Naudin, Wallace and Darwin — the tree idea

Charles Darwin's most poetic published words concern his image of the Tree of Life. However, he did not claim to have originated the image. For example, Alfred Russel Wallace had already used it. Recently, the Natural History Apostilles blog has mentioned another important predecessor of both Englishmen, the Frenchman Charles Naudin, who deserves wider recognition.

Darwin's well-known words from On the Origin of Species (1859) are:

The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species ... As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.

Wallace seems to have developed the Tree of Life metaphor quite independently (1855. On the law which has regulated the introduction of new species. Annals and Magazine of Natural History, 2nd series 16: 184-196):

"the analogy of a branching tree [is] the best mode of representing the natural arrangement of species ... a complicated branching of the lines of affinity, as intricate as the twigs of a gnarled oak ... we have only fragments of this vast system, the stem and main branches being represented by extinct species of which we have no knowledge, while a vast mass of limbs and boughs and minute twigs and scattered leaves is what we have to place in order, and determine the true position each originally occupied with regard to the others."

Darwin freely admitted having read Wallace's work. Moreover, he was well aware of the other of his predecessors, Charles Naudin, because on p.167 of his 'Books Read' and 'Books to be Read' notebook of 1852-1860 (see Darwin Online CUL-DAR128) he recorded:

"Revue Horticol Imp. 1852. p. 102. Naudin Consid. Phil, sur l'espèce"

Charles Naudin's words are these, roughly translated from the original French (1852. Considérations philosophiques sur l'espèce et la variété. Revue Horticole, 4th series 1: 102-109) [NB. the long convoluted sentences are in the original]:

This doctrine of inbreeding among organic beings of the same family, the same class, and perhaps of the same kingdom, is not new; men of talent, both in France as well as abroad, among them our learned Lamarck, have supported it with all of the authority of their names. We do not deny that, on more than one occasion, they have reasoned upon assumptions which were not adequately supported by observation, that they did sometimes apply to the facts forced interpretations, that finally resulted in exaggerations that have mainly helped to push their ideas. But these defects in details do not diminish the greatness and perfect rationality of the whole system that, alone, reflects, by the community of origin, the great fact of the organizational community of the other living beings of the same kingdom, the primary basis of our rankings of species into genera, families, orders and phyla. In the opposing system now in vogue, in this system which involves many partial and independent creations we recognize or think we recognize as distinct species, one is forced to be logical, to admit the similarities exhibited by these species are only fortuitous coincidence, that is to say an effect without a cause, concluding that the reason is not acceptable. In our own [system], on the contrary, these similarities are both the consequence and proof of a relationship, not metaphorical, but real, that they hold a common ancestor, which they left at times more or less remote and through a series of intermediaries greater or fewer in number; so they express the true relationships between species by saying that the sum of their mutual similarities is the expression of their degree of relationship, as the sum of the differences is that of the distance they are from the common stock from which they derive their origin.

Considered from this point of view, the plant kingdom would present, not as a linear series whose terms would increase or decrease in organizational complexity, according as we consider starting with one end or the other; it would not be more of a disordered tangle of intersecting lines, like a geographical map, whose regions, different in shape and size, would touch by a greater or lesser number of points; it would be a tree the roots of which, mysteriously hidden in the depths of cosmological time, would have given birth to a limited number of successively divided and subdivided stems. These first stems would represent the primordial types of the kingdom; their last ramifications would be the current species.

It follows from there that a perfect and rigorous classification of the other organized beings of the same kingdom, of the same order, of the same family, if something other than the family tree even of the species, indicates the relative age of each, its degree of speciation and the line of ancestors from which it descended. Thereby would be represented, in a manner of some sort so palpable and material, the different degrees of relationship of the species, such as that of groups of varying degrees, dating back to the primordial kinds. Such a classification, summarized in a graphical table, would be seized with much facility by the mind through the eyes, and present the most beautiful application of this principle generally accepted by naturalists: that nature is avaricious [stingy?] of causes and prodigal of effects.

This is quite clearly a description of a modern phylogenetic tree, and the taxonomic consequences of adopting that conception.

It is, however, rather a pity that he explicitly rejects a network ("a disordered tangle of intersecting lines") as a suitable model, along with the chain ("a linear series").