[填空题]

Long before man lived on the Earth, there were fishes, reptiles, birds, insects, and some mammals. Although some of these animals were ancestors of kinds living today, others are now extinct, that is, they have no descendants alive now. Nevertheless, we know a great deal about many of them because their bones and shells have been preserved in the rocks as fossils.
41.______That kind of rock in which the remains are found tells us much about the nature of the original land, often of the plants that grew on it, and even of its climate.
When an animal dies, the body, its bones, or shell, may often be carried away by streams into lakes or the sea and there get covered up by mud. If the animal lived in the sea its body would probably sink and be covered with mud. More and more mud would fall upon it until the bones or shell become embedded and preserved. 42．______Thus it follows that there must be many kinds of mammals, birds, and insects of which we know nothing,

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