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Essentially, a theory is an abstract, symbolic representation of what is conceived to be reality.

Thanks to modern irrigation, crops now grow abundantly in areas where once nothing but cacti and sagebrush could live.

Anthropologists have discovered that fear, happiness, sadness, and surprise are universally reflected in facial expressions.

Of the millions who saw Haley’s Comet in 1986, how many people will live long enough to see it return in the twenty-first century.

The greater the population there is in a locality, the greater the need there is for water, transportation, and disposal of refuse.

Perfectly matched pearls, strung into a necklace, bring a far higher price than the same pearls sold individually.

The development of mechanical timepieces spurred the search for more accurate sundials with which to regulate them.

Studies of the gravity field of the Earth indicate that its crust and mantle yield when unusual weight is placed on them.

Quails typically have short rounded wings that enable them to spring into full flight instantly when disturbed in their hiding places.

Fungi are important in the process of decay, which returns ingredients to the soil, enhances soil fertility, and decomposes animal debris.

Most substance contract when they freeze so that the density of a substance’s solid is higher than the density of its liquid.

By the middle of the twentieth century, painters and sculptors in the United States had begun to exert a great worldwide influence over art.

Although apparently rigid, bones exhibit a degree of elasticity that enables the skeleton to withstand considerable impact.

Finally, the coughing reflex in reaction to irritants in the airway produces not a cough during sleep but a cessation of breathing.

But when consumers do not know they are being lobbied, they may accept claims they would otherwise be suspicious of.

Buyers and sellers should be aware of new developments in technology which can and does affect marketing activities.

With modern offices becoming more mechanized, designers are attempting to personalize them with warmer, less severe interiors.

The worker and soldier castes of termites consist of both males and females, and the queen lives permanently with a male consort.

Children with parents whose guidance is firm, consistent, and rational are inclined to possess high levels of self-confidence.

Lucretia Mott’s influence was so significant that she has been credited by some authorities as the originator of feminism in the United States.

No social crusade aroused Elizabeth Williams’ enthusiasm more than the expansion of educational facilities for immigrants to the United States.

Since wind-pollinated flowers have no need to attract insects or other animals, they have dispensed with bright petals, nectar, and scent.

In the absence of a tradition of classical stone-carving and building, the desire to develop Roman amenities would have been difficult to fulfill.

Out in remote areas of the countryside, in rural locations, music was more traditional; the same songs were enjoyed by previous generations.

In the second half of the millennium, in the south around the city of Uruk, there was an enormous escalation in the area occupied by permanent settlements.

By fossils, we mean traces of prehistoric animals such as bones, which become mineralized, or impressions of bones or organs that are left in stone.

The application of electronic controls made possible by the microprocessor and computer storage have multiplied the uses of the modern typewriter.

Even though the precise qualities of hero in literary words may vary over time, the basic exemplary function of the hero seems to remain constant.

When Henry Ford first sought financial backing for making cars, the very notion of farmers and clerks owning automobiles was considered ridiculous.

Many people who wanted to become archaeologists ended up pursuing other careers and contributing to archaeological research only as unpaid amateurs.

Even before the discovery of germs, beliefs that disease spread by “miasmas” （noxious forms of bad air） prompted attention to sewers and open garbage.

Since the consumer considers the best fruit to be that which is the most attractive, the grower must provide products that satisfy the discerning eye.

With no oceans to dissolve it, outgassed carbon dioxide began to accumulate in the atmosphere, intensifying the greenhouse effect even more.

Buildings contribute to human life when they provide shelter, enrich space, complement their site, suit the climate, and are economically feasible.

Research into the dynamics of storms is directed toward improving the ability to predict these events and thus to minimize damage and avoid loss of life.

Their streamlined bodies, the absence of hind legs, and the presence of a fluke and blowhole cannot disguise their affinities with land dwelling mammals. ① streamline /ˈstriːmlaɪn/ vt. 使成流线型 ② hind /haɪnd/ a. 后面的，在后的 ③ presence /ˈprezns/ n. 存在；出现

Scientists do not know why dinosaurs became extinct, but some theories postulate that changes in geography, climate, and sea levels were responsible.

The elimination of inflation would ensure that the amount of money used in repaying a loan would have the same value as the amount of money borrowed.

Futurism, an early twentieth-century movement in art, rejected all traditions and attempted to glorify contemporary life by emphasizing the machine and motion.

Acids are chemical compounds that, in water solution, have a sharp taste, a corrosive action on metals, and the ability to turn certain blue vegetable dyes red.

In other instances, such as the Indus River Valley, the cities lacked a royal precinct and the ostentatious palaces that marked their contemporaries elsewhere.

Offshore drilling platforms extend the search for oil to the ocean’s continental shelves—those gently sloping submarine regions at the edges of the continents.

Even those artists who were most dependent on photography became reluctant to admit that they made use of it, in case this compromised their professional standing.

According to anthropologists, the earliest ancestors of humans that stood upright resembled chimpanzees facially, with sloping foreheads and protruding brows.

Anarchism is a term describing a cluster of doctrines and attitudes whose principal uniting feature is the belief that government is both harmful and unnecessary.

A rotary engine attached to the steam engine enabled shafts to be turned and machines to be driven, resulting in mills using steam power to spin and weave cotton.

Self-awareness, even in its earliest stages, might entail an awareness of others, the ability to see their perspective and to look at the world from another’s point of view.

Various committees acted as an executive branch, implementing policies of the assembly and supervising, for instance, the food and water supplies and public buildings.

A mountain range may emerge and gradually split a population of organisms that can inhabit only lowland lakes; certain fish populations might become isolated in this way.

Think of it this way, humans originally needed to have a stronger sensitivity to bitter-tasting foods so they could learn what plants were good for them and which ones might be poisonous.

Because eastern plows could not penetrate the densely tangled roots of prairie grass, the earliest settlers erected farms along the boundary separating the forest from the prairie.

If the artificial Sun remained stationary, the birds would shift their direction with respect to it at a rate of about 15 degrees per hour, the Sun’s rate of movement across the sky.

The Industrial Revolution had several roots, one of which was a commercial revolution that, beginning as far back as the sixteenth century, accompanied Europe’s expansion overseas.

The jaw joints of many advanced herbivores, such as cows, lie at a different level than the tooth row, allowing transverse tearing, shredding, and compression of plant material.

So musical instruments evolved in ways that optimize their acoustical properties—how the instrument vibrates and sends those vibration through the air to our eardrums.

German printers had the disadvantage of working with the complex typeface that the Italians sneeringly referred to as “Gothic” and that later became known as black letter.

Elizabeth Blackwell, the first woman medical doctor in the United States, founded the New York Infirmary, an institution that has always had a completely female medical staff.

We’ve gone over where coral reefs are usually formed—along the edges of shallow ocean banks in tropical or subtropical regions, and the fact that they are declining at an alarming rate.

Raised beaches often consist of areas of sand, pebbles, or dunes, sometimes containing seashells or piles of debris comprising shells and bones of marine animals used by humans.

But as more and more accumulations of strata were cataloged in more and more places, it became clear that the sequences of rocks sometimes differed from region to region and that no rock type was ever going to become a reliable time marker throughout the world.

This was an important innovation, because the camel’s abilities to thrive in harsh desert conditions and to carry large loads cheaply made it an effective and efficient means of transportation.

These people exploited the landscape intensively, foraging on hill slopes for wild cereal grasses and nuts, while hunting gazelle and other game on grassy lowlands and in river valleys.

The recognition that many aspects of the modern flora and fauna were present on both sides of the Bering Sea as remnants of the ice-age landscape led to this region being named Beringia.

Thus, the recalling of an event （a hunt, battle, or other feat） is elaborated through the narrator’s pantomime and impersonation and eventually through each role being assumed by a different person. ① feat /fiːt/ n. 功绩，壮举 ② elaborate /ɪˈlæbəreɪt/ v. 详述；详细制定 /ɪˈlæbərət/ a. 精心制作的，复杂精美的

The extreme seriousness of desertification results from the vast areas of land and the tremendous numbers of people affected, as well as from the great difficulty of reversing or even slowing the process.

Before starting on a sea voyage, prudent navigators learn the sea charts, study the sailing directions, and memorize lighthouse locations to prepare themselves for any conditions they might encounter.

The question of why infantile amnesia occurs has intrigued psychologists for decades, especially in light of ample evidence that infants and young children can display impressive memory capabilities.

Researchers can drill core samples, uh, basically get a cross-section of a tree without having to kill it, look at the rings and get a picture of what the climate was like in the basin for each of the tree’s years.

Great Britain’s better-developed banking and credit system also helped speed the industrial progress, as did the fact that it was the home of an impressive array of entrepreneurs and inventors.

Confronted with a multitude of unreliable figures, economists have compared the population records with the aggregate data for cultivated land area and grain production in the six centuries since 1368.

Apparently, species-rich plots were likely to contain some drought-resistant plant species that grew better in drought years, compensating for the poor growth of less-tolerant species.

Even more shocking is the fact that the number and rate of imprisonment have more than doubled over the past twenty years, and recidivism—that is the rate for re-arrest—is more than 60 percent.

These plants propagate by producing spores—tiny fertilized cells that contain all the instructions for making a new plant—but the spores are unprotected by any outer coating and carry no supply of nutrient.

Meanwhile, the deadliest strains of the virus perished with their hosts as natural selection favored strains that could infect hosts but not kill them. Thus, natural selection stabilized this host-parasite relationship.

The residence time is the average length of time that any particular molecule of water remains in the lake, and it is calculated by dividing the volume of water in the lake by the rate at which water leaves the lake.

An eye is a collection of cells that are specialized for light detection through the presence of photosensitive pigment as well as a means of restricting the direction of incoming light that will strike the photosensitive cells.

Sociologists view primary groups as bridges between individuals and the larger society because they transmit, mediate, and interpret a society’s cultural patterns and provide the sense of oneness so critical for social solidarity.

Importing the grain, which would have been expensive and time consuming for the Dutch to have produced themselves, kept the price of grain low and thus stimulated individual demand for other foodstuffs and consumer goods.

While the men who make faithful copies of sand paintings from the past represent the principle of stability in Navajo thought, women embody dynamism and create new designs for every weaving they make.

In turn, a deep attachment to the land, and to the stability which rural life engenders, fostered the Roman virtues: gravitas, a sense of responsibility, pietas, a sense of devotion to family and country, and iustitia, a sense of the natural order.

In contrast to mammals and birds, amphibians are unable to produce thermal energy through their metabolic activity, which would allow them to regulate their body temperature independent of the surrounding or ambient temperature.

This is because the gaps among the original grains are often not totally plugged with cementing chemicals; also, parts of the original grains may become dissolved by percolating groundwater, either while consolidation is taking place or at any time afterwards.

The growth of mutual trust among merchants facilitated the growth of sales on credit and led to new developments in finance, such as the bill of exchange, a device that made the long, slow, and very dangerous shipment of coins unnecessary.

Widely reported, if somewhat distrusted, accounts by figures like the famous traveler from Venice, Marco Polo, of the willingness of people in China to trade with Europeans and of the immensity of the wealth to be gained by such contact made the idea irresistible.

In the wake of the Roman Empire’s conquest of Britain in the first century A.D., a large number of troops stayed in the new province, and these troops had a considerable impact on Britain with their camps, fortifications, and participation in the local economy.

While many programs designed for preschoolers focus primarily on social and emotional factors, some are geared mainly toward promoting cognitive gains and preparing preschoolers for the formal instruction they will experience when they start kindergarten.

Hero compiled descriptions and sketches of 70 some clever little, mechanical devices, most of which utilized compressed air to cause water, or in some cases, wine to flow from one place to another, or sometimes to squirt or to make some kind of noise.

Introducing genetically modified trees, designed for fast growth and high yield in given geographic conditions, would satisfy the demand for wood in many of those areas and save the endangered native trees, which often include unique or rare species.

In the 1760s, James Watt perfected a separate condenser for the steam, so that the cylinder did not have to be cooled at every stroke; then he devised a way to make the piston turn a wheel and thus convert reciprocating （back and forth） motion into rotary motion.

Unlike short-term financial cooperation between investors for a single commercial undertaking, joint-stock companies provided permanent funding of capital by drawing on the investments of merchants and other investors who purchased shares in the company.

This was justified by the view that reflective practice could help teachers to feel more intellectually involved in their role and work in teaching and enable them to cope with the paucity of scientific fact and the uncertainty of knowledge in the discipline of teaching.

There were no other large public buildings, suggesting that the priests who were in charge of the temples also were responsible for governing the city and organizing people to work in the fields and on irrigation projects building and maintaining systems of ditches and dams.

Inequalities of wealth and rank certainly exist, and have probably existed in most pastoralist societies, but except in periods of military conquest, they are normally too slight to generate the stable, hereditary hierarchies that are usually implied by the use of the term class.

Many ecologists now think that the relative long-term stability of climax communities comes not from diversity but from the “patchiness” of the environment; an environment that varies from place to place supports more kinds of organisms than an environment that is uniform.

According to conventional theory, yawning takes place when people are bored or sleepy and serves the function of increasing alertness by reversing, through deeper breathing, the drop in blood oxygen levels that are caused by the shallow breathing that accompanies lack of sleep or boredom.

Contrary to the arguments of some that much of the Pacific was settled by Polynesians accidentally marooned after being lost and adrift, it seems reasonable that this feat was accomplished by deliberate colonization expeditions that set out fully stocked with food and domesticated plants and animals.

The Athenian philosopher Socrates （470-399 B.C.） was the first person in Greece to propose a morality based on individual conscience rather than the demands of the state, and for this he was accused of not believing in the city’s gods and so corrupting the youth, and he was condemned to death.

Two additional kinds of evidence support the hypothesis that petroleum is a product of the decomposition of organic matter: oil possesses optical properties known only in hydrocarbons derived from organic matter, and oil contains nitrogen and certain compounds believed to originate only in living matter.

Though it may be difficult to imagine from a later perspective, a strain of critical opinion in the 1920s predicted that sound film would be a technical novelty that would soon fade from sight, just as had many previous attempts, dating well back before the First World War, to link images with recorded sound.

In the multiplicity of small-scale local egalitarian or quasi-egalitarian organizations for fellowship, worship, and production that flourished in this laissez-faire environment, individuals could interact with one another within a community of harmony and ideological equality,