Human Migration
Human migration: the term is vague. What people usually think of is the permanent movement of people from one home to another. More broadly, though, migration means all the ways—from the seasonal drift of agricultural workers within a country to the relocation of refugees from one country to another.
Migration is big, dangerous, and compelling. It is 60 millionEuropeans leaving home from the 16th to the 20th century. It is some 15 million Hindus, Sikhs, and Muslims swept up in a tumultuous shuffle of citizens between India and Pakistan after the partition of the subcontinent in 1947.
Migration is the dynamic undertow of population change: everyone’s solution, everyone’s conflict.As the century turns, migration, with its inevitable economic and political turmoil, has been called "one of the greatest challenges of the coming century".
But it is much more than that. It is, as has always been, the great adventure of human life. Migration helped create humans, drove us to conquer the planet, shaped our societies, and promises to reshape them again.
"You have a history book written in your genes," said Spencer Wells. The book he’s trying to read goes back to long before even the first word was written, and it is a story of migration.
Wells, a blond geneticist at Stanford University, spent the summer of 1998 exploring remote parts of Transcaucasia andCentralAsia with three colleagues in a Land Rover, looking for drops of bloo
D、In the blood, donated by the people he met, he will search for the story that genetic markers can tell of the long paths human life has taken across theEarth.
A、[■]But however the paths are traced, the basic story is simple: people have been moving since they were people.
B、[■] If early humans hadn’t moved and intermingled as much as they did, they probably would have continued to evolve into different species.
C、[■] From beginnings inAfrica, most researchers agree, groups of hunter-gatherers spread out, driven to the ends of theEarth.
D、[■]
To demographer KingsleyDavis, two things made migration happen. First, human beings, with their tools and language, could adapt to different conditions without having to wait for evolution to make them suitable for a new niche. Second, as populations grew, cultures began to differ, and inequalities developed between groups. The first factor gave us the keys to the door of any room on the planet; the other gave us reasons to use them.
Over the centuries, as agriculture spread across the planet, people moved toward places where metal was found and worked to centers of commerce that then became cities. Those places were, in turn, invaded and overrun by people in later generations called barbarians.
In between, these storm surges were steadier but similarly profound tides in which people moved out to colonize or were captured and brought in as slaves. For a while the population ofAthens, that city of legendary enlightenment was as much as 35 percent slaves.
"What strikes me is how important migration is as a cause and effect in great world events. " Mark Miller, co-author of TheAge of Migration and a professor of political science at the University ofDelaware, told me recently.
It is difficult to think of any great events that did not involve migration. Religions spawned pilgrims or settlers; wars drove refugees before them and made new land available for the conquerors; political upheavals displaced thousands or millions; economic innovations drew workers and entrepreneurs like magnets; environmental disasters like famine or disease pushed their bedraggled survivors anywhere they could replant hope.
"It’s part of our nature, this movement," Miller said, "It’s just a fact of the human condition. " The word intermingled in Paragraph 7 is c

Ceratopsia
Ceratopsia thrived in NorthAmerica andAsia during theCretaceous Period (about 146 to 65.5 million years ago). Their name means "horned face" and indeed many of them did feature bony horns projecting from their skulls. ■
A、The horns may look quite predatory, butCeratopsians were herbivores¹, so their horns were defensive in nature and may have been used to enforce order in the group. ■
B、Not allCeratopsians had horns, but all had beaklike mouths. ■
C、The fossil record has revealed three distinct families amongCeratopsia: Psittacosaurids, Protoceratopsids, andCeratopsids. Triceratops, a member of theCeratopsid family, is probably the best known species ofCeratopsiA、With its facial horns and neck display, it exhibits many of the features commonly associated with this dinosaur group. ■
D、However, there are species ofCeratopsia that are quite different from the familiar Triceratops.
The Psittacosaurids comprise the Psittacosaurus genus² and the Hongshanosaurus genus.Although not much fossil data are available for Hongshanosaurus, comparisons with Psittacosaurus can be made.Evidence indicates that both were roughly deer-sized bipeds dwelling inEastAsiA、Hongshanosaurus had beak points on both upper and lower jaws, while only the upper jaw of Psittacosaurus was pointeD、Much more fossil evidence is available for Psittacosaurus species.At least one species had long quills along its back and tail. Researchers speculate these quills were used for display during mating or fighting.As earlyCeratopsians, the Psittacosaurids had many anatomical features that would show up in similar or interestingly modified forms in later species, but none of the Psittacosaurids had the neck frills or facial horns that came to be associated with theCeratopsians.
Although their name suggests that the Protoceratopsids were the first of this dinosaur group to have horned faces, they, in fact, lacked the well-developed horns of later species. While they more closely resembled theCeratopsids on the whole, they were still relatively small and hornless. Protoceratops, a six-foot-long quadrupedal herbivore, is a representative ProtoceratopsiD、Fossil examples found inChina and Mongolia show that it had a turtle-like beak and a neck frill, a version of which would show up among the more familiarCeratopsids. While Protoceratops did have some knobby points on its skull, the points only slightly hinted at the impressive horns that appeared on theCeratopsids.
With the arrival of Triceratops and other advancedCeratopsids,Ceratopsia reached new levels of sophistication, variety, and size.AllCeratopsid fossils have been found in western NorthAmerica and reveal that they were quadrupedal herbivores with beaked jaw tips and rows of teeth specialized for shearing. The family featured a remarkable array of horns, neck frills, and spines. These neck frills, manifesting in various shapes and sizes, probably were too thin to be practical as a defense against predator species, and researchers speculate that they were used during mating displays, for anchoring jaw muscles, for regulating body temperature, or for a combination of these purposes. MostCeratopsids had two long brow horns and a short horn on the nasal ridge.Because theCeratopsids were vegetarian and fairly large (up to 30 feet long and 10 feet tall), researchers think that the horns, along with the neck frills, largely served functions of competition, whether in displays or combat. Moreover, since intraspecific conflicts often are tests of strength rather than fights to the death, thin neck frills may have been sturdy enough to provide some protection during such confrontations.
There is evidence ofCeratopsian species that do not neatly fall into the familial categories as describeD、While these species appear closely related toCeratopsia, they tend to be

Types of Money
The functions of money as a medium of exchange and a measure of value greatly facilitate the exchange of goods and services and the specialization of production. Without the use of money, trade would be reduced to barter, or the direct exchange of one commodity for another. This was the means of exchange used in primitive societies, and bartering is still practiced in some parts of the world today. In a barter economy, a person having something to trade must find another who wants it and has something acceptable to offer in exchange. In a money economy, the owner of a commodity may sell it for money, which is acceptable in payment for a wide range of other goods or services, thus avoiding the time and effort that would be required to find someone who could make an acceptable trade. Money may thus be regarded as a keystone of modern economic life.
The most important types of money are commodity money, credit money, and fiat money. The value of commodity money is about equal to the value of the material contained in it. The principal materials used for this type of money have been gold, silver, and copper. In ancient times, various articles made of these metals, as well as of iron and bronze, were used as money, while among primitive societies commodities such as shells, beads, elephant tusks, furs, skins, and livestock served as mediums of exchange. The gold coins that circulated in the United States before 1933 were examples of commodity money because the value of the gold contained in the coin was about equal to the value of the coin.
Credit money is paper backed by promises by the issuer, whether a government or a bank, to pay an equivalent value in the standard monetary metal, such as gold or silver. Paper money that is not redeemable in any other type of money and the value of which is fixed merely by government edict is known as fiat money. This is the type of money found today in the United States in the form of both coins and dollar bills.
Credit money becomes fiat money when the issuing government suspends the convertibility of credit money into precious metal. Most fiat money began as credit money, such as the U.S. note known as the greenback which was issued during theAmericanCivil War. Most minor coins in circulation are also a form of fiat money, because the value of the material of which they are made is usually less than their value as money. For example, the amount of nickel in a nickel coin today is less than its value as money.
Both the fiat and credit forms of money are generally made acceptable through a government decree that all creditors must take the money in settlement of debts; the money is then referred to as legal tender. If the supply of paper money is not excessive in relation to the needs of trade and industry and people feel confident that this situation will continue, the currency is likely to be generally acceptable and to be relatively stable in value. If, however, such currency is issued in excessively large volume in order to finance government needs, confidence is destroyed and it rapidly loses value. Such depreciation of the currency is often followed by formal devaluation, or reduction of the official value of the currency, by governmental decree.
The basic money of a country into which other forms of money may be converted and which determines the value of other kinds of money is called the money of redemption or standard money.A、[■]Modern standards have been either commodity standards, in which either gold or silver has been chiefly used as standard money, or fiat standards, consisting of inconvertible currency paper units.B、[■]Most monetary systems of the world at the present time, including those inChina and the United States, are fiat systems.C、[■]They do not allow free convertibility of the currency into a metallic standard, and money is given value by government fiat

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