identifiable members of the native fauna of the United States. The great number of tales, legends, and myths about these birds indicates that people have been exceptionally interested in them for a long time. On the other hand, when it comes to substantive─particularly behavioral─information, crows are less well known than many comparably common species and, for that matter, not a few quite uncommon ones, the endangered California condor, to cite one obvious example .There are practical reasons for this.
Crows are notoriously poor and aggravating subjects for field research. Keen observers and quick learners, they are astute about the intentions of other creatures, including researchers, and adept at avoiding them. Because they are so numerous, active, and monochromatic, it is difficult to distinguish one crow from another. Bands, radio transmitters, or other identifying devices can be attached to them, but this of course requires catching live crows, who are among the wariest and most untrappable of birds.
Technical difficulties aside, crow research is daunting because the ways of these birds are so complex and various. As preeminent generalists, members of this species ingeniously exploit a great range of habitats and resources, and they can
quickly adjust to changes in their circumstances. Being so educable, individual birds have markedly different interests and inclinations, strategies and scams. For example, one pet crow learned how to let a dog out of its kennel by pulling the pin on the door. When the dog escaped, the bird went into the kennel and ate its food. Key 乌鸦
乌鸦可能是美国本土动物中最常见、也是最容易识别的成员（物种）。关于它的奇闻异事和神话传说（故事、传说和神话）流传众多，可见，人们对它的浓厚兴趣由来已久。另一方面，在提及对它的本性——特别是行为方面的了解时，与其他同样普通和一些非常罕见的物种——一个显而易见的例子是加州濒临灭绝的秃鹫——相比，它更鲜为人知。这其中有一些实际的原因。
众所周知，野外的乌鸦研究成果甚少，并让研究者苦恼不已。它们观察敏锐，领悟快捷，善于分辨其他动物和研究人员的意图，并能巧妙地避开他们。由于乌鸦数量众多、颜
色单一、机警灵活，人们很难把它们区分开来。人们当然可以把脚环、无线电发射机和其他识别装置缚在乌鸦身上，但这却要满足一个前提，那就是活抓乌鸦——但这却是最为机警、最难抓到的鸟类之一。
乌鸦的研究之所以令人却步，撇开技术上的困难不谈，其原因之一便是它们生活方式复杂多变。它们是超凡的多面手，其种族中的一些成员创造性地开发了大量的栖息地、利用大量的资源，而且能很快适应环境的变化。由于善于学习，每个乌鸦个体都有迥然不同的兴趣、爱好，策略和花招。比如，有一只宠物乌鸦，学会将狗窝上的门闩拉开放狗出窝。狗出窝后，它便进去，享用了狗的美食。 Passage 4 Life on Earth
Life originated in the early seas less than a billion years after the Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life’s transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis
of life.
What forms of life were able to make such a drastic change in lifestyle The traditional view of the first terrestrial organisms is based on megafossils--relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plats first colonized the margins of continental waters, followed by animals that fed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million years ago.
Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence from sediments that were deposited near the shores of the ancient
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