The causes behind the rapid development of the Minoan and Mycenaean civilizations in the Aegean during the late third and second millennia B.C.E. have intrigued scholars for years. Until recently, most explanations attributed Aegean development to outside influence. Civilization had emerged in Mesopotamia by 3000 B.C.E, and, some archeologists argued, Mesopotamian trade introduced civilized ideas and technological innovations into nearby, less advanced areas. Others hypothesized that civilization was brought to the Aegean by invasion from some adjacent region, of which Anatolia in modern Turkey seemed the most plausible. 

　　In a work published in 1972, Professor Colin Renfrew approached the problem from a different viewpoint. He argued that the scanty available evidence for invasion or immigration from Anatolia into Greece in the early Bronze Age （about 3300-2200 B.C.E.） showed that, at most, such incursion was limited, and that it could not be regarded as responsible for the transformation of society there. Trade, though clearly documented, was also an inadequate explanation in itself. To understand the major changes in social organization and complexity that took place, it was necessary, said Renfrew, to determine the impact that new variables emerging in the early Bronze Age may have had on every interrelated aspect of the local social system. The two new major developments he considered were changes in the subsistence economy and the introduction of bronze metallurgy. 

　　The economy of Neolithic Greece was based on growing grains and raising sheep. Early in the third millennium B.C.E., cultivation of grape vines and olive trees also became important in southern Greece and the Aegean Islands. Both crops were eminently suitable for trade and storage in the form of olive oil and wine. They were grown on land that was not suitable for grain farming. Their cultivation required work at a different time of year from that needed by grain crops, and much of this work, such as harvesting, was relatively light. As a result, agricultural yields were substantially increased without disrupting established agricultural practice. That increase in turn allowed, or stimulated, population growth. For the first time there was enough demand for specialized crafts and services to justify the existence of full-time craftspeople, who could be supported from the extra agricultural output. 

　　Some copper artifacts were made during the fourth millennium B.C.E, but there were not many   of them and they had little economic or social significance. When,  in  the  third  millennium, copper began to be mixed with tin to produce the relatively hard alloy bronze, demand for metal goods grew. Bronze could be used to make a range of useful new tools and weapons and a variety of impressive ornaments. The demand for metalwork stimulated further specialization in crafts such as tool making and jewelry making. The new tools promoted the development of other crafts, like carpentry and shipbuilding. Competition for prestigious or useful craft products and for control of their producers helped to heighten both social differences within communities and conflicts between them, resulting in the emergence of local chieftains, who were also in many instances warriors. These chieftains regulated agricultural and craft production, operating a distribution system through which the farmers could obtain tools or ornaments they needed or wanted. The organizational demands of controlled distribution made it necessary to develop methods of measurement and recording, which led to the development of writing. 

　　Renfrew argued that any single innovation would have had a limited or negligible effect on social organization because the inherently conservative nature of societies acts to minimize change. However, the interaction of several simultaneous developments created a multiplier effect. In the Aegean, increased agricultural productivity provided the means to support craft specialization, while bronze metallurgy provided the technology for producing highly valued new products. These factors set in motion a series of changes in other subsystems of society. Those changes in turn resulted in what, in a term borrowed from electronics, are called positive feedback loops—alterations in the workings of a social system that serve to reinforce themselves. Thus Aegean society was transformed from one consisting of basically self-sufficient and egalitarian farming villages to one of prosperous, hierarchical chiefdoms, with palace-dwelling rulers, actively competing with one another both at home and in international trade.

　　justify= provide a rational basis for 

　　promoted= encouraged

　　several simultaneous development= developments that occurred at the same time

　　hierarchical= having several levels of authority

　　A central hypothesis of animal cognition is that many animals make use of cognitive maps—internal representations or codes—of the spatial relationships among objects in their surroundings. In a broad sense, most animals that migrate probably have some internal map of the way external objects are arranged. 

　　The most extensive studies of cognitive maps have been made for animals exhibit seasonal migration, the regular back-and-forth movement of animals between two geographic areas at particular times of the year.

　　Seasonal migration enables many species to access rich food resources throughout the year and to breed or winter in areas that favor survival. One long-distance traveler is the gray whale. During summer, these giant aquatic mammals feast on small, bottom-dwelling invertebrates that abound in northern oceans. In the autumn, they leave their northern feeding grounds and begin a long trip south along the North American coastline. Arriving in warm, shallow lagoons off Baja California（Mexico）in the winter months, they breed, and pregnant females give birth to young before migrating back north. The yearly round-trip, some 20,000 kilometers, is the longest for any mammal.

　　Among insects, the monarch butterfly has one of the most remarkable seasonal migrations. During winter, these insects decorate certain trees at the western tip of Cuba, in a few mountain valleys of central Mexico, and at a few sites along the California coast. All monarchs alive at the end of summer fly south to reach their wintering sites. With the onset of spring, monarchs mate at the wintering sites and begin migrating northward. As they arrive at summer destinations, they lay eggs and then die. Two or more generations are produced during the summer, repopulating the United States and southern Canada. With the approach of fall, the summer’s last generation of monarchs flies south to the wintering grounds. They migrate as far as 4,000 kilometers and end up at a specific site, although they have not flown the route before.

　　Researchers have found that migrating animals stay on course by using a variety of environmental cues. Gray whales, for instance, seem to use coastal landmarks to pilot their way north and south. Migrating south in the autumn, they orient with the North American coastline on their left. Migrating north in the spring, they keep the coast on their right. Whale watchers sometimes see gray whales stick their heads straight up out of the water, perhaps to obtain a visual fix on land. Many birds migrate at night, navigating by the stars the way ancient human soldiers did. In contrast, monarch butterflies migrate during the day, resting in trees and bushes at night, genetic programming may enable them to use the Sun as a compass.

　　Navigating by the Sun or by stars requires an ability to keep track of compass direction. Many migrators also must have an internal timing mechanism that compensates for the continuous daily movement of Earth relative to celestial objects. The timing mechanism must also allow for the apparent change in position of celestial objects as the animal moves over its migration route. Almost  nothing  is  known  yet  about  the  nature  of  these  timing  mechanisms. At  least one night-migrating bird, the indigo bunting, seems to avoid the need for a timing mechanism by fixing on the North Star, the one bright star in northern skies that appears almost stationary. Researcher have found that buntings learn a star map and fix on a stationary star when navigating at night.

　　Another interesting, and more or less open, question about migration is how birds continue navigating when the Sun or stars are obscured by clouds. These is strong evidence that some birds can orient to Earth’s magnetic field. Magnetite, the iron-containing mineral once used by sailors as a crude compass, is probably involved in sensing the field. The mineral has been found in the heads of pigeons, in the abdomens of bees, and in certain bacteria that orient to a magnetic field. Future research may show that magnetic sensing is a widespread, important part of a complex navigation mechanism in many animals.

　　exhibit = display

　　compensate for = make up for  

　　crude= primitive

　　Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases （water vapor, ammonia, methane, carbon dioxide, and carbon monoxide） that hold together small pieces of rocky and metallic materials. Many comets travel in very elongated orbits that carry them far beyond Pluto. These long-period comets take hundreds of thousands of years to complete a single orbit around the Sun. However, a few short-period comets （those having an orbital period of less than 200 years）， such as Halley’s Comet, make a regular encounters with the inner solar system. 

　　When a comet first becomes visible from Earth, it appears very small, but as it approaches the Sun, solar energy begins to vaporize the frozen gases, producing a glowing head called the coma. The size of the coma varies greatly from one comet to another. Extremely rare ones exceed the size of the Sun, but most approximate the size of Jupiter. Within the coma, a small glowing nucleus with a diameter of only a few kilometers can sometimes be detected. As comets approach the Sun, some develop a tail that extends for millions of kilometers. Despite the enormous size of their tails and comas, comets are relatively small members of the solar system. 

　　The observation that the tail of a comet points away from the Sun in a slightly curved manner led early astronomers to propose that the Sun has a repulsive force that pushes the particles of the coma away, thereby forming the tail. Today, two solar forces are known to contribute to this formation. One, radiation pressure, pushes dust particles away from the coma. The second, known as solar wind, is responsible for moving the ionized gases, particularly carbon monoxide. Sometimes a single tail composed of both dust and ionized gases is produced, but often two tails—one of dust, the other, a blue streak of ionized gases—are observed. 

　　As a comet moves away from the Sun, the gases forming the coma recondense, the tail disappears, and the comet returns to distant space. Material that was blown from the coma to form the tail is lost from the comet forever. Consequently, it is believed that most comets cannot survive more than a few hundred close orbits of the Sun. Once all the gases are expelled, the remaining material—a swarm of tiny metallic and stony particles—continues the orbit without a coma or a tail. 

　　Comets apparently originate in two regions of the outer solar system. Most short-period comets are thought to orbit beyond Neptune in a region called the Kuiper belt, in honor of the astronomer Gerald Kuiper. During the past decade over a hundred of these icy bodies have been discovered. Most Kuiper belt comets move in nearly circular orbits that lie roughly in the same plane as the planets. A chance collision between two comets, or the gravitational influence of  one of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—may occasionally alter the orbit of a comet in these regions enough to send it to the inner solar system and into our view. 

　　Unlike short-period comets, long-period comets have elliptical orbits that are not confined to the plane of the solar system. These comets appear to be distributed in all directions from the Sun, forming a spherical shell around the solar system, called the Oort cloud, after the Dutch astronomer Jan Oort. Millions of comets are believed to orbit the Sun at distances greater than 10,000 times the Earth-Sun distance. The gravitational effect of a distant passing star is thought to send an occasional Oort cloud comet into a highly eccentric orbit that carries it toward the Sun. However, only a tiny portion of the Oort cloud comets have orbits that bring them into the inner solar system. 

　　The most famous short-period comet is Halley’s Comet, named after English astronomer Edmond Halley. Its orbital period averages 76 years, and every one of its 30 appearances since 240 B.C. has been recorded by Chinese astronomers. When seen in 1910, Halley’s Comet had developed a tail nearly 1.6 million kilometers （1 million miles） long and was visible during daylight hours. Its most recent approach occurred in 1986.

　　exceed=go beyond

　　detected=noticed

　　propose=offer the theory

　　roughly=approximately

　　In the northern American colonies, especially New England, tight-knit farming families, organized in communities of several thousand people, dotted the landscape by the mid-eighteenth century. New Englanders staked their future on a mixed economy. They cleared forests for timber used in barrels, ships, houses, and barns. They plumbed the offshore waters for fish to feed local populations. And they cultivated and grazed as much of the thin-soiled, rocky hills and bottomlands as they could recover from the forest. 

　　The farmers of the middle colonies-Pennsylvania, Delaware, New Jersey, and New York-set their wooden plows to much richer soils than New Englanders did. They enjoyed the additional advantage of setting an area already partly cleared by Native Americans who had relied more on agriculture than had New England tribes. Thus favored, mid-Atlantic farm families produced modest surpluses of corn, wheat, beef, and pork. By the mid-eighteenth century, ships from New York and Philadelphia were carrying these foodstuffs not only to the West Indies, always  a primary market, but also to areas that could no longer feed themselves-England, Spain, Portugal, and even New England. 

　　In the North, the broad ownership of land distinguished farming society from every other agricultural region of the Western world. Although differences in circumstances and ability led gradually toward greater social stratification, in most communities, the truly rich and terribly poor were few and the gap between them small compared with European society. Most men other than indentured servants （servants contracted to work for a specific number of years） lived to purchase or inherit a farm of at least 50 acres. With their family’s labor, they earned a decent existence and provided a small inheritance for each of their children. Settlers valued land highly, for owning land ordinarily guaranteed both economic independence and political rights. 

　　By the eighteenth century, amid widespread property ownership, a rising population pressed against a limited land supply, especially in New England. Family farms could not be divided and subdivided indefinitely, for it took at least fifty acres（of which only a quarter could usually be cropped） to support a single family. In Concurd, Massachusetts, for example, the founders had worked farms averaging about 250 acres. A century later, in the 1730s, the average farm had shrunk by two thirds, as farm owners struggled to provide an inheritance for the three or four sons that the average marriage produced. 

　　The decreasing fertility of the soil compounded the problem of dwindling farm size in New England. When land had been plentiful, farmers planted crops in the same field for three years and then let it lie fallow （unplanted） in pasture seven years or more until it regained its fertility. But on the smaller farms of the eighteenth century, farmers had reduced fallow time to only a year or two. Such intense use of the soil reduced crop yields, forcing farmers to plow marginal land or shift to livestock production. 

　　The diminishing size and productivity of family farms forced many New Englanders to move to the frontier or out of the area altogether in the eighteen century. "Many of our old towns are too full of inhabitants for husbandry, many of them living on small shares of land, " complained one writer. In Concurd, one of every four adult males migrated from town every decade from the 1740s on, and in many towns migration out was even greater. Some drifted south to New York and Pennsylvania. Others sought opportunities as artisans in the coastal towns or took to the sea. More headed for the colonies, western frontier or north into New Hampshire and the eastern frontier of Maine. Several thousand New England families migrated even farther north to the Annapolis Valley of Nova Scotia. Throughout New England after the early eighteenth century, most farmers' sons knew that their destiny lay elsewhere. 

　　Wherever they took up farming, northern cultivators engaged in agricultural work routines that were far less intense than in the south. The growing season was much shorter, and the cultivation of cereal crops required incessant labor only during spring planting and autumn harvesting. This less burdensome work rhythm let many northern cultivators to fill out their calendars with intermittent work as clockmakers, shoemakers, carpenters, and weavers.

　　modest=moderate

　　indefinitely=without limit

　　compounded=added to 

　　burdensome=difficult

　　Following the Roman Empire conquering the area in the first century A.D., there is a great deal of archaeological evidence for the economic growth of the British Isles. Prior to this event, the economy of the British Isles, which was based on manufacturing, was centered mainly on the household and on craft skills, and where the best quality and greatest range of goods were largely a monopoly of the tribal aristocracies. This was the nature of the economy which lasted in regions of Britain that were unconquered by the Roman Empire, even though some Roman products were utilized in such areas. The majority of these Roman artifacts were glass vessels, pots, as well as small metal objects that were dispersed over a vast region. They perhaps held a symbolic value and were not necessarily used for their originally designed purposes. The spread of Roman objects beyond Roman Britain does not seem to have happened on an enormous scale. In areas where artifacts are more numerous, it is likely due to gift giving during close interactions between the Roman government and the tribes.

　　In regions that experienced direct economic control under the Romans, however, economic growth is clearly notable. There was an enormous increase in the number and variety of goods in circulation and the range of settlements in which they were found. This is clearly true in the overwhelming majority of excavated sites in Roman Britain, with the only exceptions being some rural regions that continued the pre-Roman, Iron Age pattern. The majority of sites resulted in the discovery of an abundance of iron, glass, and pottery, and good quantities of copper alloys, lead, tin, silver, and occasionally gold. For example, the humble iron nail is found in numbers not repeated until the Industrial Revolution.

　　The technology levels and range of the manufacturing of these objects also developed alongside the sheer increase in their quantity. During the Iron Age, the typical household objects were usually manufactured using a low technology of craft manufacture. Later, this changed to more specialized and larger-scale production methods. During this time, specialized workers could utilize equipment manufactured through'time and resource investments. In these regions, small-scale workshops used by specialized craftsmen betoken full-time employment in this work. Regardless of the large increase in the scale of manufacturing, there is little evidence of major growth in the size of productive units. We are left with the impression of an economy still based on small-scale craft production.

　　Where we do see an important change is in the removal of any exclusive association between the best traditional craftsmen and the governing elite. The powerful could show off their status in new ways, particularly by using Roman architecture and domestic decoration, but the traditional classes of decorative metalwork manufacture no longer seem to have been under the control of the tribal leaders. Rich objects from a wide range of archaeological sites imply the deterioration of this monopoly. There are a number of contributing factors. The control of precious metals moved to the imperial government immediately after the conquest, and gold and silver were also removed from circulation when captured as booty during the invasion. Similarly, changes in taste and the fashions of wealth and status display were stimulated by the arrival of new things like Roman dress, architecture, and sculpture.

　　These changes in manufacture were accompanied by increased distances over which many goods were transported to their consumers. ■ The bulk of pottery and other items originated locally, during the Iron Age; but after the Roman invasion, these objects had been produced over a far greater range of distances. ■ In this way, vast regions of the Roman province were incorporated into a society where there was wide access to material wealth. _ New changes in manufacturing production were coupled with huge increase in the importation of goods from elsewhere in the empire. ■ These commodities, which included Mediterranean foodstuffs such as olive oil as well as comparatively low-value objects such as decorated pottery, also achieved a wide distribution and are found in many different types of site.

　　Before about 4500 B.C., lower Mesopotamia, the region between the Tigris and Euphrates rivers just north of the Persian Gulf, was much less densely populated than other inhabited regions of the Near and Middle East. Its marshy soil, subject to annual inundations （floods） from the rivers, was not suited to the primitive hoe culture of early agriculture, in which land was cultivated without domestic animals or beasts. Moreover, the land was virtually treeless and lacked building stone and mineral resources. During the next thousand years, however, this unpromising area became the seat of Sumer, the first great civilization known to history, with large concentrations of people, bustling cities, monumental architecture, and a wealth of religious, artistic, and literary traditions that influenced other ancient civilizations for thousands of years. The exact sequence of events that led to this culmination is unknown, but it is clear that the economic basis of this first civilization lay in its highly productive agriculture. 

　　The natural fertility of the rich black soil was renewed annually by the silt left from the spring floods of the Tigris and Euphrates rivers. Harnessing its full productive power, however, required an elaborate system of drainage and irrigation, which in turn required a large and well-disciplined workforce as well as skilled management and supervision. The latter were supplied by a class of priests and warriors who ruled a large population of peasants and artisans. Through taxation and other means the rulers extracted wealth from the population and then used it to construct temples and other public buildings and to create works of art. That gave them （or some of them） the leisure to perfect the other refinements of civilization. 

　　The rise of civilization brought with it a far more complex division of labor and  system of economic organization. Full-time artisans specialized in the manufacture of textiles and pottery, metalworking, and other crafts. The professions of architecture, engineering, and medicine, among others, were born. Weights and measures were systematized, mathematics was invented, and primitive forms of science emerged. Since Sumer was virtually devoid of natural resources other than its rich soil, it traded with other people, thereby contributing to the diffusion of Sumerian civilization. The scarcity of stone, for tools as well as for buildings, probably hastened the adoption of copper and bronze. Copper, at least, was already known before the rise of Sumerian civilization, but lack of demand for it among the Stone Age peasant villages inhibited its widespread use. In Sumerian cities, on the other hand, stone imported by sea through the Persian Gulf from Oman and downriver from the mountains of Anatolia and the Caucasus had to compete with imported copper, and the latter proved more economical and effective for a variety of uses. Thereafter metallurgy, the technology of separating metals from their ores and purifying them, was regarded as one of the hallmarks of civilization. 

　　Sumer’s greatest contribution to subsequent civilizations, the invention of writing, likewise grew out of economic necessity. The early cities—Eridu, Ur, Uruk, and Lagash—were temple cities: both economic and religious organizations centered on the temple of the local patron deity, represented by a priestly hierarchy. Members of the hierarchy directed the construction and maintenance of irrigation and drainage systems, oversaw agricultural activities, and supervised the collection of produce as taxation or tribute （money or other wealth given as a sign of submission or in return for protection）。 The need to keep records of the sources and uses of this tribute led to the use of simple pictographs on clay tablets sometime before 3000 B.C. By about 2800 B.C. the pictographs had been stylized into the system of writing known as cuneiform （using wedge-shaped marks on clay）， a distinctive characteristic of Mesopotamian civilization. It is one of the few examples in history of a significant innovation issuing from a bureaucratic organization. 

　　Although writing originated in response to the need for administrative bookkeeping, it soon found multiple religious, literary, and economic uses. In a later phase of development, after the strict temple-centered organization of the economy had given way to greater freedom of enterprise, clay tablets were used for recording the details of contracts, debts, and other commercial and financial transactions.