Last updated on 22 Jun 2018
Very famously, Darwin came up with the idea of the evolutionary tree. What is not often realized is that it is the tree that is more uniquely Darwin’s than natural selection, for which he is better known. This radical idea has been called “tree thinking”, but even this has not been fully understood. In the pre-Darwinian view of human nature, the sequence of organisms, either along a scale of organisation, or a temporal sequence in which simple organisms shade into more complex ones, was the usual view, called by historian E. A. Lovejoy the Great Chain of Being.
This was a commonly held view in the Arabian scientific period that preceded the European renaissance, and which found expression in the work of Raimond Lull (or Lullius), a Spanish humanist of the 16th century. In the figure above, Lull shows the steps of ontological grades or ranks from stones (Lapis) through flames, plants, beasts, humans, and then heaven, angels and finally God himself.
In the Enlightenment, Lull’s ideas triggered a number of philosophers and naturalists to try to rank living things from simple and nonmoral to complex and moral. The moral aspect of the chain is evident in this 16th century woodcut:
The left hand side follows the standard Arisotelian “souls” (psuche), that exist as material forms of life – the vegetable soul which takes nourishment and grows, the sensitive soul which responds to the world (sometimes called “irritability”), and the rational soul, which only humans (Homo) has.
The right hand side shows the moral equivalents of these souls: the virtuous (rational), the sensory (luxuria), the gluttonous (gula) and the simply existing (acedia) who is almost literally “stoned” (mineralis). The states of being here are existent (Est), alive (Vivit), sentient (Sentit) and thinking (Intelligit). Well before Kant or even the Enlightenment, to be moral was to be rational.
In the 17th century, Lord Monboddo regarded the newly-discovered (by Europeans) orangutan as the final step beneath humans in that scale of nature (it was called the scala naturae, but the Latin actually means “ladder”), and the final flowering of Chainism was in the work of the entomologist Charles Bonnet, who listed the grades of organisation extensively but implausibly, in the late 18th century.
Bonnet in 1745 tried hard to put all living things in a chain, and published a now-famous figure. This is a translated version from my book Species: A History of the Idea:
It clearly has a lot of arbitrariness in it, and Bonnet’s scale was soon taken to task by Peter Simon Pallas, who in his work on “zoophytes” (plant-animals, 1766), came up for the first time with the idea that rather than a single linear arrangement of kinds of living things, a better metaphor was that of a tree (with two trunks – one for plants and one for animals, joined at his zoophytes). On this view, the arrangement by Linneaus earlier that century of species and groups of species into a nested hierarchy made sense. Every species was closely related to other species, and less closely to even more. The tree was a good way to conceive of these “affinities”, or sets of shared traits, and the differences.
Around the turn of the 19th Century, a French botanist named Angier literally drew such a taxonomic tree. It was, so far as I can tell, the first graphical representation of taxonomic arrangements.
Others used the typographical convention of using braces in tables to indicate the grouping of smaller taxonomic groups under larger ones, which Darwin called “group subordinate to group” in the Origin. This arrangement of taxa in a systematic fashion came to be called systematics, and it was Darwin’s primary problem. With his taking the tree to represent a historical process of the development of biodiversity, a new way to conceive of relationship was presented, and it had implications that we are still in the process of working out.
An early 19th century table of characters, using the braces to indicate logical relationships, or “affinities”.
The use of a hierarchy or tree to represent relations had a rather interesting set of implications: traits were not acquired as things got complex (in a non temporal sort of way), but in terms of being related more closely to other species. This set up the problem that Darwin solved with the idea that a series of shared traits indicates a temporal sequence and common descent.
Next, I will consider how this affects the notion that moral behaviour is derived from reasoning. It may be that the opposite is the case, a view originally proposed by David Hume.