Older histories of biology are often full of useful and interesting facts. One of my all-time favourites is Eric Nordenskiöld’s history, but I came across an earlier one by Louis Compton Miall in which I found this text:
Bonnet in 1745 traced the scale of nature in fuller detail than had been attempted before. He made Hydra a link between plants and animals, the snails and slugs a link between mollusca and serpents, flying fishes a link between ordinary fishes and land vertebrates, the ostrich, bat, and flying fox links between birds and mammals. Man, endowed with reason, occupies the highest rank; then we descend to the half-reasoning elephant, to birds, fishes, and insects (supposed to be guided only by instinct), and so to the shell-fish, which shade through the zoophytes into plants. The plants again descend into figured stones (fossils) and crystals. Then come the metals and demi-metals, which are specialised forms of the elemental earth. Water, air, and fire, with perhaps the aether of Leibnitz, are placed at the bottom of the scale.
In Bonnet’s hands the scale of nature became an absurdity, by being traced so far and in so much detail. It was not long before a reaction set in. The great German naturalist, Pallas, in his Elenchus Zoophytorum (1766) showed that no linear scale can represent the mutual relations of organised beings; the branching tree, he said, is the appropriate metaphor. Cuvier taught that the animal kingdom consists of four great divisions which are not derived one from another, and his authority overpowered that of Lamarck, who still maintained that all animals form a single graduated scale. A complete reversal of opinion ensued, so complete that at length the theologians, who had once seen in the scale of nature a proof of the wisdom of Providence, were found fighting with all their might against the insensible gradations which, according to Darwin’s Origin of Species must have formerly connected what are now perfectly distinct forms of life. [p46]
Naturally, I was intrigued. According to the received history, tree diagrams were either entirely new with Darwin or go back to Aristotle, depending on how you define things. So I did a little digging, with some help from my friends.
Thanks to the largesse of the Internet Archive, Pallas’ book was easily accessible. But it’s in Latin, and I just learned enough Latin to fail it at university. So I tried to look for sections that might be what Miall is talking about, and came across the phrase arbore quasi vitali on page 8. Success! “Tree of life” – couldn’t be clearer. Only, there were a few hints that I was wrong; as the section (page 8f) is talking about the circulatory system. My net friend Reed Cartwright – another of those overeducated scientists who put us humanists to shame – reads Latin and he informed me of my error. So I was starting to give up, thinking that yet another myth had found its way into the history of biology, when I had an idea. I tasked (well, begged) Reed to look through it, and yes, he found the passage on pages 23 and following, just as Miall had said.
There’s something to be said for the exhaustive and careful sort of scholarship of the late nineteenth and early twentieth century; generally if a good scholar like Nordenskiöld or Miall says X, then you had best believe that X is right until you have further evidence to the contrary. So here are a couple of passages that Reed has roughly translated, with my editorialising interventions. Pallas is discussing the great chain of being idea that species can be arranged in a single series from minerals to man. Dismissing it he says:
But on the contrary, let the System of all organic Bodies be well represented by a the likeness of a Tree, which at once from the base, out of the simplest plants and animals, twofold, it brings forth diversely a contiguous trunk, Animal and Vegetable.
With this figure, let organic Bodies be also shown to be neither successive nor adjacent with beasts but to stand like a great tree on the ground. According to (this) principle, the trunk, full of series of adjacent genera, puts out little branches, having been brought together by those lateral affinities not able to be set between.
It could hardly be more direct. There is no diagram, but it is clearly a taxonomic tree that he is talking about.
This arises in the context of a debate that spans two centuries, from around 1840 to now, on what counts as a “natural” arrangement of species. While Linnaeus had arranged his as “group within group”, his presentation was simply as a hierarchical scheme useful for diagnosis. He himself had said that it was a conventional scheme, although he held that genera and species were natural:
Naturae opus semper est Species et Genus; Culturae sapius Varietas; Naturae et Artis Class et Ordo.
The work of nature is always Species and Genus. Cultivation understands varieties; of nature and skill [are] Class and Order.
So what Pallas is asserting is that the best analogy for a natural arrangement of genera (i.e., natural groupings) is a tree, not a ladder. This sets up the problem for which Darwin’s notion of descent with modification is a solution.