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tions for food getting and escape from enemies, the seasonal change in color of the weasel is an example of an adaptation which serves both of the above purposes. This is only one of hundreds of others that might be mentioned.

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The California sea lion (Zalophus californianus). Photographed in the Philadelphia Zoological Gardens by Davison.

Economic Importance. - The Carnivora as a group are of much economic importance as the source of most of our fur. The fur seal fisheries alone amount to millions of dollars annually. Otters, skunks, sables, weasels, and minks are of considerable importance as fur products. In India tigers do considerable harm in some localities, and in our own country wolves, pumas, and wild cats do some damage.

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Ungulates: Hoofed Mammals. This group includes the domesticated animals as the horse, cow, sheep, and pig. A group of animals which originally roamed wild, they eventually came under the subjugating influence of man. Now they form a source of the world's wealth, and are an important part of the wealth of the United States.

The order of ungulates is a very large one. It is characterized by the fact that the nails have grown down to become thickened as hoofs. In some cases only two (the third and fourth) toes are largely developed. Such animals have a cleft hoof, as in the ox, deer, sheep, and pigs. These form the even-toed ungulates.

Even-toed Ungulates. -The deer family are the largest in number of species and individuals among our native forms, and in fact the world over. Among them are the common Virginia deer of the Eastern states, the whitetailed deer of our Adirondack forests. All males of the deer family possess horns which are solid and are shed annually. The antelopes and giraffes have also solid horns, but

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Virginia deer. From photograph loaned by the
American Museum of Natural History.

do not shed them. The bison, or buffalo, is nearly related to the deer and wild cattle. Formerly bisons existed in enormous numbers on our Western plains. They were hunted by whites and Indians for the hides and tongues only, and thousands of carcasses were left to rot after a hunt. They are now almost extinct.

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The buffalo (bison). Photographed in the Philadelphia Zoological Gardens by Davison.

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Odd-toed Ungulates. In some ungulates the middle toe of the foot has become largely developed, with the result that the animal stands on it. Such are the zebra and the horse, and the rhinoceros, which has also the second and fourth toes in each foot.

Geologic History of the Horse. - We have, from time to time, made reference to the fact that certain forms of life, now almost extinct, flourished on the earth in former geologic periods. One type of animal which under domestication has become greatly improved is the horse. It is interesting

to note that America was the original home of the horse. Ages ago, the ancestor of the present horse was not larger than a fox terrier, and instead of having one toe prolonged into a hoof, it walked on five toes. Later changes probably caused the little horse to abandon the life it led in the swamp for that on drier land. At that time it was about the size of a sheep and had three toes in each foot. Later the longer-legged and single-toed horses probably became speedier, thus escaping from the numerous carnivores which must have preyed upon them. So ultimately by very gradual variation the present horse was evolved. This purely hypothetical history was probably repeated with variations in the case of many other species of animals.

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.

REFERENCE BOOKS

FOR THE PUPIL

American Book Company.

Davison, Practical Zoology, pages 261–292. American Book Company.
Herrick, Text-book in General Zoology, Chap. XXIV.
Ingersoll, Wild Neighbors. The Macmillan Company.
Ingersoll, Life of the Mammals. The Macmillan Company.

FOR THE TEACHER

Dodge, General Zoology, pages 177-202. American Book Company.
Riverside Natural History. Houghton, Mifflin, and Company.

PART III. HUMAN PHYSIOLOGY

XXVII. FOODS

Why we need Food. We have already defined food as anything that forms material for the growth or repair of the body of a plant or animal, or that furnishes energy for it. Food, then, not only furnishes our body with material to grow, but also gives us the energy we expend in the acts of walking, running, breathing, and even in thinking.

Nutrients. Certain nutrient materials form the basis of food of both plants and animals. These have been stated to be carbohy drates (starches, sugars, gums, etc.), hydrocarbons (fats and oils, both animal and vegetable), proteids (such as lean meat, eggs, the gluten of bread), and mineral matter and water. Oxygen, although not a nutrient, ought to be considered as food because it enters into the composition of the body, and without it no energy could be released. Let us apply this general knowledge with reference to the human body in order to determine the uses made of food taken into the body; for parts of the human body, be they muscle, blood, nerve, bone, or gristle, are built up from the nutrients in our food.

The Body a Machine. The body has been likened to a machine in that it turns over the latent or potential energy stored up in food into kinetic energy (mechanical work and heat) which is manifested when we perform work. One great difference exists between an engine and the human body. The engine uses fuel unlike the substance out of which it is made. The human body, on the other hand, uses for fuel the same substances out of which it is formed; it may, indeed, use part of its own substance for food. Let us now consider more in detail the nutrients used by man as food, and determine the use of each to him.

Proteids. As we know, proteids, in some manner unknown to us, are manufactured in the leaves of plants. Proteid sub

stances contain the element nitrogen. Hence such foods are called nitrogenous foods. Although about four fifths of the atmosphere is composed of nitrogen, yet plants are unable to take it from the air, but are forced to absorb it through the roots in the form of nitrates dissolved in the water in the soil. Herbivorous animals eat the plants, take into their bodies the stored nitrogenous foods, and change this food into protoplasm. Man himself must form the protoplasm of his body (that is, the muscles, tendons, nervous system, blood corpuscles, the living parts of the bone and the skin, etc.) from nitrogenous food. Some of this he obtains by eating the flesh of animals, and some he obtains directly from plants (for example, peas and beans). Because of their chemical composition, proteids are considered to be flesh-forming foods. They are, however, oxidized to release energy if occasion requires it.

Hydrocarbons or Fats. Hydrocarbons (fats and oils) are the materials from which the body derives most of its energy. The chemical formula of a fat shows that, compared with other food substances, there is very little oxygen present; hence the greater capacity of this substance for uniting with oxygen. The rapid burning of fat compared with the slower combustion of a piece of meat or a piece of bread illustrates this. A pound of butter releases over twice as much energy to the body as does a pound of sugar or a pound of steak. Fats and oils are stored in the human body as fatty tissue. This material may be drawn on as reserve fuel when the body requires it. The Arctic-living tribes, as is well known, exist almost entirely on the blubber of the walrus or the whale. Thus they transform this fatty material into heat and are enabled to withstand the intense cold of these regions.

Carbohydrates. We see that the carbohydrates, like the fats, contain carbon, hydrogen, and oxygen. Here, however, the oxygen and hydrogen are united in the molecule in the same proportion as are hydrogen and oxygen in water. Carbohydrates are essentially energy producing foods. They are, however, believed to be of some use in building up or repairing tissue. Some experiments seem to indicate that carbohydrates may be formed directly into tissue. It is certainly true that in both plants and

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