ment, 84.-The laws or general facts of development, 86.-The significance of the facts of development, 89.-Metamorphosis, 90.-Metamorphosis among insects, 90.-Metamorphosis of the toad, 94.-Metamorphosis among other animals, 96.- Duration of VI. THE PRIMARY CONDITIONS OF ANIMAL LIFE. Primary conditions and special conditions, 106.-Food, 106.- Oxygen, 107.-Temperature, pressure, and other conditions, 108. -Difference between animals and plants, 111.-Living organic matter and inorganic matter, 112. The crowd of animals, 114.-The struggle for existence, 116. -Selection by Nature, 117.-Adjustment to surroundings a re- sult of natural selection, 120.-Artificial selection, 120.-Depend- Origin of adaptations, 123.-Classification of adaptations, 123. -Adaptations for securing food, 125.-Adaptations for self-de- fense, 128.-Adaptations for rivalry, 135.-Adaptations for the defense of the young, 137.-Adaptations concerned with sur- roundings in life, 143.-Degree of structural change in adapta- Man not the only social animal, 149.-The honey-bee, 149.- The ants, 155.-Other communal insects, 158.-Gregariousness and mutual aid, 163.-Division of labor and basis of communal life, 168.-Advantages of communal life, 170. Association between animals of different species, 172.-Com- Relation of parasite and host, 179.-Kinds of parasitism, 180. -The simple structure of parasites, 181.-Gregarina, 182.-The tape-worm and other flat-worms, 183.-Trichina and other round- worms, 184.-Sacculina, 187.-Parasitic insects, 188.-Parasitic vertebrates, 193.-Degeneration through quiescence, 193.-De- generation through other causes, 197.-Immediate causes of de- Protective resemblance defined, 201.-General protective or aggressive resemblance, 202.-Special protective resemblance, 207.-Warning colors and terrifying appearances, 212.-Alluring coloration, 216.-Mimicry, 218.-Protective resemblances and Importance of the special senses, 224.-Difficulty of the study of the special senses, 224.-Special senses of the simplest ani- mals, 225.—The sense of touch, 226.-The sense of taste, 228.- The sense of smell, 229.-The sense of hearing, 232.-Sound-mak- Irritability, 240.-Nerve cells and fibers, 240.-The brain or sensorium, 241.-Reflex action, 241.-Instinct, 242.-Classifica- tion of instincts, 243.-Feeding, 244.-Self-defense, 245.-Play, 247.-Climate, 248.-Environment, 248.-Courtship, 248.-Repro- duction, 249.-Care of the young, 250.-Variability of instincts, Importance of care of the young, 257.-Care of the young and communal life, 257.-The invertebrates (except spiders and in- sects), 258.-Spiders, 259.-Insects, 262.-The vertebrates, 264. XVI.-GEOGRAPHICAL DISTRIBUTION OF ANIMALS Geographical distribution, 272.-Laws of distribution, 274.- Species debarred by barriers, 274.-Species debarred by inability to maintain their ground, 275.-Species altered by adaptation to new conditions, 276.-Effect of barriers, 283.-Relation of species to habitat, 283.-Character of barriers to distribution, 288.-Bar- riers affecting fresh-water animals, 294.-Modes of distribution, 296. Fauna and faunal areas, 296.-Realms of animal life, 297.— Subordinate realms or provinces, 303.-Faunal areas of the sea, THE LIFE OF THE SIMPLEST ANIMALS 1. The simplest animals, or Protozoa. The simplest animals are those whose bodies are simplest in structure and which do the things done by all living animals, such as eating, breathing, moving, feeling, and reproducing in the most primitive way. The body of a horse, made up of various organs and tissues, is complexly formed, and the various organs of the body perform the various kinds of work for which they are fitted in a complex way. The simplest animals are all very small, and almost all live in the water; some kinds in fresh water and many kinds in the ocean. Some live in damp sand or moss, and still others are parasites in the bodies of other animals. They are not familiarly known to us; we can not see them with the unaided eye, and yet there are thousands of different kinds of them, and they may be found wherever there is water. In a glass of water taken from a stagnant pool there is a host of animals. There may be a few water beetles or water bugs swimming violently about, animals half an inch long, with head and eyes and oar-like legs; or there may be a little fish, or some tadpoles and wrigglers. These are evidently not the simplest animals. There will be many very small active animals barely visible to the unaided eyes. These, too, are animals of considerable complexity. But if a single drop of the water be placed on a glass slip or in a watch glass and examined with a compound microscope, there will be seen a number of extremely small creatures which swim about in the water-drop by means of fine hairs, or crawl slowly on the surface of the glass. These are among our simplest animals. There are, as already said, many kinds of these "simplest animals," although, perhaps strictly speaking, only one kind can be called simplest. Some of these kinds are spherical in shape, some elliptical or football-shaped, some conical, some flattened. Some have many fine, minute hairs projecting from the surface; some have a few longer, stronger hairs that lash back and forth in the water, and some have no hairs at all. There are many kinds and they differ in size, shape, body covering, manner of movement, and habit of food-getting. And some are truly simpler than others. But all agree in one thing-which is a very important thing-and that is in being composed in the simplest way possible among animals. 2. The animal cell.-The whole body of any one of the simplest animals or Protozoa is composed for the animal's whole lifetime of but a single cell. The bodies of all other animals are composed of many cells. The cell may be called the unit of animal (or plant) structure. The body of a horse is complexly composed of organs and tissues. Each of these organs and tissues is in turn composed of a large number of these structural units called cells. These cells are of great variety in shape and size and general character. The cells which compose muscular tissue are very different from the cells which compose the brain. And both of these kinds of cells are very different from the simple primitive, undifferentiated kind of cell seen in the body of a protozoan, or in the earliest embryonic stages of a many-celled animal. The animal cell is rarely typically cellular in character -that is, it is rarely in the condition of a tiny sac or box. of symmetrical shape. Plant cells are often of this char |