 The Principle of Extinction further explains the gaps we find in biodiversity. As groups become more distinctive and better adapted to their environment, their parental and sister groups are pushed to extinction. This extinction of “intermediates,” as Darwin calls them, causes the observed gaps among taxa (1859 [1964],121ff.). Extinction, in other words, prunes branches on tree of life so that it has the shape we observe. Together, the Principles of Character Divergence and Extinction explain the origin of varieties and species, and the observed patterns of biodiversity in the world. The relevant point for our discussion of Darwin is that there is no special speciation mechanism that marks the difference between species and varieties. As Kohn (2008) notes, Darwin did not use the word ‘speciation’ in the Origin. For Darwin, the origin of varieties and species is due to divergent selection. As Darwin writes: “The origin of the existence of groups subordinate to groups, is the same with varieties as with species, namely, closeness of descent with various degrees of modification” (1859 [1964], 423).