Tuesday, May 31, 2016

Darwin's coral and seaweed

When challenging previous ideas about biological organization, Charles Darwin insisted upon both the origin of new biological forms and the extinction of some of the old forms. He used a multi-stemmed bush as his published metaphoric icon for these processes (in 1859), but we have always referred to it as a tree.

However, as noted in an earlier blog post (Charles Darwin's unpublished tree sketches), Darwin's first tree-like diagram (dated 1837-1838) was actually a drawing of a coral, accompanied by the text:
The tree of life should perhaps be called the coral of life, [with the] base of [the] branches dead; so that [the] passages cannot be seen
Darwin's specimen 1143, labelled Corallina officinalis.

As a geologist, Darwin had studied corals extensively in the Pacific and Indian Oceans, on the Beagle voyage (1831-1836). In May 1837 he read a paper before the Geological Society of London about his ideas for the development of reefs. This was then published in their journal:
Darwin, C.R. (1837) On certain areas of elevation and subsidence in the Pacific and Indian oceans, as deduced from the study of coral formations. Proceedings of the Geological Society of London 2: 552-554.
He subsequently published his book on the development of coral reefs in 1842 (this was first monograph):
The Structure and Distribution of Coral Reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. Smith, Elder and Co., London.
After this first use of a coral image, Darwin also tried a different marine metaphor:
a tree not [a] good simile — endless piece of sea weed dividing
He seems to have done nothing further with this particular idea.

What is most interesting for us is that the coral metaphor is not a strictly divergent model of evolutionary history. After all, there are many types of coral that form anastomoses. Indeed, there are also corals that do not even form a branching pattern. The neat divergent tree metaphor does not match the world of corals.

This point has been made at length by:
Horst Bredekamp (2003) Darwins Korallen: Frühe Evolutionsmodelle und die Tradition der Naturgeschichte. Verlag Klaus Wagenbach.
Horst Bredekamp (2005) Darwins Korallen: Die frühen Evolutionsdiagramme und die Tradition der Naturgeschichte. Verlag Klaus Wagenbach, second edition.
This book deals with the "aesthetic and political dimension of the coral", which the author (a philosopher and art historian) sees as "a model of anarchic evolution" that opposes the hierarchical metaphor of a tree.

Biologically, Darwin should have stuck to his original idea! However, it is undoubted that the Biblical association of the tree image was more likely to capture his readers' imaginations.