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desired by a force as unchanging as that by which a stream turns a mill.
From the wild animals about him man has developed the domestic animals which he finds useful. The dog which man trains to care for his sheep is developed by selection from the most tractable progeny of the wolf which once devoured his flocks. By the process of artificial selection those individuals that are not useful to man or pleasing to his fancy have been destroyed, and those which contribute to his pleasure or welfare have been preserved and allowed to reproduce their kind. The various fancy breeds of pigeons—the carriers, pouters, tumblers, ruff-necks, and fan-tails—are all the descendants of the wild dove of Europe (Columba livia). These breeds or races or varieties have been produced by artificial selection. So it is with the various breeds of cattle and of hogs and of horses and dogs.
In this artificial selection new variations are more rapidly produced than in Nature by means of intercrossing different races, and by a more rapid weeding out of unfavorable—that is, of undesirable—variations. The rapid production of variations and the careful preservation of the desirable ones and rigid destruction of undesirable ones are the means by which many races of domestic animals are produced. This is artificial selection.
252. Dependence of species on species.—There was introduced into California from Australia, on young orange trees, a few years ago, an insect pest called the cottony cushion scale (Icerya purchasi). This pest increased in numbers with extraordinary rapidity, and in four or five years threatened to destroy completely the great orange orchards of California. Artificial remedies were of little avail. Finally, an entomologist was sent to Australia to find out if this scale insect had not some special natural enemy in its native country. It was found that in Australia a certain species of lady-bird beetle attacked and fed on the cottony cushion scales and kept them in check. Some of these lady-birds (Vedalia cardinalis) were brought to California and released in a scale-infested orchard. The lady-birds, having plenty of food, thrived and produced many young. Soon the lady-birds were in such numbers that numbers of them could be distributed to other orchards. In two or three years the Vedalias had become so numerous and widely distributed that the cottony cushion scales began to diminish perceptibly, and soon the pest was nearly wiped out. But with the disappearance of the scales came also a disappearance of the lady-birds, and it was then discovered that the Vedalias fed only on cottony cushion scales and could not live where the scales were not. So now, in order to have a stock of Vedalias on hand in California it is necessary to keep protected some colonies of the cottony cushion scale to serve as food. Of course, with the disappearance of the predaceous lady-birds the scale began to increase again in various parts of the State, but with the sending of Vedalias to these localities the scale was again crushed. How close is the interdependence of these two species !
Similar relations can be traced in every group of animals. When the salmon cease to run in the Sacramento River in California the otter which feeds on them takes, it is said, to robbing the poultry-yards; and the bear, which also feeds on fish, strikes out for other game, taking fruit or chickens or bee-hives, whatever he may find.
253. Origin of adaptations.—The strife for place in the crowd of animals makes it necessary for each one to adjust itself to the place it holds. As the individual becomes fitted to its condition, so must the species as a whole. The species is therefore made up of individuals that are fitted or may become fitted for the conditions of life. As the stress of existence becomes more severe, the individuals fit to continue the species are chosen more closely. This choice is the automatic work of the conditions of life, but it is none the less effective in its operations, and in the course of centuries it becomes unerring. When conditions change, the perfection of adaptation in a species may be the cause of its extinction. If the need of a special fitness can not be met immediately, the species will disappear. For example, the native sheep of England have developed a long wool fitted to protect them in a cool, damp climate. Such sheep transferred to Cuba died in a short time, leaving no descendants. The warm fleece, so useful in England, rendered them wholly unfit for survival in the tropics. It is one advantage of man, as compared with other forms of life, that so many of his adaptations are external to his structure, and can be cast aside when necessity arises.
254. Classification of adaptations.—The various forms of adaptations may be roughly divided into five classes, as follows : (a) food securing, (b) self-protection, (c)rivalry, (a) defense of young, (e) surroundings. The few examples which are given under each class, some of them striking, some not especially so, are mostly chosen from the vertebrates and from the insects, because these two groups of animals are the groups with which beginning students of zoology are likely to be familiar, and the adaptations referred to are therefore most likely to be best appreciated. Quite as good and obvious examples could be selected from any other groups of animals. The student
Fig. 156. --The deep-sea angler (Corynolophus reinhardti), which has a dorsal spine
modified to be a luminons “fishing rod and lure,” attracting lantern-fishes (Echiostoma and Æthoprora). An extraordinary adaptation for securing food. (The angler is drawn after a figure of LÜTKEN'S.)
will find good practice in trying to discover examples shown by the animals with which he may be familiar. That all or any part of the body structure of any animal can be called with truth an example of adaptation is plain from what we know of how the various organs of the animal body have come to exist. But by giving special attention to such adaptations as are plainly obvious, beginning stu
dents may be put in the way of independent observation along an extremely interesting and attractive line of zoological study. .
255. Adaptations for securing food. — For the purpose of capture of their prey, some carnivorous animals are provided with strong claws, sharp teeth, hooked beaks, and other structures familiar to us in the lion, tiger, dog, cat, owl, and eagle. Insect Fig. 157.-The brown pelican, showing gular eating mammals have sac, which it uses in catching and holding contrivances especially fishes that form its food.
adapted for the catching of insects. The
ant-eater, for example, has a curious, long sticky tongue which it thrusts forth from its cylindrical snout deep into the recesses of the anthill, bringing it out with its sticky surface covered with ants. Animals which feed on nuts are fitted with strong teeth or beaks for cracking them. Similar teeth are found in those fishes which feed on crabs, snails, or sea-ur
chins. Those mammals like Fig. 158.- Foot of the bald eagle, the horse and cow, that
showing claws for seizing its prey. (CHAPMAN.)
feed on plants, have usually