As early as eighteen thirty-seven, he imagined this process as a tree with branches representing new species.

VOA: special.2009.02.25

I'd like to go by climbing a birch tree, And climb black branches up a snow-white trunk toward heaven, till the tree could bear no more, But dipped its top and set me down again.

我真想去爬白桦树，沿着雪白的树干，爬上乌黑的树枝，爬向那天心，直到树身再支撑不住，树梢碰着地，把我放下来。

耶鲁公开课 - 现代诗歌课程节选