Species that died without leaving descendants Darwin called 'utterly extinct'. They far outnumber the ancestors of all living things, so they resemble the dark matter of modern cosmology, which far outweighs visible matter. He realized in 1837 that their absence is what creates the groups in a natural classification. In his Notebook B he combined the idea that species multiply with the idea that ancestors' relatives must mostly be extinct. The fossil Megatherium was utterly extinct. The iconic branching 'I think' diagram shows extinction causing the origin of genera by eliminating intermediate species. Darwin's concept of taxonomic ranks, starting with the genus, was informed by his interaction with taxonomists. Based on his familiarity with demography, Darwin reasoned that the survival of transitional forms was unlikely, which helped him decide to focus at the species level. When drafting his theory in the 1840s, he left out these speculative ideas, but they emerged again in the 1850s when he realized his theory needed a cause for branches to diverge. His ecological answer worked at the species level, but his Principle of Divergence was unconvincing at higher taxonomic levels. In the Origin, Darwin repeatedly insisted on the importance of utter extinction..
