“Graphs, Maps, Trees: Abstract Models for a Literary History, Ch. 3: Trees”, 2005 (; backlinks; similar):
After the quantitative diagrams of the first chapter, and the spatial ones of the second, evolutionary trees constitute morphological diagrams, where history is systematically correlated with form. And indeed, in contrast to literary studies—where theories of form are usually blind to history, and historical work blind to form—for evolutionary thought morphology and history are truly the two dimensions of the same tree: where the vertical axis charts, from the bottom up, the regular passage of time (every interval, writes Darwin, ‘one thousand generations’), while the horizontal one follows the formal diversification (‘the little fans of diverging dotted lines’) that will eventually lead to ‘well-marked varieties’, or to entirely new species.
The horizontal axis follows formal diversification… But Darwin’s words are stronger: he speaks of ‘this rather perplexing subject’—elsewhere, ‘perplexing & unintelligible’4—whereby forms don’t just ‘change’, but change by always diverging from each other (remember, we are in §‘Divergence of Character’).5 Whether as a result of historical accidents, then, or under the action of a specific ‘principle’,6 the reality of divergence pervades the history of life, defining its morphospace—its space-of-forms: an important concept, in the pages that follow—as an intrinsically expanding one.
From a single common origin, to an immense variety of solutions: it is this incessant growing-apart of life forms that the branches of a morphological tree capture with such intuitive force. ’A tree can be viewed as a simplified description of a matrix of distances’, write Cavalli-Sforza, Menozzi and Piazza in the methodological prelude to their History and Geography of Human Genes; and Figure 29, with its mirror-like alignment of genetic groups and linguistic families drifting away from each other (in a ‘correspondence [that] is remarkably high but not perfect’, as they note with aristocratic aplomb),7 makes clear what they mean: a tree is a way of sketching how far a certain language has moved from another one, or from their common point of origin.
And if language evolves by diverging, why not literature too?