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rivals requiring the same food and locality, or enemies, for the vast majority of animals are carnivorous. Offspring are continually varying from their parents, for better or worse. If feebly adapted to the conditions of existence, they will finally go to the wall. But those forms having the slightest advantage over others inhabiting the same region, being hardier or stronger, more agile or sagacious, will survive. Should this advantageous variation become hereditary and intensified, the new variety will gradually extirpate or replace other kinds. This is what Mr. Darwin means by Natural Selection, and Herbert Spencer by the Survival of the Fittest.

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CHAPTER XXIV

THE DISTRIBUTION OF ANIMALS

LIFE is everywhere. In the air above, the earth beneath, and the waters under the earth, we are surrounded with life. Nature lives: every death is only a new birth, every grave a cradle. The air swarms with birds, insects, and invisible animalcules. The waters are peopled with innumerable forms, from the protozoan, millions of which would not weigh a grain, to the whale, so large that it seems an island as it sleeps upon the waves. The bed of the sea is alive with crabs, mollusks, polyps, starfishes, and Foraminifera. Life everywhere—on the earth, in the earth, crawling, creeping, burrowing, boring, leaping, running.

Nor does the vast procession end here. The earth we tread is largely formed of the débris of life. The quarry of limestone, the flints which struck the fire of the old Revolutionary muskets, are the remains of countless skeletons. The major part of the Alps, the Rocky Mountains, and the chalk cliffs of England are the monumental relics of bygone generations. From the ruins of this living architecture we build our Parthenons and Pyramids, our St. Peters and Louvres. So generation follows generation. But we have not yet exhausted the survey. Life cradles within life. The bodies of animals are little worlds having their own fauna and flora. In the fluids and tissues, in the eye, liver, stomach, brain, and muscles, parasites are found; and these parasites often have their parasites living on them.

Even the unicellular forms, Stylonychia, for example, have been found to be infested with parasitic protozoans.

Thus the ocean of life is inexhaustible. It spreads in every direction, into time past and present, flowing everywhere, eagerly surging into every nook and corner of creation. On the mountain top, in the abysses of the Atlantic, in the deepest crevice of the earth's crust, we find traces of animal life. Nature is prodigal of space, but economical in filling it.177

Animals are distributed over the globe according to definite laws, and with remarkable regularity.

Each of the three great provinces, Earth, Air, and Water, as also every continent, contains representatives of all the classes; but the various classes are unequally represented. Every great climatal region contains some species not found elsewhere, to the exclusion of some other forms. Every grand division of the globe, whether of land or sea, each zone of climate and altitude, has its own fauna. In traveling over the earth and settling in new regions man has been accompanied by many animals which have established themselves and thriven in the land of their adoption. For example, the house, or "English," sparrow has been brought to America, and the sparrow and rabbit to New Zealand. Hence, it is necessary to distinguish between the native or indigenous fauna, and the introduced fauna, the latter depending upon human agency. In spite of the many causes tending to disperse animals beyond their natural limits, each country preserves its peculiar zoological physiognomy.

The space occupied by the different groups of animals is often inversely as the size of the individuals. Compare the coral and elephant.

The fauna now occupying a separate area is closely allied to the fauna which existed in former geologic

times. Thus, Australia has always been the home of marsupials, and South America of edentates.

It is a general rule that groups of distinct species are circumscribed within definite, and often narrow, limits. Man is the only cosmopolitan; yet even he comprises several marked races, whose distribution corresponds with the great zoological regions. The natives of Australia are as grotesque as the animals. Certain brutes likewise have a great range: thus, the puma ranges from Canada to Patagonia; the muskrat, from the Arctic Ocean to Florida; the ermine, from Bering Strait to the Himalayas; and the hippopotamus, from the Nile and Niger to the Orange River.178

Frequently, species of the same genus, living side by side, are widely different, while there is a close resemblance between forms which are antipodes. The mud eel of South Carolina and menobranchus of the Northern States have their relatives in Japan and Austria. The American tapir has its mate in Sumatra, the llama is related to the camel, and the opossum to the kangaroo.

The chief causes modifying distribution are temperature, topography, ocean and wind currents, humidity, and light. To these may be added the fact that animals are ever intruding on each other's spheres of existence. High mountain ranges, wide deserts, and cold currents in the ocean are impassable barriers to the migration of most species. Thus, river fish on opposite sides of the Andes differ widely, and the cold Peruvian current prevents the growth of coral at the Galapagos Islands. So a broad river, like the Amazon, or a deep, narrow channel in the sea, is an effectual barrier to some tribes. Thus, Borneo belongs to the Indian region, while Celebes, though but a few miles distant, is Australian in its life. The fauna of North America, on

the east coast, west coast, and the open plains between, are very different.

Animals dwelling at high elevations resemble those of colder latitudes. The same species of insects are found on Mount Washington, and in Labrador and Greenland.

The range does not depend upon the powers of locomotion. The oyster extends from Halifax to Charleston, and the snapping turtle from Canada to the equator; while many quadrupeds and birds have narrow habitats.

The distribution of any group is qualified by the nature of the food. Carnivores have a wider range

than herbivores.

Life diminishes as we depart from the equator north or south, and likewise as we descend or ascend from the level of the sea.

The zones of geography have been divided by zoölogists into narrower provinces. Three regions in the sea are recognized: the Pelagic or surface region; the Littoral, between tide marks strictly but often interpreted to conclude depths to forty fathoms; and the Abyssal, extending from the Littoral to the greatest depths of the ocean. Every marine species has its own limits of depth. It would be quite as difficult, said Agassiz, for a fish or a mollusk to cross from the coast of Europe to the coast of America as for a reindeer to pass from the arctic to the antarctic regions across the torrid zone. Marine animals congregate mainly along the coasts of continents and on soundings. The meeting place of two maritime currents of different temperatures, as on the Banks of Newfoundland, favors the development of a great diversity of fishes.

Every great province of the ocean contains some representatives of all the subkingdoms. Deep-sea life.

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