广东省肇庆市高三上学期2024届第二次教学质量检测（二模）英语试题试题详情
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If "Why?" is the first question in science, "Why not?" must be the second one. It's interesting to think about why something does not exist in nature, such as a three-legged animal.
In an online lecture, David Jackson, a biologist in Earth and Planetary Sciences, was challenged to come up with "an animal or plant that does not and cannot exist". Jackson told the audience that in nature, no species known to date has ever been born with a third leg.
Jackson pointed out that, however, many animals do use a particular three-legged pose for support. Examples include meerkats (猫鼬) resting on their tail and back feet and woodpeckers (啄木鸟) using tail feathers to weigh themselves against a tree trunk, which is more stable and requires less energy. Given that three-legged movement does work in some situations, why are there no animals with three legs?
The answer may be at the core of evolution. Over a long time in history, almost all animals have adapted themselves and developed into a body shape which is organized around a central point and can be divided into two similar halves. Butterflies are an excellent example. Not only can their body shape be divided into two matching halves, but also the patterns on each wing are exactly the same. The law of having two sides to everything has been in our DNA since the first animals appeared on the earth.
And it is important to remember that only those biological changes that prove to have brought advantages can be passed down to next generations. If there were three-legged animals, there would be some good reasons for that to happen. However, three-legged animals would not be able to run as fast as four-legged animals. And they would also not be as efficient as two-legged animals. Three legs would simply make climbing, walking, running, and sleeping more difficult.
"The non-existence of three-legged animals can also be seen as the result of natural selection, " Jackson explained to the curious audience.
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