Historically, more cold-adapted antelope species originating in Eurasia have migrated into Africa, where the climate is generally warmer, than have warm-adapted African species into Eurasia. A likely explanation for this involves the fact that intercontinental migrations require both a land bridge connecting the two continents and the suitable habitat both on and across that land bridge. During periods of climatic cooling, such as the various ice ages, the land bridge is open for a long time (because sea level remains low) and is usable by cold-adapt species because cool habitats then extend across it. Thus during cooling most migrants would be expected to travel toward Africa, which is near the equator, since this is the direction dictated by habitat changes on a cooling Earth. In contrast, when the Earth is warm, the land bridge is reduced or gone because sea level is relatively high then. Only during the short lag between onset of global temperature change and sea level response can warm-adapted species migrate from the equator toward higher latitudes.
GRE阅读文章解析
第一句 ：
Historically, more cold-adapted antelope species originating in Eurasia have migrated into Africa, where the climate is generally warmer, than have warm-adapted African species into Eurasia.
白话版讲解：
从历史情况来看，来自欧亚大陆的、能够适应寒冷气候的羚羊物种迁徙到气候更加温暖的非洲，和来自非洲的、能够适应温暖气候的非洲羚羊迁徙到欧亚大陆，这两类羚羊相比，前者数量更多。
这句话描述了一个事实，即从欧亚大陆迁徙到非洲的羚羊比从非洲迁徙到欧亚大陆的羚羊数量要多。
第二句 ：
A likely explanation for this involves the fact that intercontinental migrations require both a land bridge connecting the two continents and the suitable habitat both on and across that land bridge.
白话版讲解：
对这种情况进行合理的解释需要考虑到这样一个事实，那就是跨大洲的迁徙既需要两个洲之间有陆地桥梁，又需要在陆地桥梁上、以及跨越桥梁的沿线都有适宜的栖息地存在。
这句话给出了跨洲迁徙所需要具备的必要条件。
第三句 ：
During periods of climatic cooling, such as the various ice ages, the land bridge is open for a long time (because sea level remains low) and is usable by cold-adapt species because cool habitats then extend across it.
白话版讲解：
在气候变冷的时期，比如各种各样的冰河时期，陆地桥梁长时期都是长期开放的（因为海平面很低）；而且能够适应寒冷气候的物种也能用到这个陆地桥梁，因为比较冷的栖息地是可以延伸直到横跨过桥梁。
第四句 ：
Thus during cooling most migrants would be expected to travel toward Africa, which is near the equator, since this is the direction dictated by habitat changes on a cooling Earth.
白话版讲解：
在气候变冷的时候，地球上的栖息地也会更多地朝赤道附近移动，受这个的影响，大部分的迁徙动物也就会随着往靠近赤道的非洲移动。
第五句 ：
In contrast, when the Earth is warm, the land bridge is reduced or gone because sea level is relatively high then.
白话版讲解：
相反，当地球温度比较高的时候，海平面相对来说会比较高一些，所以陆地桥梁就缩小了，甚至是消失了。
第六句 ：
Only during the short lag between onset of global temperature change and sea level response can warm-adapted species migrate from the equator toward higher latitudes.
白话版讲解：
只有在全球气温开始变化和水位做出反应之间的这短暂的时间差里，适应温暖气候的物种才能从赤道地区迁徙到纬度更高的地区。
先有全球气温开始变化，然后才有海平面水位做出反应（升高/降低），所以两种现象之间有时间差。只有在这段时间差里（也就是气温开始变了，但海平面还没有变化），适应温暖气候的物种才能机会迁徙到更冷的地方。
原文第三到第六句详细解释了为什么往温度比较高的地方迁徙的物种要多于往温度比较低的地方迁徙的物种。总结来说，是因为相比于气候温暖的时候，大陆桥梁更容易在气候寒冷的时候存在，而气候寒冷的时候，动物们会从气候寒冷的地方迁徙到气候温暖的地方。
GRE阅读题目解析
1. A ccording to the passage, which of the following is true of an Eurasia-Africa land bridge during ice ages?
A. I t offers suitable habitats for cold-adapted species of antelope.
B. I t encourages migration from Africa to Eurasia of cold-adapted species of antelope.
C. I t allows the survival of warm-adapted species of antelope that might otherwise home extinct.
D. I t is more likely to exist early in an ice age than later in an ice age.
E. T he habitats it offers change more quickly than do those offered by land bridges during other periods.
白话版讲解：
第一题：根据文章内容，以下哪个关于冰河时期的欧亚大陆和非洲之间的大陆桥梁的说法是正确的？
这道题的正确答案是 A。
A. 它为适应寒冷气候的羚羊物种提供了舒适的栖息地。（该选项是对原文第三句的同义改写。）
B. 它鼓励适应寒冷气候的羚羊物种从非洲迁徙到欧亚大陆。
C. 它让适应温暖气候的羚羊物种存活了下来，否则这些羚羊物种就可能会灭绝。
D. 它更可能存在于冰河世纪的早期，而不是冰河世纪的晚期。
E. 它提供的栖息地比其他时期的大陆桥梁所提供的栖息地变化得更快。
2. The author of the passage implies that during the “short lag”, a land bridge between Africa and Eurasia would
A. be in ha bited primarily by species of antelope originating in Eurasia.
B . be characterized by areas of widely varying elevation above sea level.
C. be inhabited by a wide diversity of antelope species.
D. contain habitats that could sustain warm-adapt antelope species.
E. contain habitats similar to habitats at much higher latitudes in Eurasia.
白话版讲解：
第二题：作者认为，在短暂的时间差（short lag）里，连接非洲和欧亚大陆之间的大陆桥梁会怎么样？
这道题的正确答案是 D。
A. 主要是来自欧亚大陆的羚羊物种在那里栖息。
B. 那边不同区域之间的高度相差很大（以海平面为标准）。
C. 不同种类的羚羊都在那里栖息。
D. 那里有可以支持适应温暖气候的羚羊物种生存的栖息地。（该选项是对原文第六句的同义改写。）
E. 那里的栖息地和欧亚大陆的相似。
3. According to the passage, which of the following best accounts for the apparent bias in antelope-migration direction?
A. Warm-adapted antelope species are rarely abl e to tolerate cool habitats, whereas cold-adapted antelope species usually can tolerate warm habitats.
B. During global warming periods, land bridges, when present, lack habitats suitable for sustaining warm-adapted antelope species.
C. Under most climatic conditions, Africa offers a larger number of suitable antelope habitats than does Eurasia.
D. Many more species of antelope have originated in Eurasia than have originated in Africa.
E. Land bridges are more likely to exist when climate change favors migration to warmer climates than when climate changes favors migration to cooler climates.
白话版讲解：
第三题：根据文章内容，以下哪个选项最能解释羚羊物种的迁徙方向上的倾向？
这道题应该选 E。
A. 适应温暖气候的羚羊无法忍受寒冷的栖息地，而适应寒冷气候的羚羊往往可以忍受比较炎热的栖息地。
B. 在全球气温上升的时候，大陆桥梁没有适合适应温暖气候的羚羊生存的栖息地。
C. 在大部分的气候条件下，相比于欧亚大陆，非洲能为羚羊提供更多的适宜的栖息地。
D. 源自欧亚大陆的羚羊物种数量比源自非洲的羚羊物种数量多。
E. 相比起气候变化有利于动物往温度更低的地方迁徙的时候，气候变化有利于动物往温度更高的地方迁徙的时候，陆地桥梁更容易存在。（根据原文第三到第六句的内容，陆地桥梁在气候比较寒冷的时候更容易存在，而气候变冷的时候，地球上的栖息地也会更多地朝赤道附近移动。而在气候比较温暖的时候，陆地桥梁会缩小，甚至是消失没有了。）
市面上流传的GRE阅读机经真题有三百多篇，同时又存在多种版本的答案和解析，有些疑难题目让考生不知道听谁的，还有些版本的解析像是用谷歌翻译把同题目和选项翻译了一下。TD GRE教研组认为，GRE阅读的解析，可以讲得再清楚一点。
GRE阅读不光有技巧，不光有快速解题法，GRE更加考察大家的逻辑硬实力，文字理解实打实的深度。良好的GRE备考应当在课堂上向老师学习快速解题法，取同取反逻辑的同时，课下扎扎实实的读懂每一句话。读懂，是一切技巧使用的前提。皮之不存，毛将焉附？
有感于此，我们尝试用比较长的时间，原创一个解析，力求做到超详细，大白话，包你懂。之前为大家推送了GRE Passage 1 - Passage 67的解析（?：GRE Passage 1 - Passage 67 文章合集），今天继续为大家推出GRE阅读真题Passage 68（短阅读）解析，这篇阅读一共有1段，不同场次的考试共提取出3道题，TD版超详细「包你懂」系列对此篇文章进行了清楚地解析。
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