Chapter 13: Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs

From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups.

Isn't that a good sentence? It's the first of this chapter. There's music in the way the Biblical ring of "From the first dawn of life", falls towards the swallowed repetition of "groups under groups", which itself mirrors and explains the descending degrees of resemblance that gives the sentence its scientific filling.

The next line is just as good: "This classification is evidently not arbitrary like the grouping of the stars in constellations." The Origin doesn't always slip down easily, but Darwin's strength as a writer, reflected most clearly in his choice of analogies, is that he uses words to serve the idea, not so he can get high on his own cleverness.

He's following the advice that George Orwell gave in his 1946 essay 'Politics and the English Language' (which is also the best how-to guide to science writing that I know of).

A scrupulous writer, in every sentence that he writes, will ask himself at least four questions, thus: 1. What am I trying to say? 2. What words will express it? 3. What image or idiom will make it clearer? 4. Is this image fresh enough to have an effect?


Chapter 13 labours under the title of 'Mutual affinities of organic beings: Morphology: Embryology: Rudimentary organs'. In it, Darwin tackles the science of classification -- perhaps more than in any other part of the book, I sensed that he was addressing his fellow pros (or gentlemen amateurs).

Naturalists, in attempting to assign living things to species and higher groups, have unwittingly been collecting evidence for evolution, says Darwin. Evolution, in turn, can help them understand what they have produced and guide their future efforts.

Stephen Shapin described the issue in a recent essay in the London Review of Books, (subscription required):

By the late 18th century...many botanists wanted to find a 'natural' classification - one that flowed from plants' overall morphology. The system of expert classification then really would reflect God's creative order; it would be objective and 'philosophical', not a mere pragmatic sorting device. In the first part of the 19th century, there were botanists who wished to stick with the Linnaean system and those who recommended a natural system. But there were many candidates for classificatory 'naturalness'[.]

Classification is a formalism describing patterns in biological systems, in the same way that Newtonian mechanics or the gas laws are (more precise) formalisms describing patterns in physical systems. None of these laws captures the mechanisms underlying the patterns they describe, but that doesn't make them less useful or powerful.

In this chapter Darwin nominates his theory as a candidate for classificatory naturalness, shows how it explains the patterns in classification, and mounts a takeover bid for biology.

"[T]he natural system is founded on descent with modification ... the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike."


Far from settling things, however, this laid the ground for a century of schism in taxonomy, between those who believe that classification should reflect evolutionary descent, and those who look at traits in isolation from history, and create groups by the putting together and separating of objects more or less alike.

There weren't been many things in post-Darwinian biology powerful enough to cause Richard Dawkins and Stephen Jay Gould to stop taking pot-shots at one another and stand back in awe, but the ferocity of this debate was one of them. To quote Dawkins' Blind Watchmaker, taxonomy is "one of the most rancorously ill-tempered of biological fields. Stephen Gould has well characterized it with the phrase 'names and nastiness'".

Why the fuss? Partly it's that classification is fundamental to biology. That's true in a historical sense, in that, crudely put, today's biology began with Linnaeus. And it's true in an intellectual sense. Whatever you do in biology, you need to know what species you're doing it to.

There's also something very emotive about naming. To give something a name brings it into being, puts your mark on it, and lays a claim to posterity, even immortality. That's the sort of thing that people care about. As proof, let us turn to Genesis 2:19:

"And out of the ground the LORD God formed every beast of the field, and every fowl of the air; and brought them unto Adam to see what he would call them: and whatsoever Adam called every living creature, that was the name thereof."

God, the Old Testament suggests, did not lack assertiveness. But He knew better than to get into a fight with humanity over taxonomy.


After laying out his case, Darwin spends this chapter looking at differing kinds of traits and asking: What does my theory say about why these are useful in classification?

He devotes the longest section to embryology, perhaps because "it has been strongly urged by those great naturalists, Milne Edwards and Agassiz, that embryonic characters are the most important of any in the classification of animals; and this doctrine has very generally been admitted as true".


Three words hover over this section: ontogeny recapitulates phylogeny. The influential idea that a species' embryonic development replays its evolutionary history is most closely associated with Ernst Haeckel, who proposed it in 1866 (one of his drawings is above). Darwin, however, attributes something very similar to Louis Agassiz:

"As the embryonic state of each species and group of species partially shows us the structure of their less modified ancient progenitors, we can clearly see why ancient and extinct forms of life should resemble the embryos of their descendants, our existing species. Agassiz believes this to be a law of nature; but I am bound to confess that I only hope to see the law hereafter proved true."

Instead, Darwin concentrates on the resemblances between embryos and juveniles of contemporary species (rather than the resemblance of contemporary embryos to ancient species). Organisms, he says, look more alike in youth than adulthood; he presents some measurements of domestic animals as evidence.

This, Darwin suggests, is because the adaptations that fit each species to its place in nature accumulate throughout development. Adaptation and genealogy are opposing forces. Natural selection erodes the signal of descent by building specialized organs, and embryology is especially useful in classification because less of this erosion has taken place: "Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals."


After this comes a section on "Rudimentary, atrophied, or aborted organs", such as the 'bastard-wing' in birds, the stunted limbs of some snakes, and "the presence of teeth in foetal whales, which when grown up have not a tooth in their heads".

Such organs, Darwin realizes, are some of the best evidence for his theory, and cannot be explained in any other way. Just as descent with modification builds structures up, little by little, so it can dismantle them. Indeed, if you were to imagine a clincher for evolution, it's the sort of thing you might come up with.

Writing about an earlier chapter, I mentioned that Darwin seemed to be following the conventions of narrative structure -- set up, confrontation, resolution. That thought is reinforced by this final, triumphant flourish. I've saved, says Darwin, the decisive blow for last -- I could ditch the preceding 12 chapters, and this alone would do the job.

Finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments.

On Wednesday, the last chapter: Recapitulation and Conclusion.


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This is a terrific idea for a blog and brilliantly executed.

When I look at the Haeckel sketches you show above, my mind makes the mental link with the "Worst Journey in the World" made by Cherry-Garrard, Wilson and Bowers in 1911 from McMurdo Sound to Cape Crozier in order to collect Emperor penguin eggs. I understand that they hoped to test ideas about some kind of link between birds and reptiles. But it is said that the idea was already largely discredited by the time Cherry returned back home with three eggs, as he vividly describes in his trip to the Natural History Museum.

Could you comment on this famous incident, and where we stand now when we look at those sketches by Haeckel ? I am not really familiar with this kind of detailed evidence for evolution.

Great blog John and super post on chapter 13. Sam's comment above is one I've thought about too. For those that don't know the story she's referring to, you can't beat reading Cherry-Garrard's personal account - The Worst Journey in the World. Here's a kind of summary...

Three of the scientists on Robert Falcon Scottâs ill-fated Terra Nova expedition to the South Pole (one of them Cherry-Garrard) set out across the icy wastes of the Antarctic winter on a mission to collect Emperor penguin eggs. This was the only time of year they'd get them. It took them nearly three weeks to drag their equipment and provisions some 60 miles to Cape Crozier, then the only known Emperor penguin colony where the temperature was âanywhere below seventy degrees of frost, and the blizzards blowing, always blowingâ.

The men were driven by the then-widespread belief that the embryonic development of an organism reveals its evolutionary secrets. Studying the embryo of the Emperor penguin, then considered to be the most primitive bird on earth, might expose the evolutionary origins of our feathered friends. Three eggs but only one of the men â Aspley Cherry-Garrard â made it back to Britain. The other two perished along with Scott on their abortive assault on the South Pole.

When Cherry-Garrard went, alone, to deposit the three eggs at the Natural History Museum in London, he received little recognition for his Antarctic achievement. âThis ain't an egg-shop,â came the gruff response of the clerk, who reluctantly took the three precious specimens. They were not formally described until 1932, by which time it was increasingly clear that the similarities between embryology and evolutionary history were not clear-cut.

I'm not sure exactly when the ontology-recapitulating-phylogeny idea slipped out of intellectual favour, but it's almost certainly some time between the expedition in 1912 and the description of the eggs 20 years later.

It is, as Sam says, a marvellous slice of history. If it weren't for Haeckel (or Agassiz), we would have been denied this extraordinary tale of human endeavour.

That's disappointing. It was proven, and admitted by Ernest Haeckel, that at least seven out of the nine embryo drawings are false. I'd begin to wonder at the first sound of the word "drawings." Of course evolutionist can only seem true in the poor artists interpretations of these embryos. If you look at the pictures of those same embryos you will be amazed to realize the truth of the matter. The Haeckel Fraud is SO out of date. Come on, its too east for the creationists to prove wrong. Try to give us a challenge...

By Jake Doberenz (not verified) on 02 Jul 2011 #permalink

My particular favorite sentence from this chapter is "All classification is genealogical." This is a landmine, nicely camouflaged among the amassed facts (assembled by Darwin's contemporary and preceding biologists and taxonomists) of classification that were beyond dispute. Concealed within this explosive little sentence are such implications as "And thus we are related to chimps, with whom we share an ancestor;" or "And thus all living species trace back to a common ancestral species;" or "And thus among living species there is no 'higher' or 'lower,' since all are cohorts of the same genealogical generation..."

By Melinda M (not verified) on 07 Jan 2012 #permalink