From
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© 1999 by Doubleday. Used with permission.
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From by © 1999 by Doubleday. Used with permission. |
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Editor's note: In Almost Like a Whale, Steve Jones takes on a challenge that most would find daunting - a re-examination of Darwin's On the Origin of Species, one of the most monumental works of this or any time. Jones looks at the Origin in light of today's scientific knowledge, so much of which Darwin could not have imagined, and finds that the theory put forth almost a century and a half ago is still making sense on the threshold of the millennium.
Here Jones discusses the geographic distribution of species, and introduces the excerpt for us as follows:
The Origin begins with travel: "When aboard HMS Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America …". That sentence is full of meaning. It was the door to what became a science of its own. Darwin realised that to prove the fact of evolution all that is needed is to go somewhere else. If life was created, why was it so different in different places? Maps of species distribution held an important lesson: each part of the world had its own history. To allow one animal to move at once changed the fate of another.
Except in gardens, there are no cactuses around the Mediterranean, or wattles in Chile. Such plants are confined to their native land because they are confined behind barriers - the Sahara or the Andes - that prevent them from spreading further. All plants or animals face obstacles that exclude them from places where they might otherwise do well. No two marine faunas are more distinct, with hardly a fish, shell or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, Isthmus of Panama. Any plant or animal will migrate to wherever it can, given the chance. As soon as an impediment is removed, life pours through and destroys any less adapted forms in the way. To breach a barrier is to experiment with evolution.
In the 1980s, Israelis were forced to give up bathing. Their beaches were clogged with immigrants: poisonous jellyfish, twenty in every cubic yard of water. They choked power-station inlets and forced fishermen to stay in port because their nets were filled with decayed flesh. The culprit was a native of the Red Sea that had broken through a five-million-year-old barrier.
The Mediterranean has an unexpected history. It has been not one, but several, seas (and, now and again, deserts). Five million years ago, it was dry, because the last of many great evaporations had left a layer of salt a mile thick across its floor. Some of its inundations were from its eastern end and brought warm-water plants and animals that were well adapted to the tepid waters of what is, of its nature, a subtropical ocean. The Mediterranean's modern waters, though, came from the Atlantic, over the great Falls of Gibraltar. Because it last filled from the west, the Mediterranean is now a warm sea filled with the descendants of plants and animals from a cold ocean. At the end of the last ice age, just eighteen thousand years ago, there were polar bears in the South of France, and even today the Mediterranean has more than three thousand whales (together with a tiny remnant of its once abundant seals).
The Suez Canal was opened in 1869. It was the successor to several earlier links with the Red Sea, the first made three thousand years ago by Rameses II of Egypt (the builder of the Abu Simbel temples, themselves inundated by today's engineers). The present Canal is half a mile across in parts of its hundred-mile length. It acts like a giant new Mississippi as its waters flow downhill from the Red Sea to its younger cousin. The connection between the seas allowed their animals, confined for millennia, to move.
The traffic was one way. Three hundred Red Sea natives made it to the Mediterranean, almost none the other way. The migration continues at a rate of ten new forms a year, with the newcomers flourishing at the expense of the locals. A third of the Israeli catch now consists of Red Sea fish and the wave of aliens has reached Sicily. The native prawns have almost gone and many fish are in decline. The survivors manage only because they leave space for the invaders. The local mullets, for example, live in deeper waters than do their newly arrived relatives.
The immigrants succeeded because they are more adapted to today's Mediterranean than are its natives. The locals have had no time - nor, in the absence of competition, much need - to respond to the challenges presented by its warm and salty waters since the sea last filled. As a result, they were soon driven out by the subtropical outsiders. The breach in the barrier allowed animals to swarm into the new space. In time they will fill it - as far as they are able, and until they are stopped by the cold or by others more suited to local conditions. Wherever it arises, and however it travels, a species will move on until something restrains it. As it does, it must evolve or die.
As a result, the continents generate their own mixtures of inhabitants. On each one, successive groups of beings, specifically distinct yet clearly related, replace each other. On the plains of La Plata we see the agouti and bizcacha, animals with nearly the same habits as our hares and rabbits, but they plainly display an American type of structure. We look to the waters and we do not find the beaver or muskrat, but the coypu and capybara, rodents of the American type. We see in these facts some deep organic bond, prevailing throughout space and time. The naturalist must feel little curiosity who is not led to enquire what this bond is.
The bond is inheritance, modified by natural selection. Such community of descent casts its net wide. The world can be divided into great provinces - the New World Arctic and tropics, their equivalents in the Old World, Africa, the Far East and Australia, with more domains beneath the sea. Each has its own identity, shared not just by mammals, but by insects, snails, worms and trees. The differences are not absolute, and many creatures range over more than one province, but the existence of such huge areas of affinity is evidence of a shared past.
Means of Dispersal. Once evolved, plants and animals face a constant struggle against the pressure of their own numbers. They move as far as they can, by their own efforts or with the help of others. Some can travel for huge distances by land, sea or air, while others are confined to the place where they were born.
Often, a hopeful migrant faces an impassable barrier. The capacity of migrating across the sea is more distinctly limited in terrestrial mammals than perhaps in any other organic beings; and, accordingly, we find no inexplicable cases of the same mammal inhabiting distant points of the world. For other animals the sea is a highway. Pytheas of Massalia was the first sailor to venture beyond the Pillars of Hercules (and the first Greek to visit the British Isles). As he explored the Atlantic shore of Spain he noticed that the ocean flowed south, like an immense river, an okeanos. His river runs on as the Canary Current, part of a great girdle of water flowing around the North Atlantic. The Gulf Stream has the force of three hundred Amazons; a river forty miles across that takes a month to cross the Atlantic and is but one of many great conveyor belts bearing flotsam across the world.
All oceans have their currents. About a tenth of their waters are always on the move. Most flow on the surface (although slow streams in the deeps take a thousand years to carry icy water from the Antarctic to the Galapagos). The top ten feet of the ocean store as much heat as does the entire atmosphere. The movements of water are started by heat, by winds and by the rotation of the Earth. Because water at the Equator is warm, it expands. As a result, the sea in the Caribbean is three inches higher than at Newfoundland. The warm water flows downhill towards the Poles, and is twisted in a clockwise direction in the northern hemisphere (and its reverse on the other side of the Equator) as the world turns. The wind obeys the same rules, and its storms help generate the sea's drift as they spin round the globe.
Some animals float to a new home. The surface of the water between the Galapagos Islands carries dozens of insects aphids, cicadas, ants and mosquitoes. Most will drown, but a few will make the journey. Others hitch lifts on the many vehicles that pass by. Every day, ten million pieces of garbage - bottles, bags and plastic sheets - are dumped from ships. From the land, the sea receives much more. The island of Pitcairn is three thousand miles from the nearest mainland. Its best-known detritus was the Bounty mutineers, who landed in 1789. The island was so remote that their refuge was not discovered until all but one had died. Nowadays, its beaches are as filthy as any in Europe, with a piece of rubbish every yard. Pitcairn has the European mix of buoys, bottles and bags, but a relative shortage of disposable nappies. The whisky bottles suggest that many of the migrants come from South America. Not all the flotsam is useless, as the local land crabs are fond of shoes as shelters, but it is a dismal reminder of how the most remote places have been forced to join the modern world.
Some marine debris is still mysterious: why are twice as many left shoes as right washed ashore in The Netherlands, while the opposite is true for Scottish beaches? All has a message for evolution. Nowhere is isolated. Given the chance, plants and animals will float, fly or drift through the air to reach the most remote parts of the earth.
Plenty of animals travel on rafts (shoes and bottles included). Off the coast of Cuba float substantial islands of vegetation that may bear mature trees. On Christmas Island, three thousand miles from North America, American redwood, fir and walnut are used as firewood because so many come ashore. Canoe-builders on coral atolls in the deep Pacific once depended on logs that had floated halfway across the world. A complex etiquette determined who got the biggest. The natives procured stones for their tools, solely from the roots of drifted trees, these stones being a valuable royal tax. Many of the unsinkable vagrants bore not just stones, but animals and plants. Great clumps of seaweed full of shore animals have been found a hundred miles from land. Glaciers, too, deliver their contents as they float from the ice caps. Antarctic boulders on the sea floor off Cape Town were brought within the past ten thousand years. Among them are the remains of penguins. However, the sea can be a formidable barrier. Few freshwater fish can manage to cross a strait more than a couple of miles across, elephants stop swimming at thirty miles, and tortoises and snakes cannot manage more than five hundred.
To find a raft is not enough, because the travellers must survive their journey. Most land plants find it hard to deal with seawater. Their seeds do rather better. Some simple experiments prove how tough they are. Often, dried seeds do best. To dry stems and branches of many different plants and to place them on sea water shows that some when dried floated; for instance, ripe hazel-nuts when dried floated for ninety days and then germinated. As many currents run at sixty miles a day, plants might be floated across miles of sea; and, when stranded, if blown to a favourable spot by an inland gale, they would germinate.
Seeds are the genetic memory of the plants that bore them. Many survive not for weeks, but for years, with germination sparked off by a change in the environment.
An unplanned test of the power of the seed began when the wheatfields of northern France were abandoned after the economic collapse that followed the Franco-Prussian War of 1870. The crop was in those days full of weeds. Poppies were everywhere. The plant can generate thirty thousand seeds in a square yard of soil. After the French collapse the farms of Flanders stayed grazed and flowerless until 1914. Then, the land was cultivated again not with ploughs, but with swords, shells and blood. Once disturbed, the poppies bloomed at once, from long-buried seeds. A quarter of a century later, in the next round of human folly, the Natural History Museum in London was bombed. The fire-hoses caused many seeds to germinate, among them a mimosa, collected in China in 1713, and revived in a sudden flood two centuries on and five thousand miles away. The poppy is now much used as a symbol of war's destruction; but it bears a more hopeful message about how well life can survive in the face of adversity.
With a third of the world's plants in danger, there is a new interest in conservation. The toughness of seeds is a great help - and is a reminder of their vital role as containers for genes. A Millennium Seed Bank at Kew aims to store dried and frozen seeds of a tenth of the world's kinds of plant (together with the whole of the British flora). Most should last for centuries. Simply to dry the seeds of beet, rice or elm allows them to survive for a decade and more. Some weed genes allow the plants to lie low even in good times. They sprout over weeks or months rather than all at once - which is useful when it comes to long and risky journeys.
Plenty of travellers fly, rather than float, across the globe. Every few years, after an Atlantic storm, dazed North American birds reach Europe. Most die, but the survivors carry a cargo. The crops of birds do not secrete gastric juice and so do not in the least injure the germination of seeds. All the grains do not pass into the gizzard for twelve or even eighteen hours. A bird in this interval might easily be blown five hundred miles, and hawks are known to look out for tired birds. Some bolt their prey whole, and after an interval disgorge pellets, which, as seen in experiments made in the Zoological Gardens, include seeds capable of germination.
Locusts soar upwards as the sun falls, and are caught in the winds of the upper air, to move fifty miles a night. They concentrate on the edge of weather systems to form plagues as they descend to earth. Less conspicuous things also move - as anyone who suffers from hay-fever knows. Every summer afternoon, on the Costa del Sol, a great cloud of marijuana pollen descends from the illicit fields of Morocco. A constant rain of pollen and spores from South America, mixed with the odd seed and insect, falls on Signy Island, on the edge of the Antarctic. Few places on the Antarctic continent itself are warm enough for mosses and liverworts to grow, but where they can - around hot springs and the like - they do, evidence that nowhere, remote though it may seem, is safe from migrants. The presence of the same form in distant places is not evidence that it was created twice, but that it can move.
Steve Jones is professor of genetics at University College London, and has worked at universities in the United States, Australia, and Africa. He is a regular columnist for the Daily Telegraph and frequently appears on radio and television. His earlier books include The Language of the Genes and The Cambridge Encyclopedia of Human Evolution.
Illustration © Sea World.
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Endlinks
Invasive Alien Species - a summary of the Yale-sponsored symposium that explored the economic and environmental impacts of invasive species.
Invasive Species - a gateway to information relating to non-native plant and animal species present in U.S. ecosystems. Includes a nonindigenous species database. From the National Biological Information Infrastructure.
Costly Interlopers - a report on the economic consequences of introduced species in the United States. From the February 15, 1999 issue of Scientific American.
Database on Introductions on Aquatic Species - a fully searchable database of introduced aquatic species, with worldwide maps. Sponsored by the United Nations Food and Agricultural Organization.
United States Geological Survey: Nonindigenous Aquatic Species - a central repository for accurate and spatially referenced biogeographic accounts of nonindigenous aquatic species.
How Does Your Garden Stow? The Millennium Seed Bank - an HMS Beagle Profile of the Millennium Seed Bank project.