259. Adaptations concerned with surroundings in life.-A large part of the life of the animal is a struggle with the environment itself; in this struggle only those that are adapted live and leave descendants fitted like themselves. The fur of mammals fits them to their surroundings. As the fur differs, so may the habits change. Some animals are active in winter; others, as the bear, hibernate, sleeping in caves or hollow trees or in burrows until conditions are favorable for their activity. Most snakes and lizards. hibernate in cold weather. In the swamps of Louisiana,

FIG. 180.-The giant gall of the white oak (California), made by the gall insect Andricus californicus. The gall at the right cut open to show tunnels made by the insects in escaping from the gall.-From photograph.

in winter, the bottom may often be seen covered with water snakes lying as inert as dead twigs. Usually, however, hibernation is accompanied by concealment. Some animals in hibernation may be frozen alive without apparent injury. The blackfish of the Alaska swamps, fed to dogs when frozen solid, has been known to revive in the heat of the dog's stomach and to wriggle out and escape. As animals resist heat and cold by adaptations of structure or habits, so may they resist dryness. Certain fishes hold reservoirs

of water above their gills, by means of which they can breathe during short excursions from the water. Still others (mud-fishes) retain the primitive lung-like structure of the swim-bladder, and are able to breathe air when, in the dry season, the water of the pools is reduced to mud.

Another series of adaptations is concerned with the places chosen by animals for their homes. The fishes that live in water have special organs for breathing under water (Fig. 182). Many of the South American monkeys have the tip of the tail adapted for clinging to limbs of trees or to the bodies of other monkeys of its own kind. The hooked claws of the bat hold on to rocks, the bricks of chimneys, or to the surface of hollow trees where the bat sleeps through the day. The tree-frogs (Fig. 183) or tree-toads have the tips of the toes swollen, forming little pads by which they cling to the bark of trees.

Among other adaptations relating to special surroundings or conditions of life are the great cheek pouches of the pocket gophers, which carry off the soil dug up by the large shovel-like feet when the gopher excavates its burrow.

Those insects which live underground, making burrows or tunnels in the soil, have their legs or other parts adapted for digging and burrowing. The mole cricket (Fig. 184) has its legs stout and short, with broad, shovel-like feet. Some water-beetles (Fig. 185) and water-bugs have one or more of the pairs of legs flattened and broad to serve as oars or paddles for swimming. The grasshoppers or locusts, who leap,

have their hind legs greatly enlarged and elongated, and provided with strong muscles, so as to make of them "leaping legs." The grubs

FIG. 182.-Head of rainbow trout (Salmo irideus) with gill cover bent back to show gills, the breathing organs.

or larvæ of beetles which live as "borers" in tree-trunks have mere rudiments of legs, or none at all (Fig. 186). They have great, strong, biting jaws for cutting away the hard wood. They move simply by wriggling along in their burrows or tunnels.

Insects that live

in water either come up to the surface to breathe or take down air underneath their wings, or in some other way, or have gills for breathing the air which is mixed with the water. These gills are special adap

tive structures which present a great variety of form and appearance. In the young of the May-flies they are delicate plate-like flaps projecting from the sides of the body. They are kept in constant motion, gently waving back and

forth in the water so as to maintain currents to bring fresh water in contact with them. Young mosquitoes (Fig. 187) do not have gills, but come up to the surface to breathe. The larvæ, or wrigglers, breathe through a special

FIG. 184.-The mole cricket (Gryllotalpa), with fore feet modified for digging.

tube at the posterior tip of the body, while the pupæ have a pair of horn-like tubes on the back of the head end of the body.

260. Degree of structural change in adaptations. While among the higher or vertebrate animals, especially the fishes and reptiles, most remarkable cases of adaptations occur, yet the structural changes are for the most part external, never seriously affecting the development of the internal organs other

than the skeleton. The organization of these higher animals is much

less plastic than among FIG. 186.-Wood-boring beetle larva (Prionus). the invertebrates. In

general, the higher the type the more persistent and unchangeable are those structures not immediately exposed

to the influence of the struggle for existence. It is thus the outside of an animal that tells where its ancestors have lived. The inside, suffering little change, whatever the surroundings, tells the real nature of the animal.

261. Vestigial organs.-In general, all the peculiarities of animal structure find their explanation in some need of adaptation. When this need ceases, the structure itself tends to disappear or else to serve some other need. In the bodies of most animals there are certain incomplete or rudimentary organs or structures which serve no distinct useful purpose. They are structures which, in the ancestors of the animals now possessing them, were fully developed functional organs, but which, because of a change in habits or conditions of living, are of no further need, and are gradually dying out. Such organs are called vestigial organs. Examples are the disused ear muscles of man, the vermiform appendix in man, which is the reduced and now useless anterior end of the large intestine. In the lower animals, the thumb or degenerate first finger of the bird with its two or three little quills serves as an example. So also the reduced and elevated hind toe of certain birds, the splint bones or rudimentary side toes of the horse, the rudimentary eyes of blind fishes, the minute barbel or beard of the horned dace or chub, and the rudimentary teeth of the right whales and sword-fish.

FIG. 187.-Young stages of the mosquito. a, larva (wriggler); b, pupa.