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From both the origin of species, and the early notebooks of Charles Darwin, it can be seen that the metaphor of the branching tree of evolution was integral not only in assisting in the explanation of the theory of transmutation of species', but also in the organisation of Darwin's ideas long before the publication of the origin. When Darwin suggested that “organised beings represent a tree, irregularly branched, some branches far more branched” in the B notebook in 1837, where we first hear of his ideas concerning the branching tree, the concept of systematic tree diagrams was relatively new. One might argue that it was in the rapidly maturing field of linguistics, or philology, that it was developed and used first.
August Schleicher, a major contributor to the field, was the first to emphasise the metaphor of family trees of languages, and to include actual tree diagrams in his work. Although linguistic thinking had been traditionally centered in Germany, this new, comparative philology postulated by Schleicher began to find its way into England by the early 1830s, and became strong point of focus for many British intellectuals. Probably the most significant of these was Hensleigh Wedgwood, Darwin's cousin and brother-in-law It's important to realise that the branching tree concept, as applied to the theory of transmutation of species, was not the result of a sudden flash of inspiration for Charles Darwin. Rather, it was gradually conceived after much consideration and discussion with his peers and, in particular, with Hensleigh Wedgwood. Shortly after returning from the Beagle voyage in 1836, Darwin maintained very frequent correspondence with his cousin, and this was no doubt one of the major contributing elements concerning his application of the linguistic branching tree model to his own work.
The possibility that this novel approach might be applied to elements of natural history was noticed by many other individuals between the 1830s and the publishing of the "descent of man", including the linguistic thinkers Max Muller and August Schleicher, the botanist Asa Gray, the biologist Thomas Huxley, and the geologists Louis Agassiz and rather significantly, Charles Lyell. In conjunction with John Herschel in 1836, Lyell postulated that "words are to the anthropologist what pebbles are to the geologist - battered relics of past ages often containing within them indelible records capable of intelligent interpretation" This idea had a profound impact on Darwin, and it most likely inspired him to experiment with his own linguistic analogies with which to illustrate his theory of the evolution of species.
The diversification of species from a common ancestor, but under isolated conditions such as those found on Galapagos, was what first led Darwin to view evolution as a diversifying process which branched outwards much like a tree does. Having been highly interested and well informed in the study of the recent developments in comparative linguistics, the structural similarities between trees of branching languages and the divergent branching of species must have made a strong impression on Darwin. It’s important to emphasise, though, that it is extremely unlikely that comparative philology actually inspired Darwin to formulate the idea of descent - rather, he alluded to philology’s branching thesis in order to help him elegantly express a number of different aspects of his theory at once
Also in notebook B, Darwin ponders that “the tree of life should perhaps be called the coral of life”. This metaphor, though slightly less accessible than the branching tree, seemed to Darwin to have significance because it not only illustrated the evolution of new species, but also their extinction. The way in which the living part of the coral forms only the tip, the majority of the organism being dead, would have been very familiar to Darwin from the beagle voyage during which he had spent a great deal of time formulating theories on the creation of coral reefs. The structure of coral must have seemed a fitting analogy for the marriage of lyell’s theories on extinction with Darwin’s new theory on the divergent evolution of species, but in the end as we see in the origin, the branching tree took over, probably because the coral explanation would have lacked, for his 19th century audience, the cultural resonance possessed by the tree. On this note, it might be worth mentioning that the choice of the tree over the more detailed and encompassing coral metaphor for the origin was in line with the high popularity of genealogical family trees in the mid 19th century, and this tallies with Darwin’s wish for the origin to seem as “user friendly” as possible
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