397 Vitamines, 109 Voit, Carl, food doctrines of, 90, 93, 99 Volume of cell in relation to growth, 265 Voluntary muscles, 219 Volvox, 276 Von Baer's Law of Recapitulation, 278 Vries, Hugo de, 440, 442, 461, 462, 473 Walking-leaf insect, 354 Walking-stick insect, 353 Wallaby and young, 334 Wallace, Alfred Russel, 472 War, the Great, 64, 108, 130, 133, 142, 216, 243, 354, 383, 410, 514 Warm-blooded animals, 337 Wastes of organisms, 201 ff. See also Excretion Water, essential to gametes, 328 ; in food-making, 53 ff. ; in growth of plant, 38 ; and life, 50, 52, 338 ; with meals, 114, 117; related to sprouting, 27 Water power and forest, 380 Water supply, 122, 123, 391 ; and forest, 378 ; and typhoid fever, 392 Wax, 413 Waxwing, cedar, 428 Wealth, biology related to, 4. See also Economics Weapons and tools, 499 Weevils, 416; cotton-boll, 418 Weismann, August, 459 Wheat, breeding of, 450 ; immunity to rust in, 451 ; rust of, 411 Wheelworms, 482 Whiteness of fur related to tem- perature, 353 Wood, conservation of, 384 ; in roots, 43, 44; in stems, 174, 175; supply of, 382, 383 ; uses of, 377 Woodpecker, red-headed, 428 Work, alcohol and, 136; hours of, 213; rate of, 211; relation of, to food requirements, 100-101 ; and rest, 214 Work conditions, regulation of, 394, 395; standardizing, 216 Wounds, treatment of, 191 Xylem, 176 Yeast, 23, 133, 291-292 Yellow fever, 406; reduction of, in Cuba, 409 Yellow-bellied sapsucker, 427 Young, protection of, 371. See also Development and Infancy Youth, 335 Zygospore, 297 Zygote, 296, 297, 332, 459 ; origin of, 457 University of Toronto Library DO NOT REMOVE THE CARD FROM THIS POCKET Acme Library Card Pocket Under Pat. "Ref. Index FUe" Made by LIBRARY BUREAU