The tree is among our most versatile and powerful visual metaphors. It provides a familiar, organic model for visualizing relationships, whether between individuals in a genealogy, species in a tree of life, branches of knowledge in an arbor scientiae, or options in a decision tree. Three recent books consider the tree metaphor as it has been used in different fields and across different eras:
1. Manuel Lima's (2012) Book of Trees
2. Theodore Pietsch's (2012) Trees of Life
3. David Archibald's (2014) Aristotle's ladder, Darwin's tree
Flipping through these handsomely (and abundantly) illustrated books, what I found most striking was how tree diagrams appear to have evolved over time. There seem to be two general trends, both noted by Lima:
Loss of detail. At first the trees are leafy, irregular, clothed in bark—in other words, they look like trees. But, over time, the trees become more abstract: leaves and bark disappear, the trunk is no thicker at the bottom than at the top, roots are nowhere in evidence. The trees have become schematic shells of their former selves.
Disorientation. Trees in nature grow upward, and tree diagrams seemed to start out this way, too. But, over time, the idea that they should be oriented upward, or even oriented in any single direction, seems to have been relaxed. Tree diagrams today grow every which way—genealogies now often unfold from left to right (despite our language of "descendants"), and the recently updated biological "tree of life" has no one discernible direction of growth.
Edward Hitchcock's paleontological chart from 'Elementary Geology' (1840).
Both trends are consistent with the idea that the tree started out as a faithful visual metaphor, but slowly, over centuries of use, became a fully abstract visualization tool. And this evolution makes sense. The communicative and cognitive power of the tree has nothing to do with its bark, leaves, or roots—these are, if anything, distracting. Its real power lies in how it offers a model for thinking about relationships.
Notes
1. There is also a rich tradition in religion and mythology of using the tree as a symbol, which Lima sketches in the introduction to his book. In such cases, the tree as a whole stands in for some other idea—life itself, fecundity, nature, or the like—and its distinctive branching structure is beside the point. In its use as a visual metaphor, the branching structure is essential.
2. For Lima's discussion of these trends, see pages 40-41 of his book. Interestingly, he also notes that the phenomenon of "multidirectional trees" may actually have ancient roots.
3. Brian Bowdle and Dedre Gentner have shed light on the "career" of verbal metaphors. Visual metaphors no doubt have careers, too—it would be interesting to look more systematically at whether the two kinds of metaphors follow similar evolutionary trajectories.