XIV. DIVISION OF LABOR. THE VARIOUS FORMS OF PLANTS AND ANIMALS
Problems.—The development and forms of plants.
The development of a simple animal.
What is division of labor? In what does it result?
How to know the chief characters of some great animal groups.
Laboratory Suggestions
A visit to a botanical garden or laboratory demonstration.—Some of the forms of plant life. Review of essential facts in development of bean or corn embryo.
Demonstration.—Charts or models showing the development of a many-celled animal from egg through gastrula stage.
Demonstration.—Types which illustrate increasing complexity of body form and division of labor.
Museum trip.—To afford pupil a means of identification of examples of principal phyla. This should be preceded by objective demonstration work in school laboratory.
Reproduction in Plants.—Although there are very many plants and animals so small and so simple as to be composed of but a single cell, by far the greater part of the animal and plant world is made up of individuals which are collections of cells living together.
A cell of pond scum. How might it divide to form a long thread made up of cells?
In a simple plant like the pond scum, a string or filament of cells is formed by a single cell dividing crosswise, the two cells formed each dividing into two more. Eventually a long thread of cells is thus formed. At times, however, a cell is formed by the union of two cells, one from each of two adjoining filaments of the plant. At length a hard coat forms around this cell, which has now become a spore. The tough covering protects it from unfavorable changes in the surroundings. Later, when conditions become favorable for its germination, the spore may form a new filament of pond scum. In molds, in yeasts, and in the bacteria we also found spores could be formed by the protoplasm of the plant cutting up into a number of tiny spores. These spores are called asexual (without sex) because they are not formed by the union of two cells, and may give rise to other tiny plants like themselves. Still other plants, mosses and ferns, give rise to two kinds of spores, sexual and asexual. All of these collectively are called spore plants.
The formation of spores in pond scum. zs, zygospore; f, fusion in progress.
Reproduction in Seed Plants.—Another great group of plants we have studied, plants of varied shapes and sizes, produce seeds. They bear flowers and fruits.
The formation and growth of a plant embryo. 1, the sperm and egg cell uniting; 2, a fertilized egg; 3, two cells formed by division; 4, four cells formed from two; 5, a many-celled embryo; 6, young plant; H, hypocotyl; P, plumule; C, cotyledons.
The embryo develops from a single fertilized "egg," growing by cell division into two, four, eight, and a constantly increasing number of cells until after a time a baby plant is formed, which as in the bean, either contains some stored food to give it a start in life, or, as in the corn, is surrounded with food which it can digest and absorb into its own tiny body. We have seen that these young plants in the seed are able to develop when conditions are favorable. Furthermore, the young of each kind of plant will eventually develop into the kind of plant its parent was and into no other kind. Thus the plant world is divided into many tribes or groups.
A colony of trilliums, a flowering plant. (Photograph by W. C. Barbour.)
Plants are placed in Groups.—If we plant a number of peas so that they will all germinate under the same conditions of soil, temperature, and sunlight, the seedlings that develop will each differ one from another in a slight degree.[27] But in a general way they will have many characters in common, as the shape of the leaves, the possession of tendrils, form of the flower and fruit. A species of plants or animals is a group of individuals so much alike in their characters that they might have had the same parents. Individuals of such species differ slightly; for no two individuals are exactly alike.
Rock fern, polypody. Notice the underground stem giving off roots from its lower surface, and leaves (C), (S), from its upper surface.
Species are grouped together in a larger group called a genus. For example, many kinds of peas—the wild beach peas, the sweet peas, and many others—are all grouped in one genus (called Lathyrus, or vetchling) because they have certain structural characteristics in common.
Plant and animal genera are brought together in still larger groups, the classification based on general likenesses in structure. Such groups are called, as they become successively larger, Family, Order, and Class. Thus both the plant and animal kingdoms are grouped into divisions, the smallest of which contains individuals very much alike; and the largest of which contains very many groups of individuals, the groups having some characters in common. This is called a system of classification.
Classification of the Plant Kingdom.—The entire plant kingdom has been divided into four sub-kingdoms by botanists:—
| 1. Spermatophytes. | Angiosperms, true flowering plants. Gymnosperms, the pines and their allies. |
| 2. Pteridophytes. | The fern plants and their allies. |
| 3. Bryophytes. | The moss plants and their allies. |
| 4. Thallophytes. | The Thallophytes form two groups: the Algæ and the Fungi; the algæ being green, while the fungi have no chlorophyll. |
Rockweed, a brown algæ, showing its distribution on rocks below highwater mark.
The extent of the plant kingdom can only be hinted at; each year new species are added to the lists. There are about 110,000 species of flowering plants and nearly as many flowerless plants. The latter consist of over 3500 species of fernlike plants, some 16,500 species of mosses, over 5600 lichens (plants consisting of a partnership between algæ and fungi), approximately 55,000 species of fungi, and about 16,000 species of algæ.
A moss plant. G, the moss body; S, the spore-bearing stalk (fruiting body).
Development of a Simple Animal.—Many-celled animals are formed in much the same way as are many-celled seed plants. A common bath sponge, an earthworm, a fish, or a dog,—each and all of them begin life in the same manner. In a many-celled animal the life history begins with a single cell, the fertilized egg. As in the flowering plant, this cell has been formed by the union of two other cells, a tiny (usually motile) cell; the sperm, and a large cell, the egg. After the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the blastula is formed; later this ball sinks in on one side, and a double-walled cup of cells, now called a gastrula, results. Practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg.
In animals the body consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called ectoderm, which later gives rise to the skin, nervous system, etc.; an inner layer, developed from the inner layer of the gastrula, the endoderm, which forms the lining of the digestive organs, etc.; a middle layer, called the mesoderm, lying between the ectoderm and the endoderm, is also found. In higher animals this layer gives rise to muscles, the skeleton, and parts of other internal structures.
Stages in the development of a fertilized egg into the gastrula stage. Read your text, then draw these stages and name each stage.
Photograph of a living vorticella, showing the contractile stalk and the cilia around the mouth. Compare this figure with that of the paramœcium. Which cell shows greater division of labor?
Physiological Division of Labor.—If we compare the amœba and the paramœcium, we find the latter a more complex organism than the former. An amœba may take in food through any part of the body; the paramœcium has a definite gullet; the amœba may use any part of the body for locomotion; the paramœcium has definite parts of the cell, the cilia, fitted for this work. Since the structure of the paramœcium is more complex, we say that it is a "higher" animal. In the vorticella, a still more complex cell, part of the cell has grown out like a stalk, has become contractile, and acts like muscle.
As we look higher in the scale of life, we invariably find that certain parts of a plant or animal are set apart to do certain work, and only that work. Just as in a community of people, there are some men who do rough manual work, others who are skilled workmen, some who are shopkeepers, and still others who are professional men, so among plants and animals, wherever collections of cells live together to form an organism, there is division of labor, some cells being fitted to do one kind of work, while others are fitted to do work of another sort. This is called physiological division of labor.
Enlarged lengthwise section of the hydra, a very simple animal which shows slight division of labor. ba, base; b, bud; m, mouth; ov, ovary; sp, spermary.
Different forms of tissue cells. C, bone making cells; E, epithelial cells; F, fat cells; L, liver cells;M, muscle cell; i, involuntary; v, voluntary; N, nerve cell; C B, cell body; N.F., nerve fiber; T.B., nerve endings; W, colorless blood cells.
As we have seen, the higher plants are made up of a vast number of cells of many kinds. Collections of cells alike in structure and performing the same function we have called a tissue. Examples of animal tissues are the highly contractile cells set apart for movement, muscles; those which cover the body or line the inner parts of organs, the skin, or epithelium; the cells which form secretions or glands and the sensitive cells forming the nervous tissues.
Frequently several tissues have certain functions to perform in conjunction with one another. The arm of the human body performs movement. To do this, several tissues, as muscles, nerves, and bones, must act together. A collection of tissues performing certain work we call an organ.
Part of a sponge, showing how cells perform division of labor. ect, ectoderm; mes, mesoderm; end, endoderm; c.c., ciliated cells, which take in food by means of their flagellæ or large cilia (fla).
In a simple animal like a sponge, division of labor occurs between the cells; some cells which line the pores leading inward create a current of water, and feed upon the minute organisms which come within reach, other cells build the skeleton of the sponge, and still others become eggs or sperms. In higher animals more complicated in structure and in which the tissues are found working together to form organs, division of labor is much more highly specialized. In the human arm, an organ fitted for certain movements, think of the number of tissues and the complicated actions which are possible. The most extreme division of labor is seen in the organism which has the most complex actions to perform and whose organs are fitted for such work, for there the cells or tissues which do the particular work do it quickly and very well.
In our daily life in a town or city we see division of labor between individuals. Such division of labor may occur among other animals, as, for example, bees or ants. But it is seen at its highest in a great city or in a large business or industry. In the stockyards of Chicago, division of labor has resulted in certain men performing but a single movement during their entire day's work, but this movement repeated so many times in a day has resulted in wonderful accuracy and speed. Thus division of labor obtains its end.
Organs and Functions Common to All Animals.—The same general functions performed by a single cell are performed by a many-celled animal. But in the many-celled animals the various functions of the single cell are taken up by the organs. In a complex organism, like man, the organs and the functions they perform may be briefly given as follows:—
(1) The organs of food taking: food may be taken in by individual cells, as those lining the pores of the sponge, or definite parts of a food tube may be set apart for this purpose, as the mouth and parts which place food in the mouth.
(2) The organs of digestion: the food tube and collections of cells which form the glands connected with it. The enzymes in the fluids secreted by the latter change the foods from a solid form (usually insoluble) to that of a fluid. Such fluid may then pass by osmosis, through the walls of the food tube into the blood.
(3) The organs of circulation: the tubes through which the blood, bearing its organic foods and oxygen, reaches the tissues of the body. In simple animals, as the sponge and hydra, no such organs are needed, the fluid food passing from cell to cell by osmosis.
(4) The organs of respiration: the organs in which the blood receives oxygen and gives up carbon dioxide. The outer layer of the body serves this purpose in very simple animals; gills or lungs are developed in more complex animals.
(5) The organs of excretion: such as the kidneys and skin, which pass off nitrogenous and other waste matters from the body.
(6) The organs of locomotion: muscles and their attachments and connectives; namely, tendons, ligaments, and bones.
(7) The organs of nervous control: the central nervous system, which has control of coördinated movement. This consists of scattered cells in low forms of life; such cells are collected into groups and connected with each other in higher animals.
(8) The organs of sense: collections of cells having to do with the reception and transmission of sight, hearing, smell, taste, touch, pressure, and temperature sensations.
(9) The organs of reproduction: the sperm and egg-forming organs.
Almost all animals have the functions mentioned above. In most, the various organs mentioned are more or less developed, although in the simpler forms of animal life some of the organs mentioned above are either very poorly developed or entirely lacking. But in the so-called "higher" animals each of the above-named functions is assigned to a certain organ or group of organs. The work is done better and more quickly than in the "lower" animals. Division of labor is thus a guide in helping us to determine the place of animals in the groups that exist on the earth.
The glasslike skeleton of a radiolarian, a protozoan. (From model at American Museum of Natural History.)
The Animal Series.—We have found that a one-celled animal can perform certain functions in a rather crude manner. Man can perform these same functions in an extremely efficient manner. Division of labor is well worked out, extreme complexity of structure is seen. Between these two extremes are a great many groups of animals which can be arranged more or less as a series, showing the gradual evolution or development of life on the earth. It will be the purpose of the following pages to show the chief characteristics of the great groups of the animal kingdom.
I. Protozoa.—Animals composed of a single cell, reproducing by cell division.
The following are the principal classes of Protozoa, examples of which we may have seen or read about:—
Class I. Rhizopoda (Greek for root-footed). Having no fixed form, with pseudopodia. Either naked as Amœba or building limy (Foraminifera) or glasslike skeletons (Radiolaria).
Class II. Infusoria (in infusions). Usually active ciliated Protozoa. Examples, Paramœcium, Vorticella.
Class III. Sporozoa (spore animals). Parasitic and usually nonactive. Example, Plasmodium malariæ.
II. Sponges.—Because the body contains many pores through which water bearing food particles enters, these animals are called Porifera. They are classed according to the skeleton they possess into limy, glasslike, and horny fiber sponges. The latter are the sponges of commerce. With but few exceptions sponges live in salt water and are never free swimming.
A horny fiber sponge. Notice that it is a colony. One fourth natural size.
III. Cœlenterates.—The hydra and its salt-water allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the Cœlenterata. The word "cœlenterate" (cœlom = body cavity, enteron = food tube) explains the structure of the group. They are animals in which the real body cavity is lacking, the animal in its simplest form being little more than a bag. Some examples are the hydra, shown on page 179, salt-water forms known as hydroids, colonial forms which have part of their life free swimming as jellyfish; sea anemones and coral polyps, tiny colonial hydra like forms which build a living or secreted covering.
Sea anemones. One half natural size. The right hand specimen is expanded and shows the mouth surrounded by the tentacles. The left hand specimen is contracted. (From model at the American Museum of Natural History.)
IV. Worms.—The wormlike animals are grouped into flatworms, roundworms, and segmented or jointed worms.
(a) Flatworms are sometimes parasitic, examples being the tapeworm and liver fluke. They are usually small, ribbon- or leaf-like and flat and live in water.
(b) Roundworms, minute threadlike creatures, are not often seen by the city girl or boy. Vinegar eels, the horsehair worm, the pork worm or trichina and the dread hookworm are examples.
(c) Segmented worms are long, jointed creatures composed of body rings or segments. Examples are the earthworm, the sandworm (known to New York boys as the fishworm), and the leeches or bloodsuckers.
A jointed worm. The sandworm. Slightly reduced.
The common starfish seen from below to show the tube feet. About one half natural size.
V. Echinoderms.—These are spiny-skinned animals, which live in salt water. They are still more complicated in structure than the worms and may be known by the spines in their skin. They show radial symmetry. Starfish or sea urchins are examples.
The crayfish, a crustacean. A, antenna; M, mouth; E, compound stalked eye; Ch, pincher claw; C.P., cephalothorax; Ab, abdomen; C.F., caudal fin. A little reduced.
VI. Arthropods.—These animals are distinguished by having jointed body and legs. They form two great groups. The higher forms of the Crustacea have only two regions in the body, a fused head and thorax, called the cephalothorax, and an abdominal region. A second group is the Insecta, of which we know something already. Crustacea breathe by means of gills, which are structures for taking oxygen out of the water, while adult insects breathe through air tubes called trachea.
A common snail, a mollusk. (From a photograph by Davison.)
Two smaller groups of arthropods also exist, the Arachnida, consisting of spiders, scorpions, ticks, and mites, and the Myriapoda, examples being the "thousand leggers" found in some city houses.
VII. Mollusca.—Another large group is the Mollusca. This phylum gets its name from the soft, unsegmented body (mollis = soft). Mollusks usually have a shell, which may be of one piece, as a snail, or two pieces or valves, as the clam or oyster.
The skeleton of a dog; a typical vertebrate.
VIII. The Vertebrates.—All of the animals we have studied thus far agree in having whatever skeleton or hard parts they possess on the outside of the body. Collectively, they are called Invertebrates. This exoskeleton differs from the main or axial skeleton of the higher animals, the latter being inside of the body. The exoskeleton is dead, being secreted by the cells lining the body, while the endoskeleton is, in part at least, alive and is capable of growth, e.g. a broken arm or leg bone will grow together. But a man has certain parts of the skeleton, as nails or hair, formed by the skin and in addition possesses inside bones to which the muscles are attached. Some of the bones are arranged in a flexible column in the dorsal (the back) side of the body. This vertebral column, as it is called, is distinctive of all vertebrates. Within its bony protection lies the delicate central nervous system, and to this column are attached the big bones of the legs and arms. The vertebrate animals deserve more of our attention than other forms of life because man himself is a vertebrate.
The sand shark, an elasmobranch. Note the slits leading from the gills. (From a photograph loaned by the American Museum of Natural History.)
Five groups or classes of vertebrates exist. Fishes, Amphibians, Reptiles, Birds, and Mammals. Let us see how to distinguish one class from another.
The sturgeon, a ganoid fish.
Fishes.—Fishes are familiar animals to most of us. We know that they live in the water, have a backbone, and that they have fins. They breathe by means of gills, delicate organs fitted for taking oxygen out of the water. The heart has two chambers, an auricle and a ventricle. They have a skin in which are glands secreting mucus, a slimy substance which helps them go through the water easily. They usually lay very many eggs.
Classification of Fishes
Order I. The Elasmobranchs. Fishes which have a soft skeleton made of cartilage and exposed gill slits. Examples: sharks, skates, and rays.
Order II. The Ganoids. Fishes which once were very numerous on the earth, but which are now almost extinct. They are protected by platelike scales. Examples: gars, sturgeon, and bowfin.
A bony fish.
Order III. The Teleosts, or Bony Fishes. They compose 95 per cent of all living fishes. In this group the skeleton is bony, the gills are protected by an operculum, and the eggs are numerous. Most of our common food fishes belong to this class.
Order IV. The Dipnoi, or Lung Fishes. This is a very small group. In many respects they are more like amphibians than fishes, the swim bladder being used as a lung. They live in tropical Africa, South America, and Australia, inhabiting the rivers and lakes there.
Newt. (From a photograph loaned by the American Museum of Natural History.) About natural size.
Characteristics of Amphibia.—The frog belongs to the class of vertebrates known as Amphibia. As the name indicates (amphi, both, and bia, life), members of this group live both in water and on land. In the earlier stages of their development they take oxygen into the blood by means of gills. When adult, however, they breathe by means of lungs. At all times, but especially during the winter, the skin serves as a breathing organ. The skin is soft and unprotected by bony plates or scales. The heart has three chambers, two auricles and one ventricle. Most amphibians undergo a complete metamorphosis, or change of form, the young being unlike the adults.
Order I. Urodela. Amphibia having usually poorly developed appendages. Tail persistent through life. Examples: mud puppy, newt, salamander.
Order II. Anura. Tailless Amphibia, which undergo a metamorphosis, breathing by gills in larval state, by lungs in adult state. Examples: toad and frog.
The leopard frog, an amphibian.
Characteristics of Reptilia.—These animals are characterized by having scales developed from the skin. In the turtle they have become bony and are connected with the internal skeleton. Reptiles always breathe by means of lungs, differing in this respect from the amphibians. They show their distant relationship to birds in that their large eggs are incased in a leathery, limy shell.
Classification of Reptiles
Order I. Chelonia (turtles and tortoises). Flattened reptiles with body inclosed in bony case. No teeth or sternum (breastbone). Examples: snapping turtle, box tortoise.
Order II. Lacertilia (lizards). Body covered with scales, usually having two-paired appendages. Breathe by lungs. Examples: fence lizard, horned toad.
Order III. Ophidia (snakes). Body elongated, covered with scales. No limbs present. Examples: garter snake, rattlesnake.
Order IV. Crocodilia. Fresh-water reptiles with elongated body and bony scales on skin. Two-paired limbs. Examples: alligator, crocodile.
Box tortoise, a land reptile. (From photograph loaned by the American Museum of Natural History.) About one fourth natural size.
The gila monster, a poisonous lizard. About one twelfth natural size.
The common garter snake. Reduced to about one tenth natural size.
Adaptations in the bills of birds. Could we tell anything about the food of a bird from its bill? Do these birds all get their food in the same manner? Do they all eat the same kind of food?
Birds.—Birds among all other animals are known by their covering of feathers and the presence of wings. The feathers are developed from the skin. These aid in flight, and protect the body from the cold.
The form of the bill in particular shows adaptation to a wonderful degree. A duck has a flat bill for pushing through the mud and straining out the food; a bird of prey has a curved or hooked beak for tearing; the woodpecker has a sharp, straight bill for piercing the bark of trees in search of the insect larvæ which are hidden underneath. Birds do not have teeth.
Common tern and young, showing nesting and feeding habits. (From group at American Museum of Natural History.)
The rate of respiration, of heartbeat, and the body temperature are all higher in the bird than in man. Man breathes from twelve to fourteen times per minute. Birds breathe from twenty to sixty times a minute. Because of the increased activity of a bird, there comes a necessity for a greater and more rapid supply of oxygen, an increased blood supply to carry the material to be used up in the release of energy, and a means of rapid excretion of the wastes resulting from the process of oxidation. Birds are large eaters, and the digestive tract is fitted to digest the food quickly, by having a large crop in which food may be stored in a much softened condition. As soon as the food is part of the blood, it may be sent rapidly to the places where it is needed, by means of the large four-chambered heart and large blood vessels.
The high temperature of the bird is a direct result of this rapid oxidation; furthermore, the feathers and the oily skin form an insulation which does not readily permit of the escape of heat. This insulating cover is of much use to the bird in its flights at high altitudes, where the temperature is often very low. Birds lay eggs and usually care for their young.
Classification of Birds
Order I. Cursores. Running birds with no keeled breastbone. Examples: ostrich, cassowary.
African ostrich, one of the largest living birds.
Order II. Passeres. Perching birds; three toes in front, one behind. Over one half of all species of birds are included in this order. Examples: sparrow, thrush, swallow.
Order III. Gallinæ. Strong legs; feet adapted to scratching. Beak stout. Examples: jungle fowl, grouse, quail, domestic fowl.
Order IV. Raptores. Birds of prey. Hooked beak. Strong claws. Examples: eagle, hawk, owl.
Order V. Grallatores. Waders. Long neck, beak, and legs. Examples: snipe, crane, heron.
Order VI. Natatores. Divers and swimmers. Legs short, toes webbed. Examples: gull, duck, albatross.
Order VII. Columbinæ. Like Gallinæ, but with weaker legs. Examples: dove, pigeon.
Order VIII. Pici. Woodpeckers. Two toes point forward, two backward, and adaptation for climbing. Long, strong bill.
Order IX. Psittaci. Parrots, hooked beak and fleshy tongue.
Order X. Coccyges. Climbing birds, with powerful beak. Examples: kingfisher, toucan, and cuckoo.
Order XI. Macrochires. Birds having long-pointed wings, without scales on metatarsus. Examples: swift, humming bird, and goatsucker.
Mammals.—Dogs and cats, sheep and pigs, horses and cows, all of our domestic animals (and man himself) have characters of structure which cause them to be classed as mammals. They, like some other vertebrates, have lungs and warm blood. They also have a hairy covering and bear young developed to a form similar to their own,[28] and nurse them with milk secreted by glands known as the mammary glands; hence the term "mammal."
The bison, an almost extinct mammal.
Adaptations in Mammalia.—Of the thirty-five hundred species, most inhabit continents; a few species are found on different islands, and some, as the whale, inhabit the ocean. They vary in size from the whale and the elephant to tiny shrew mice and moles. Adaptations to different habitat and methods of life abound; the seal and whale have the limbs modified into flippers, the sloth and squirrel have limbs peculiarly adapted to climbing, while the bats have the fore limbs modeled for flight.
Lowest Mammals.—The lowest are the monotremes, animals which lay eggs like the birds, although they are provided with hairy covering like other mammals. Such are the Australian spiny anteater and the duck mole.
All other mammals bring forth their young developed to a form similar to their own. The kangaroo and opossum, however, are provided with a pouch on the under side of the body in which the very immature, blind, and helpless young are nourished until they are able to care for themselves. These pouched animals are called marsupials.
The other mammals may be briefly classified as follows:—
Classification of Higher Mammals
Order I. Edentata. Toothless or with very simple teeth. Examples: anteater, sloth, armadillo.
Order II. Rodentia. Incisor teeth chisel-shaped, usually two above and two below. Examples: beaver, rat, porcupine, rabbit, squirrel.
Order III. Cetacea. Adapted to marine life. Examples: whale, porpoise.
Order IV. Ungulata. Hoofs, teeth adapted for grinding. Examples: (a) odd-toed, horse, rhinoceros, tapir; (b) even-toed, ox, pig, sheep, deer.
Order V. Carnivora. Long canine teeth, sharp and long claws. Examples: dog, cat, lion, bear, seal, and sea lion.
Order VI. Insectivora. Example: mole.
Order VII. Cheiroptera. Fore limbs adapted to flight, teeth pointed. Example: bat.
Order VIII. Primates. Erect or nearly so, fore appendage provided with hand. Examples: monkey, ape, man.
The geological history of the horse. (After Mathews, in the American Museum of Natural History.) Ask your teacher to explain this diagram.
Increasing Complexity of Structure and of Habits in Plants and Animals.—In our study of biology so far we have attempted to get some notion of the various factors which act upon living things. We have seen how plants and animals interact upon each other. We have learned something about the various physiological processes of plants and animals, and have found them to be in many respects identical. We have found grades of complexity in plants from the one-celled plant, bacterium or pleurococcus, to the complicated flowering plants of considerable size and with many organs. So in animal life, from the Protozoa upward, there is constant change, and the change is toward greater complexity of structure and functions. An insect is a higher type of life than a protozoan, because its structure is more complex and it can perform its work with more ease and accuracy. A fish is a higher type of animal than the insect for these same reasons, and also for another. The fish has an internal skeleton which forms a pointed column of bones on the dorsal side (the back) of the animal. It is a vertebrate animal.
The evolutionary tree. Modified from Galloway. Copy this diagram in your notebook. Explain it as well as you can.
The Doctrine of Evolution.—We have now learned that animal forms may be arranged so as to begin with very simple one-celled forms and culminate with a group which contains man himself. This arrangement is called the evolutionary series. Evolution means change, and these groups are believed by scientists to represent stages in complexity of development of life on the earth. Geology teaches that millions of years ago, life upon the earth was very simple, and that gradually more and more complex forms of life appeared, as the rocks formed latest in time show the most highly developed forms of animal life. The great English scientist, Charles Darwin, from this and other evidence, explained the theory of evolution. This is the belief that simple forms of life on the earth slowly and gradually gave rise to those more complex and that thus ultimately the most complex forms came into existence.
The Number of Animal Species.—Over 500,000 species of animals are known to exist to-day, as the following table shows.
| Protozoa | 8,000 | Arachnids | 16,000 |
| Sponges | 2,500 | Crustaceans | 16,000 |
| Cœlenterates | 4,500 | Mollusks | 61,000 |
| Echinoderms | 4,000 | Fishes | 13,000 |
| Flat-worms | 5,000 | Amphibians | 1,400 |
| Roundworms | 1,500 | Reptiles | 3,500 |
| Annelids | 4,000 | Birds | 13,000 |
| Insects | 360,000 | Mammals | 3,500 |
| Myriapods | 2,000 | Total | 518,900 |
Man's Place in Nature.—Although we know that man is separated mentally by a wide gap from all other animals, in our study of physiology we must ask where we are to place man. If we attempt to classify man, we see at once he must be placed with the vertebrate animals because of his possession of a vertebral column. Evidently, too, he is a mammal, because the young are nourished by milk secreted by the mother and because his body has at least a partial covering of hair. Anatomically we find that we must place man with the apelike mammals, because of these numerous points of structural likeness. The group of mammals which includes the monkeys, apes, and man we call the primates.
Although anatomically there is a greater difference between the lowest type of monkey and the highest type of ape than there is between the highest type of ape and the lowest savage, yet there is an immense mental gap between monkey and man.
Instincts.—Mammals are considered the highest of vertebrate animals, not only because of their complicated structure, but because their instincts are so well developed. Monkeys certainly seem to have many of the mental attributes of man.
Professor Thorndike of Columbia University sums up their habits of learning as follows:—
"In their method of learning, although monkeys do not reach the human stage of a rich life of ideas, yet they carry the animal method of learning, by the selection of impulses and association of them with different sense-impressions, to a point beyond that reached by any other of the lower animals. In this, too, they resemble man; for he differs from the lower animals not only in the possession of a new sort of intelligence, but also in the tremendous extension of that sort which he has in common with them. A fish learns slowly a few simple habits. Man learns quickly an infinitude of habits that may be highly complex. Dogs and cats learn more than the fish, while monkeys learn more than they. In the number of things he learns, the complex habits he can form, the variety of lines along which he can learn them, and in their permanence when once formed, the monkey justifies his inclusion with man in a separate mental genus."
Evolution of Man.—Undoubtedly there once lived upon the earth races of men who were much lower in their mental organization than the present inhabitants. If we follow the early history of man upon the earth, we find that at first he must have been little better than one of the lower animals. He was a nomad, wandering from place to place, feeding upon whatever living things he could kill with his hands. Gradually he must have learned to use weapons, and thus kill his prey, first using rough stone implements for this purpose. As man became more civilized, implements of bronze and of iron were used. About this time the subjugation and domestication of animals began to take place. Man then began to cultivate the fields, and to have a fixed place of abode other than a cave. The beginnings of civilization were long ago, but even to-day the earth is not entirely civilized.
The Races of Man.—At the present time there exist upon the earth five races or varieties of man, each very different from the other in instincts, social customs, and, to an extent, in structure. These are the Ethiopian or negro type, originating in Africa; the Malay or brown race, from the islands of the Pacific; the American Indian; the Mongolian or yellow race, including the natives of China, Japan, and the Eskimos; and finally, the highest type of all, the Caucasians, represented by the civilized white inhabitants of Europe and America.
[27] Note To Teachers.—A trip to the Botanical Garden or to a Museum should be taken at this time.
[28] With the exception of the monotremes.
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology, American Book Company.
Bulletin of U. S. Department of Agriculture, Division of Biological Survey, Nos. 1, 6, 13, 17.
Davison, Practical Zoölogy. American Book Company.
Ditmars, The Reptiles of New York. Guide Leaflet 20. Amer. Mus. of Nat. History.
Sharpe,[TN2] A Laboratory Manual in Biology, pp. 140-150, American Book Company.
Walker, Our Birds and Their Nestlings. American Book Company.
Walter, H. E. and H. A., Wild Birds in City Parks. Published by authors.
advanced
Apgar, Birds of the United States. American Book Company.
Beebe, The Bird. Henry Holt and Company.
Ditmars, The Reptile Book. Doubleday, Page and Company.
Hegner, Zoölogy. The Macmillan Company.
Hornaday, American Natural History.
Jordan and Evermann, Food and Game Fishes. Doubleday, Page and Company.
Parker and Haswell, Textbook of Zoölogy. The Macmillan Company.
Riverside Natural History. Houghton, Mifflin and Company.
Weed and Dearborn, Relation of Birds to Man. Lippincott.