新概念英語第四冊(cè)（美音版）The past life of the earth

[00:00.48]Lesson 26 [00:03.68]The past life of the earth [00:09.22]What is the main condition for the preservation of the remains of any living creature? [00:17.59]It is animals and plants which lived in or near water whose remains are most likely to be preserved, [00:24.49]for one of the necessary conditions of preservation is quick burial, [00:30.40]and it is only in the seas and rivers, and sometimes lakes, [00:34.84]where mud and silt have been continuously deposited, [00:38.72]that bodies and the like can be rapidly covered over and preserved. [00:44.25]But even in the most favourable circumstances [00:47.63]only a small fraction of the creatures that die are preserved in this way [00:52.96]before decay sets in or, even more likely, before scavengers eat them. [00:59.09]After all, all living creatures live by feeding on something else, [01:03.71]whether it be plant or animal, dead or alive, [01:07.51]and it is only by chance that such a fate is avoided. [01:11.65]The remains of plants and animals that lived on land are much more rarely preserved, [01:17.26]for there is seldom anything to cover them over. [01:20.71]When you think of the innumerable birds that one sees flying about, [01:25.56]not to mention the equally numerous small animals like field mice and voles which you do not see, [01:32.76]it is very rarely that one comes across a dead body, except, of course, on the roads. [01:39.14]They decompose and are quickly destroyed by the weather or eaten by some other creature. [01:46.22]It is almost always due to some very special circumstances that traces of land animals survive, [01:53.83]as by falling into inaccessible caves, or into an ice crevasse, [01:59.26]like the Siberian mammoths, [02:01.40]when the whole animal is sometimes preserved, as in a refrigerator. [02:06.58]This is what happened to the famous Beresovka mammoth which was found preserved and in good condition. [02:13.76]In his mouth were the remains of fir trees--the last meal that he had before he fell into the crevasse and broke his back. [02:23.19]The mammoth has now been restored in the Palaeontological Museum in St.Petersburg. [02:29.60]Other animals were trapped in tar pits, like the elephants, sabre-toothed cats, [02:35.46]and numerous other creatures that are found at Rancho la Brea, which is now just a suburb of Los Angeles. [02:43.22]Apparently what happened was that water collected on these tar pits [02:48.29]and the bigger animals like the elephants ventured out on to the apparently firm surface to drink, [02:55.69]and were promptly bogged in the tar. [02:58.49]And then, when they were dead, the carnivores, [03:01.94]like the sabretoothed cats and the giant wolves, [03:05.48]came out to feed and suffered exactly the same fate. [03:10.68]There are also endless numbers of birds in the tar as well.

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