Charles Darwin · 1809
Heavy going at CET-4 level. Not unreadable — just not readable the look-every-word-up way.
Coverage = the share of words in the text you already know. The research gives two lines: 98% before independent fluent reading is realistic, 95% for the instructional level that needs support (Hu & Nation, 2000). Proper nouns are not counted as unknown — recognising that it is a name is enough; this book has 469 of them.
Advanced — read it with glosses. 154,905 words in total; at 150 words a minute for a non-native reader, about 17 h 13 min to finish. Average sentence length 29.2 words — on the long side, so the load comes from sentence structure as much as from vocabulary. Reaching 95% coverage on this book (the floor for readability) takes roughly 19,250 word families.
The opening of On the Origin of Species By Means of Natural Selection, word for word. Tap any word you don't know and its meaning here appears beside it — no sign-up, tap as often as you like.
When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years’ work I allowed myself to speculate on the subject, and drew up some short notes; these I enlarged in 1844 into a sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily pursued the same object. I hope that I may be excused for entering on these personal details, as I give them to show that I have not been hasty in coming to a decision.
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Computed by Readii from the public-domain text (updated 2026-10-21), not estimated by hand. The Readii reading level is our own scale, anchored on unknown-word density; it is not the same system as Lexile or AR and does not convert directly. Unknown words are measured against the CET-4 ∪ Gaokao lists, so this is difficulty for a Chinese-speaking learner, not a native-child reading level.