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Since there is such an abundance of food in the sea, it is understandable that some of the efficient, highly adaptable, warm-blooded mammals that evolved on land should have returned to the sea . Those that did have flourished. Within about 50 million years -no time at all, geologically speaking -one of the four kinds of mammals that has returned to a marine environment has developed into the largest of all animal forms , the whale. A second kind, the seal , has produced what is probably the greatest population of large carnivorous mammals on Earth. This suggests that these "top dogs " of the ocean are prospering and multiplying . However , such has not been the case, at least not for the last 150 years. Trouble has closed in on these mammals in the form of equally warm-blooded and even more efficient and adaptable predators, human. At sea, as on land, humans have now positioned themselves on top of the whole great pyramid of life, and they have caused serious problems for the mammals of the sea. There is a simple reason for this. Marine mammals have the misfortune to be swimming aggregates of commodities that humans want: fur, oil, and meat. Even so, they might not be so vulnerable to human depredation if they did not , like humans, reproduce so slowly. Every year humans take more than 50 million tons of fish from the oceans without critically depleting the population of any species. But the slow-breeding mammals of the sea have been all but wiped out by humans seeking to satisfy their wants and whims. 重点单词： Of all the folk artist in the United Stares the most well known of the twentieth century is certainly Grandma Moses -Anna Mary Robertson Moses (1860-1961). She was also the most successful within her lifetime and her work was reproduced on greeting cards and calendars and in prints. As with many folk artists, her career as a painter started late in life. at the age of 67, but she continued painting until her death at the age of 101, so her active painting life still spanned over 34 years. Her subjects are based on the New England countryside and evoke a strong mood of nostalgia. Many of her early paintings are copies of , or use sections form, prints by Currier and lves that she then recomposed in her own way . In her versions the figures became more stylized and the landscapes less naturalistic. Her painting was preceded by the production of landscapes in needlework, and it was only the onset of arthritis that forced the change of medium. The images, however, continued the same, and she reexecuted some of her needlework landscapes in paint at a later date. From these early sources she began to compose original paintings such as Housick Falls, New York in Winter (1944) that relied on her surroundings and her memories of country life and activities ; these paintings display an increasing technical ability. By the 1940's her work had become a marketable commodity and collectors created a demand for her paintings. Like many painters of the nineteenth and twentieth centuries, Grandma Moses made use of photographs for information, for figures, for fragments of landscape, and for buildings, but her work, especially that of her later years, was not a slavish copying of these but compositions using them as source material. Her output was prodigious, and consequently her work is of varying quality. Although much of her public appeal is based on the emotive image of the "Grandma " figure producing naive pictures of country life, her paintings place her among the top folk painters of the nineteenth and twentieth centuries. 重点单词： In general, as soon as the newborn child's muscles, sense organs, and nerves are fully formed, the child begins to use them. But much of the human nervous system is not fully developed until the child is a year or two old, and some parts, such as the corpus callosum, continue to mature for at least the next 20 years. The general pattern of bodily development is from head to foot .Simple skills, such as head movements, appear first because the structures that control these skills are among the first to mature .More complex behavior patterns , such as crawling, standing, and walking , come much later in the developmental sequence than head movements do. The motor centers in the brain are connected by long nerve fibers(usually through one or more synapses )to the muscles in various parts of the body. Since the head muscles are closer to the brain than are the foot muscles, according to one theory, the head comes under the control of the motor centers long before the feet do. The appearance of anew motor skill (such as crawling and grasping ) always suggests that a new part of the child's body has just matured -that is, that the brain centers have just begun to control the muscles involved in the new motor skill. 重点单词： By long-standing convention all meteorites are assigned to three broad divisions on the basis of two kinds of material that they contain: metallic nickel-iron(metal)and silicates, which are compounds of other chemical elements with silicon and oxygen. As their name suggests, the iron meteorites consist almost entirely of metal. At the opposite extreme, the stony meteorites consist chiefly of silicates and contain little or no metal. A third category, stony-irons, includes those meteorites that contain similar amounts of metal and silicates. Since meteoritic metal weighs more than twice as much as the same volume of meteoritic silicates, these three kinds of meteorites can usually be distinguished by density, without more elaborate tests. The stony meteorites can also be subdivided into two categories by using nothing more complicated than a magnifying glass. The great majority of such meteorites are chondrites, which take their name from tiny , rounded objects -chondrules -that occur in most of them and are among their most puzzling features . The rest of the stony meteorites lack chondritic texture and are therefore called achondrites. Achondrites vary widely in texture, composition , and history. Irons, stony-irons, chondrites, and achondrites are by no means equally abundant among observed meteorites: chondrites are much more common than all other kinds of meteorites put together. The irons, which are usually prominent in museum displays, are really quite uncommon .Curators like to highlight iron meteorites because many of them are large and their internal structure is spectacular in polished, etched slices . A stony meteorite has a beauty of its own, but it only appears under the microscope: to the unaided eye, stony meteorites appear to be-indeed they are-rather homely black or gray rocks . To go further with meteorite classification , it is necessary to be more specific about the minerals that make up a meteorite: which silicates are present , and what kind of metal? To answer these questions ,one needs to see more detail than is visible to the unaided human eye. 重点单词： National parties in the United States have generally been weak in structure and wary of ideology. Many writers have said that American parties are the least centralized in the world. However, the argument that parties have not represented significant differences in policy can be pushed too far. For example, in this century , at least , the Republicans have been more committed than the Democrats to a market-oriented economy , while the Democrats have been more prepared to use government to address economic problems. Within both parties there has been wide variance on issues, but in general the Republicans have been the more conservative and the Democrats the more liberal . Both parties, however, have resisted reducing these tendencies in their social, economic, and moral belief systems to a rigid ideology . And neither, until recently, vested much authority in its national party structure . At state and local levels, on the other hand , party organizations often achieved impressive levels of solidarity and internal discipline. Both Democrats and Republicans maintained potent local political organizations in many cities and states. Whatever their merits or demerits the traditional organizations went into steep decline during the 1950's and 1960's .The old organizations lost the ability to maintain internal discipline .The share of voters regarding themselves as political independents, that is , people not affiliated with either of the major parties, rose . There were several reasons for the loss of effectiveness of the major party organizations. Development of a welfare state administered by the federal government established some of the services that had formerly been dispensed by the organizations as political favors. As recent immigrants became more educated they were less dependent on party workers. The inclusion of more state employees under civil service protection dried up some of the old wells of patronage , Growing unionization of public employees after 1960 struck an even more serious blow at the patronage system. Television brought candidates into voters' living rooms, thereby antiquating some of the communication and education functions of party workers. Most of all, perhaps ,the old tribal differences associated with the parties began to seem irrelevant to members of generations that sought fresh identities. 重点单词： |
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