In the text below some words are missing. Drag words from the box below to the appropriate place in the text. To undo an answer choice, drag the word back to the box below the text.
下面的文本缺了一些词。请把词框里的词拖到文中合适的位置；要撤销选择，把词拖回文本下方的词框即可。
In The Origin of Species, Darwin provided abundant evidence that life on Earth has evolved over time, and he proposed natural selection as the primary mechanism for that change. He observed that individuals in their inherited traits and that selection acts on such differences, leading to change. Although Darwin realised that variation in heritable traits is a prerequisite for , he did not know precisely how organisms pass heritable traits to their offspring. Just a few years after Darwin published The Origin of Species, Gregor Mendel wrote a groundbreaking paper on inheritance in pea plants. In that paper, Mendel proposed a model of inheritance in which organisms transmit discrete heritable units (now called genes) to their offspring. Although Darwin did not know about genes, Mendel's paper set the stage understanding the genetic differences on which evolution is based.
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