Evolution and Wheels
In the past, evolutionary biologists contemplating the absence of wheels in nature agreed that the explanation was not undesirability: wheels would be good for animals, just as they are for us. Animals were prevented from evolving wheels, the biologists reasoned, by the following dilemma: living cells in an animals body are connected to the heart by blood vessels, and to the brain by nerves. Because a rotating joint is essential to a wheel, a wheel made of living cells would twist its artery vein and nerve connections at the first revolution, making living impracticable. However, there is a flaw in the argument that the evolution of wheeled animals was thwarted by the insoluble joint problem. The theory fails to explain why animals have not evolved wheels of dead tissue with no need for arteries and nerves. Countless animals, including us, bear external structures without blood supply or nerves - for example, our hair and fingernails, or the scales, claws, and ho rns of other animals. Why have rats not evolved bony wheels, similar to roller skates?Paws might be more useful than wheels in some situations, but cats claws are retractable: why not retractable wheels?We thus arrive at the serious biological paradox flippantly termed the RRR dilemma: natures failure to produce rats with retractable roller skates.
冀教版英语八上《Unit 6 I Love Learning English》word同步测试(二)
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冀教版英语八上《Unit 1 Me and My Class》(Lesson 4 On a School Day)word同步测试
冀教版英语八上《Unit 4 My Neighbourhood》(Lesson 31 I Need a Map)word同步测试
冀教版英语八上《Unit 4 My Neighbourhood》(Lesson 28 Turn Right, Turn Left)word同步测试
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牛津深圳版英语八上8.3《Detectives and crimes》word同步测试5套
冀教版英语八上《Unit 4 My Neighbourhood》(Lesson 27 My map,Your Map,Our maps)word同步测试
冀教版英语八上《Unit 8 Celebrating Me》(Lesson 61 I Am a Good Person)word同步测试
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冀教版英语八上《Unit 6 I Love Learning English》(Lesson 43 Write a Letter in English)word同步测试
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冀教版英语八上《Unit 4 My Neighbourhood》word同步测试
冀教版英语八上《Unit 5 My Future》word同步测试
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冀教版英语八上《Unit 6 I Love Learning English》(Lesson 41)word同步测试
冀教版英语八上《Unit 6 I Love Learning English》(Lesson 47 Some News About English)word同步测试
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冀教版英语八上《Unit 7 Enjoy Your Hobby》(Lesson 53 What’s Paul’s Hobby)word同步测试
冀教版英语八上《Unit 4 My Neighbourhood》word同步测试(一)
冀教版英语八上《Unit 4 My Neighbourhood》(Lesson 29 My Neighbourhood)word同步测试
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