INFLUENCE OF OCEAN CURRENTS.
THE ZOOLOGICAL PROVINCE OF AUSTRALIA.
Although there are few places on the Australian coast where one can spend a more enjoyable summer holiday than the Abrolhos, both on account of the free and “simple life” they afford, and the great historic interest they possess as being the scene of the final catastrophe which befel the Dutch attempt to colonise Australia in 1629 under the unfortunate Pelsart, the islands themselves are of peculiar interest, and have given rise to much speculation as to their origin.
They are all composed entirely of dead coral, here and there partially covered by wind-blown sand, and for the most part not rising more than ten or twelve feet above the water, while many of them do not attain an elevation of more than three or four feet. They are situated on the 29th parallel of south latitude, and are probably further removed from the Equator than any other coral islands in the world, while the indigenous vegetation, such as it is, belongs to a latitude well within the tropics. It is a curious speculation therefore, how these islands came to exist in their present position.
When we “lay to” on our recent trip our boat drifted considerably northward, while closer to Geraldton the northerly current is still more defined. These northerly flowing waters are the cold waters of the Southern Ocean travelling into the equatorial regions, where they are warmed and supply the current which passes westerly and southerly through the Indian Ocean, attaining its greatest velocity as it passes through the Mozambique Channel, thence round the Cape of Good Hope, and north-westerly through the Carribean Sea to form the Gulf stream, which takes a north-easterly course to impart their warm, genial climate to the British islands.
The equatorial current of the Pacific is broken up and delayed in its course as it passes through the Indian Archipelago and Sunda Islands, and its waters become warmer than those of any other oceanic waters on the globe, while the current itself is split up and diverted into innumerable directions, which make navigation extremely intricate and dangerous. It finally emerges from this intricate maze of islands at about latitude 15 south and longitude 115 east, where portion of its waters join the easterly current to Mozambique, and the remainder flows about south by west, being kept from the Australian shores by the stream of cool water from the Southern Ocean, which hugs the Westralian coast line till it reaches well into the tropics, and merges into the warmer water of Oceania between Sharks Bay and Java, as a lower current. A glance at the map of the world will show exactly how the islands of the Indian Archipelago would operate in forming the intricacies of the currents along the north-west coast.
Owing to the rotation of the earth being from west to east, the tendency of all currents in the ocean is to flow westward, unless some local cause deflects them from that course. The most notable example of this deflection is the Gulf stream, while the next most important is the Pacific current in its ramifications through the islands of the Indian Archipelago.
Polar waters always flow towards the equator, still partaking, however, of the westerly direction unless deflected by local causes; and, being cold, flow beneath the warmer currents, which have been expanded by equatorial heat, until, being warmed, they gradually rise and mingle with the equatorial currents, which, in addition to their westerly motion, also flow towards the Poles, till, becoming cooled, they sink and merge with the Polar waters, thus maintaining a continued circulation.
Thus the polar current which flows northerly past Geraldton has come up from the south of Australia, and being deflected by the southern currents from the Pacific and Indian Ocean, has been pressed, as it were, against the coast line, attaining its narrowest part as it passes beyond Sharks Bay.
The longitude of the Abrolhos Islands is about 113-30E., and here we may refer to a bottle which was picked up on December 9th—two or three weeks before our visit—on East Wallaby Island, by Mr. F. Burton, which has an important bearing on the question under discussion. This bottle was thrown over from the German ship Innsbruck on the 2nd June in longitude 111-41 E. and latitude 21 S., and, as mentioned in the “Guardian” of 15th December, which quoted the written paper it contained, was for the purpose of determining the course of the current from that point. We do not know how long this bottle was on the beach before it was discovered, but we may be pretty certain that the longest portion of its period afloat would be after it arrived at the Abrolhos, in battling with the local counter currents which the islands themselves would produce. During the six months which elapsed from the time it left the Innsbruck, it had travelled through 8 degrees of latitude southward and 1-11 of longitude westward—a distance of about 600 miles—in nautical phrase, about south by quarter west. The position 111-41 E. and 21 south would be about 80 miles west of Flaming Head, and although we have no data on which to estimate the rate of the current, the bottle gives us absolute information as to its direction—south by quarter west.
Australia, and the whole of what is known as the zoological province of Australia, stands on an ocean plateau which rarely exceeds 100 fathoms in depth; while immediately beyond it, the ocean suddenly attains a depth of from 1000 to 1,200 fathoms. The edge of this plateau is at Lombock Strait, between Lombock and Sambawa Islands, to the east of Java, and passes northerly through the Strait of Macassar, between Celebes and Borneo, thence easterly, embracing the Molucca Islands and New Guinea, and southerly between Australia and New Zealand and embracing Tasmania. The Abrolhos also stand on this plateau.
Thus all the Pacific waters which pass to the eastward of Celebes and Lombock pass over a considerable portion of this plateau, where the water is only 100 fathoms deep, and therefore absorb a much larger amount of equatorial heat than the waters to the west of that line do, where they attain a depth of 1000 to 1,200 fathoms.
This eastern water, of the Australian province, heated by passing over the Australian plateau, is the water into which the captain of the Innsbruck threw his bottle. Thus, while the climate of Geraldton is tempered by the cool waters of the northern current from the Antarctic Ocean, the Abrolhos have an undue share of warmth transmitted to them by the heated waters of the Pacific as they enter the Indian Ocean. It is this undue share of heat which places these islands in the unique position of being the most southerly coral formations of the world. The coral insect can live and work in the temperature which the waters of the Pacific convey to them to a latitude beyond which the same temperature is denied elsewhere.
Thus most of the flora and fauna of these islands are distinctly tropical in their character, although their habitat is situated six degrees outside the tropics.
The Australian plateau is not a mere geographical phenomenon. It is more, and forms the boundary between the zoological provinces of Australia and Malay. To the east of Lombock Strait everything—birds, animals, fish and even molluscs—are distinctly Australian in their types. To the west they are as distinctly Malayan, and although Lombock Strait is less than twenty miles in width, yet the flora and fauna on either side of it are as distinct as though they were separated by half the diameter of the globe. The birds, which could easily bridge the distance in their flights, or the fish, which could traverse the waters in an hour, remain distinct and faithful to the type of their own zoological province. In the flora, too, the trees and shrubs, the grasses and the herbs, true to their province, retain their botanical individuality, with as much persistence as though separated from their neighbors across the strait, by the broad waters of a mighty ocean and give a contradiction to the doctrines of Darwinism that migration has anything to do with the origin of species, or that natural selection plays any part in permanency of races.
The whole surroundings go to prove the truth of the observations of Agassis—that each zoological province has brought forth life best suited to its own chemical and physical environment, without reference to the life already existing in the neighboring provinces.