桦树林中出现了一些云杉的幼苗，云杉长大后，其高度超过桦树，桦树因不耐阴而逐渐死亡。经过许多年后，这片桦树林最终被云杉取代。这一群落演替过程表明
A．种内斗争有利于物种的延续
B．云杉的结构比桦树复杂
C．云杉的进化水平比桦树高
D．适者生存是通过生存斗争来实现的
答案：D
生物进化的原因是自然选择，达尔文把通过激烈的生存竞争，适者生存，不适者被淘汰的过程叫做自然选择。
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| Earthquakes are 1 ; thousands of them happen each day. But most are too weak to feel. During a 2 earthquake, there is often a great noise first. Then the earth 3 terribly; many houses 4 down. Railway tracks break and trains go 5 lines; a great many factories are 6 ; thousands of deaths are caused, and many more lose homes... 7 the great damage and deaths caused by the earthquake 8 , other disasters such as fires often 9 . More buildings are destroyed and more 10 are caused. It is well known of the 11 of a possible earthquake, and for centuries man has been making researches on earthquakes. More than 2,000 years ago, 12 , a Chinese scientist named Zhang Heng 13 a machine which could find out from which 14 the seismic (地震的) waves had come, and this machine is still 15 by scientists today. Now we know much more about earthquakes and 16 they happen, but we still cannot 17 exactly when and where an earthquake will happen, and cannot 18 it from happening. So earthquakes are among the 19 disasters in the world. No one can stop natural earthquakes. 20 , scientists can help stop earthquakes from destroying whole cities and causing too many deaths. | ||||
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