That sign languages may be full-blown languages but they just take--they're just harder to learn because the brain and the body have adapted for speech.

手语或许是成熟的语言,但它们更难学,因为大脑和身体已经适应了口语

耶鲁公开课 - 心理学导论课程节选

The body cannot adapt to a diet that has certain properties.

身体不能适应这样的饮食

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选