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For centuries, the nature of the brain was shrouded in mystery. Aristotle is said to have 1 it was a cold sponge, whose main task was to 2 the blood. Later, Leonardo da Vinci 3 the brain as a curious void filled by three tiny bulbous structures 4 in a straight line 5 the eyeball.   Not all early theories were quite so misguided, 6 . From the first studies 7 language deficits, it was 8 that the brain played some direct part in language use. In 1836, an 9 French country doctor, Max Dax, claimed that, in forty aphasic (患失语症的) patients he had seen, 10 of language ability always 11 with damage to the left half of the brain. Thirty years later, this claim was 12 proved by the French surgeon Paul Broca. He had studied aphasic 13 in patients who were found to have brain damage 14 the left frontal lobe. Broca was struck by the contrast with right hemisphere damage, 15 seemed to have little effect on speech. The area Broca isolated and the aphasia associated with it now 16 his name, "Broca’’s aphasia."   Ten years after Broca’’s 17 , Karl Wernicke, a young researcher in Germany, made another startling 18 , which ultimately 19 him to propose not just a new language area, but an overall theory of 20 language is handled in the brain.

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A
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Solar energy is called a renewable resource because on a human time scale it is essentially inexhaustible, it is expected to last at least 6.5 billion years while the sun completes its life cycle. 46) A potentially renewable resource can be renewed fairly rapidly (hours to several decades) through natural processes, examples of which include forest trees, grassland grasses, wild animals, fresh lake and stream water, groundwater, fresh air, and fertile soil.47) One important potentially renewable resource for us and other species is biological diversity or biodiversity, which consists of the life forms that can best survive variety of conditions currently found on Earth. Kinds of biodiversity include (1) genetic diversity( variety in the genetic makeup among individuals within a single species), (2) species diversity (variety among the species or unique forms of life found in different habitats of the planet), and ( 3 ) ecological diversity (variety of forests, deserts, grasslands, streams, lakes, oceans, wetlands, and other biological communities). 48)This rich variety of genes, species, and biological communities gives us food, wood, fibers, energy, raw materials, industrial chemicals, and medicines―all of which pour hundreds of billions of dollars into the world economy each year. Earth’’s vast list ’’of life forms and biological communities also provides free recycling and purification services and natural pest control.Potentially renewable resources, however, can be exhausted. 49)The highest rate at which a potentially renewable resource can be used indefinitely without reducing its available supply is called its sustainable yield. If a resource’’s natural replacement rate is exceeded, the available supply begins to shrink―a process known as environmental degradation.Several types of environmental degradation can change potentially renewable resources into nonrenewable or unusable resources. In the United States, one-fourth of the groundwater withdrawn each year is not replenished (filled up again). Between 25% and 50% of the world’’s wetlands (55% in the U. S. ) have been drained, built upon, or seriously polluted. Almost half of the world’’s tropical forests have been cleared. Thousands of wildlife species become extinct each year, mostly because of human activities. If habitat destruction continues at present rates, as many as 1.5 million species could disappear over the next 25 years―a drastic loss in vital Earth capital. 50)These examples help explain why most environmental scientists believe that over the next few decades, the danger of degradation and exhaustion is greatest for potentially renewable resources, not nonrenewable resources ( except for petroleum and perhaps a few scarce minerals for which we can’’t find economically and environmentally acceptable substitutes).