Darwin’s Rheas and the Birth of Evolutionary Theory

Everyone knows about Darwin’s Finches, of the Galapagos Islands. But of course, Darwin made observations of birds throughout his travels on The Beagle. And, one of the most interesting sets of observations has to do with the Rhea.

Struthio Rhea

I will now give an account of … the Struthio Rhea, or South American ostrich. This bird is well known to abound over the plains of Northern Patagonia, and the united provinces of La Plata. It has not crossed the Cordillera; but I have seen it within the first range of mountains on the Uspallata plain…. The ordinary habits of the ostrich are familiar to every one. They feed on vegetable matter; such as roots and grass; but at Bahia Blanca, I have repeatedly seen three or four come down at low water to the extensive mud-banks which are then dry, for the sake, as the Gauchos say, of catching small fish. Although the ostrich in its habits is so shy, wary, and solitary, and although so fleet in its pace, it falls a prey, without much difficulty, to the Indian or Gaucho armed with the bolas. When several horsemen appear in a semicircle, it becomes confounded, and does not know which way to escape. They generally prefer running against the wind; yet at the first start they expand their wings, and like a vessel make all sail. On one fine hot day I saw several ostriches enter a bed of tall rushes, where they squatted concealed, till quite closely approached. It is not generally known that ostriches readily take to the water. Mr. King informs me that at the Bay of San Blas, and at Port Valdes in Patagonia, he saw these birds swimming several times from island to island. …When swimming, very little of their bodies appear above water, and their necks are extended a little forward: their progress is slow. On two occasions, I saw some ostriches swimming across the Santa

The following passage is thought by some Darwin scholars to reflect one of Darwin’s most significant “aha” moments, leading to his understanding of evolutionary processes. The bird described here is known as the Avestruz Petise, and was named by the ornithologist Gould as Rhea darwinii. Unfortunately, since the bird was earlier named (based on reports, not specimens) Pterocnemia pennata (the Lesser rhea), Darwin’s name does not survive today in the annals of taxonomy.

Read the passage then I’ll note its presumed significance.

…I repeatedly heard the Gauchos talking of a very rare bird which they called Avestruz Petise. They described it as being less than the common ostrich (which is there abundant), but with a very close general resemblance. … The few inhabitants who had seen both kinds, affirmed they could distinguish them apart from a long distance. … This species occurs most rarely on the plains bordering the Rio Negro; but about a degree and a half further south they are tolerably abundant. …They are said to prefer the plains near the sea. When at Port Desire, in Patagonia (lat. 48°), Mr. Martens shot an ostrich; and I looked at it, forgetting at the moment, in the most unaccountable manner, the whole subject of the Petises, and thought it was a two-third grown one of the common sort. The bird was cooked and eaten before my memory returned. Fortunately the head, neck, legs, wings, many of the larger feathers, and a large part of the skin, had been preserved. From these a very nearly perfect specimen has been put together, and is now exhibited in the museum of the Zoological Society. Mr. Gould, who in describing this new species did me the honour of calling it after my name, states, that besides the smaller size and different colour of the plumage, the beak is of considerably less proportional dimensions than in the common Rhea …

Eventually, Darwin made note of the fact that over time, distinct but similar species seemed to differ by grades (such as in size) in the fossil record, as and the same pattern could be seen across geographical space. In his notebooks, he was to eventually note that the variation across space and time seemed to be two ways of looking at the same pattern of change. He made the link between biographical variation in the Rhea and the finches on the Galapagos and similar variation seen in the fossil fauna such as discussed here, along the South American Atlantic coast.

In a note book dated to “1836 and after” (late in the voyage), Darwin wrote a passage that has been the focus of a great deal of attention. In it, he demonstrates his waffling about the nature of species. He frames his introspection in terms of “creation” and at the same time struggles with the evidence from biogeography, which suggests that closely related species would have a common ancestor. He also addresses extinction. Darwin is essentially asking “… where are the transitional forms?”

Remember, this is his notebook writing … it is very much stream of consciousness, conflicting, and hard to understand. I’ll provide you with the entire relevant passage unedited except for the removal of some geologizing. It is painful and wonderful at the same time:

Speculate on neutral ground for 2 Ostriches: bigger one encroaches on smaller;–change not progressive; produced at one blow, if one species altered. …

Should urge that extinct Llama owed its death not to change of circumstances; reversed argument, knowing it to be a desert. Tempted to believe animals created for definite time:–not extinguished by change of circumstances.

The same kind of relation that common ostrich bears to Petisse–[S. Darwinii] and difft. kinds of extinct Guanaco to recent. In former case position, in latter time (or changes consequent on lapse), being the relation, as in first cases distinct species inosculate [To pass into; to join or unite so as to become continuous; to blend] so must we believe ancient ones [did] not gradual change or degeneration from circumstances, if one species does change into another it must be per saltum–or species may perish. This representation of species important, each its own limit and represented. Chiloe creeper; Fournarius, Callandria. Inosculation alone shows not gradation.

an animal in two (gemmiparous by nature or by accident) we see an individual divided either at one moment or through lapse of ages. Therefore we are not so much surprised at seeing Zoophite producing distinct animals, still partly united, & egg which becomes quite separate. Considering all individuals of all species as each one individual divided by different methods, associated life only adds one other method where the division is not perfect.

Dogs, Cats, Horses, Cattle, Goat, Asses, have all run wild and bred, no doubt with perfect success. Showing how creation does not bear upon solely adaptation of animals. Extinction in same manner may not depend. There is no more wonder in extinction of species than of individual.

When we see Avestruz [the Petisse or smaller Ostrich, Struthio Darwinii] two species certainly different, not insensible change; yet one is urged to look to common parent? Why should two of the most closely allied species occur in same country? In botany instances diametrically opposite have been instanced–

Of this passage, Nora Barlow (in her publication on these notebooks) wrote, in 1945:

We can see the mill at work, grinding out hypotheses. … In biological fields the throes of question and doubt, of comparison of masses of facts, of discardings and reviewing, were still to continue for 23 years before the Theory of Evolution as we know it, found expression.

Well said, Nora.

Barlow, Nora ed. 1945. Charles Darwin and the voyage of the Beagle. London: Pilot Press.

Darwin, C. R. ed. 1841. Birds Part 3 of The zoology of the voyage of H.M.S. Beagle. by John Gould. Edited and superintended by Charles Darwin. London: Smith Elder and Co.

h6 style=”text-align: right;”>A Rhea from Darwin’s Voyage monograph on birds