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.
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.
[Illustration: 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?]
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 larvae which are hidden underneath. Birds do not have teeth.
[Illustration: 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.
[Illustration: African ostrich, one of the largest living birds.]
CLASSIFICATION OF BIRDS
ORDER I. _Cursores._ Running birds with no keeled breastbone. Examples: ostrich, cassowary.
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. _Gallinae._ 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. _Columbinae._ Like Gallinae, 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."
Footnote 28: With the exception of the monotremes.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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 Coelenterates 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:--