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Two opposing scenarios, the "arboreal"树木的，生于树上的 hypothesis and the "cursorial" 善于奔跑的hypothesis, have traditionally been put forward concerning the origins of bird flight. The "arboreal" hypothesis holds that bird ancestors began to fly by climbing trees and gliding down from branches with the help of incipient 初期的feathers: the height of trees provides a good starting place for launching flight, especially through gliding. As feathers became larger over time, flapping flight evolved and birds finally became fully air-borne. This hypothesis makes intuitive sense, but certain aspects are troubling. Archaeopteryx (the earliest known bird) and its maniraptoran dinosaur cousins have no obviously arboreal adaptations, such as feet fully adapted for perching栖息，置于....上. Perhaps some of them could climb trees, but no convincing analysis has demonstrated how Archaeopteryx would have both climbed and flown with its forelimbs, and there were no plants taller than a few meters in the environments where Archaeopteryx fossils have been found. Even if the animals could climb trees, this ability is not synonymous with gliding ability. (Many small animals, and even some goats and kangaroos, are capable of climbing trees but are not gliders.) Besides, Archaeopteryx shows no obvious features of gliders, such as a broad membrane膈膜 connecting forelimbs and hind limbs. The "cursorial"(running) hypothesis holds that small dinosaurs ran along the ground and stretched out their arms for balance as they leaped into the air after insect prey or, perhaps, to avoid predators. Even rudimentary feathers on forelimbs could have expanded the arm's surface area to enhance lift slightly. Larger feathers could have increased lift incrementally, until sustained flight was gradually achieved. Of course, a leap into the air does not provide the acceleration produced by dropping out of a tree; an animal would have to run quite fast to take off. Still, some small terrestrial animals can achieve high speeds. The cursorial hypothesis is strengthened by the fact that the immediate theropod dinosaur ancestors of birds were terrestrial, and they had the traits needed for high lift off speeds: they were small, agile, lightly built, long-legged, and good runners. And because they were bipedal, their arms were free to evolve flapping flight, which cannot be said for other reptiles of their time."

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When the history of women began to receive focused attention in the 1970', Eleanor Roosevelt was one of a handful of female Americans who were well known to both historians and the general public. Despite the evidence that she had been important in social-reform circles before her husband was elected President and that she continued to advocate different causes than he did, she held a place in the public imagination largely because she was the wife of a particularly influential President. Her own activities were seen as preparing the way for her husband's election or as a complement to his programs. Even Joseph Lash's two volumes of sympathetic biography, Eleanor and Franklin (1971) and Eleanor: The Years Alone (1972), reflected this assumption. Lash's biography revealed a complicated woman who sought through political activity both to flee inner misery and to promote causes in which she passionately believed. However, she still appeared to be an idiosyncratic figure, somehow self-generated not amenable to any generalized explanation. She emerged from the biography as a mother to the entire nation, or as a busybody, but hardly as a social type, a figure comprehensible in terms of broader social developments. But more recent work on the feminism of the post-suffrage years (following 1920) allows us to see Roosevelt in a different light and to bring her life into a more richly detailed context. Lois Scharf's Eleanor Roosevelt, written in 1987, depicts a generation of privileged women, born in the late nineteenth century and maturing in the twentieth, who made the transition from old patterns of female association to new ones. Their views and their lives were full of contradictions. They maintained female social networks but began to integrate women into mainstream politics; they demanded equal treatment but also argued that women's maternal responsibilities made them both wards and representatives of the public interest. Thanks to Scharf and others, Roosevelt's activities--for example, her support both for labor laws protecting women and for appointments of women to high public office--have become intelligible in terms of this social context rather than as the idiosyncratic career of a famous man's wife.

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The experience of British business in Iran between the 1860's and the 1970's is one example of the changing importance of British enterprise in Asia as a whole. Before 1914 British business established and dominated Iran's modern industrial and financial sector; in the 1920's this domination began to wane; by the 1960's British enterprise was of little importance in the Iranian economy. While in Japan and India the decline of British business was primarily a function of the rise of strong indigenous 当地的 business groups, in Iran, by contrast, the government played a large role in both challenging British commercial interests and stimulating locally owned enterprise. Periodic surges of intense Iranian economic nationalism must be understood partly as a reaction to the close relations between British business in Iran and the British government. In retrospect回顾, it is possible to see the uneasy and ambiguous nature of this relationship. It is true that the British Imperial Bank in Iran was never entirely a tool of the British government, and that the Anglo-Iranian Oil Company did not take its orders from the British government, despite the 51 percent government shareholding. However, the relationship between British business and the British government was sufficiently close that many Iranians understandably viewed the oil company and the bank as symbols of a British imperialist policy.

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Astronomers theorize that a black hole forms when a massive 巨大的object shrinks catastrophically under its own gravity重力 引力, leaving only a gravitational field so strong that nothing escapes it. Astronomers must infer the existence of black holes, which are invisible, from their gravitational influence on the visible bodies surrounding them. For example, observations indicate that gas clouds in galaxy M87 are whirling unusually fast about the galaxy's center. Most astronomers believe that the large concentration of mass at the galaxy's center is a black hole whose gravity is causing the gas to whirl. A few skeptics have argued that the concentration of mass necessary to explain the speed of the whirling gas is not necessarily a black hole: the concentration in M87 might be a cluster of a billion or so dim stars. The same hypothesis might have been applied to the galaxy NGC 4258, but the notion of such a cluster's existing in NGC 4258 was severely undermined when astronomers measured the speed of a ring of dust and gas rotating close to the galaxy's center. From its speed, they calculated that the core's density is more than 40 times the density estimated for any other galaxy. If the center of NGC 4258 were a star cluster, the stars would be so closely spaced that collisions between individual stars would have long ago torn the cluster apart.