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借款合同或对外贸易合同适用的履行合同担保形式是______
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"Two centuries ago, Meriwether Lewis and William Clark left St. Lois to explore the new lands acquired in the Louisiana Purchase, " George W. Bush said, announcing his desire for a program to send men and women to Mars. "They made that journey in the spirit of discovery... America has ventured forth into space for the same reasons. " Yet there are vital differences between Lewis and Clark"s expedition and a Mars mission. First, Lewis and Clark were headed to a place amenable to life; hundreds of thousands of people were already living there. Second, Lewis and Clark were certain to discover places and things of immediate value to the new nation. Third, the Lewis and Clark venture cost next to nothing by today"s standards. In 1989, NASA estimated that a people-to-Mars program would cost $ 400 billion, which inflates to $ 600 billion today. But the fact that a destination is tantalizing does not mean the journey makes sense, even considering the human calling to explore. And Mars as a destination for people makes absolutely no sense with current technology. Present systems for getting from Earth"s surface to low-Earth orbit are so fantastically expensive that merely launching the 1, 000 tons or so of spacecraft and equipment a Mars mission would require could be accomplished only by cutting health-care benefits, education spending or other important programs or by raising taxes. Absent some remarkable discovery, astronauts, geologists and biologists once on Mars could do little more than analyze rocks and feel awestruck beholding the sky of another world. It is interesting to note that when President Bush unveiled his proposal, he listed these recent major achievements of space exploration: pictures of the rings of Saturn and the outer planets, evidence of water on Mars and the moon of Jupiter, discovery of more than 100 planets outside our solar system and study of the soil of Mars. All these accomplishments came from automated probes or automated space telescopes. Bush"s proposal, which calls for "reprogramming" some of NASA"s present budget into the Mars effort, might actually lead to a reduction in such unmanned science, the one aspect of space exploration that"s working really well. Rather than spend hundreds of billions of dollars to hurl tons toward Mars using current technology, why not take a decade or two decades, or however much time is required researching new launch systems and advanced propulsion If new launch systems could put weight into orbit affordably, and if advanced propulsion could speed up that long, slow transit to Mars, then the dreams of stepping onto the Red Planet might become reality. Mars will still be there when the technology is ready. The drive to explore is part of what makes us human, and exploration of the past has led to unexpected glories. Dreams must be tempered by realism, however. For the moment, going to Mars is hopelessly unrealistic. According to the author, human travel to Mars
In almost all cases, the soft parts of fossils are gone for ever but they were fitted around or within the hard parts. Many of them also were attached to the hard parts and usually such attachments are visible as depressed or elevated areas, ridges or grooves, smooth or rough patches on the hard parts. The muscles most important for the activities of the animal and most evident in the appearance of the living animal are those attached to the hard parts and possible to reconstruct from their attachments. Much can be learned about a vanished brain from the inside of the skull in which it was lodged. Restoration of the external appearance of an extinct animal has little or no scientific value. It does not even help in inferring what the activities of the living animal were, how fast it could run, what its food was, or such other conclusions as are important for the history of life. However, what most people want to know about extinct animals is what they looked like when they were alive. Scientists also would like to know. Things like fossil shells present no great problem as a rule, because the hard parts are external when the animal is alive and the outer appearance is actually preserved in the fossils. Animals in which the skeleton is internal present great problems of restoration, and honest restorers admit that they often have to use considerable guessing. The general shape and contours of the body are fixed by the skeleton and by muscles attached to the skeleton, but surface features, which may give the animal its really characteristic look, are seldom restorable with any real probability of accuracy. The present often helps to interpret the past. An extinct animal presumably looked more or less like its living relatives, if it has any. This, however, may be quite equivocal. For example, extinct members of the horse family are usually restored to look somewhat like the most familiar living horses — domestic horses and their closest wild relatives. It is, however, possible and even probable that many extinct horses were striped like zebras. If lions and tigers were extinct they would be restored to look exactly alike. No living elephants have much hair and mammoths, which are extinct elephants, would doubtless be restored as hairless if we did not happen to know that they had thick, woolly coats. We know this only because mammoths are so recently extinct that prehistoric men drew pictures of them and that the hide and hair have actually been found in a few specimens. For older extinct animals we have no such clues. The muscles of a fossilized animal can sometimes be reconstructed because
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