Lamarck would have been delighted with this fact…
Charles Darwin, The Voyage of the Beagle
July 5, 1832 – The Beagle sails from Rio de Janeiro to Maldonado, a small town of little trade (now a town of some 55,000 people in Uruguay). Darwin spends 10 weeks here, exploring the region and collecting specimens. The people here had had very little contact with the outside world, and had never seen a compass, or a map, or even a match. Their world geography was a little confused – some of them thought that England was a large village in London, or that London, England, and North America were different names for the same place. As Darwin’s party ride west across the plains, Darwin tries using a “lazo” (lasso) and bolas (weighted balls tied to a rope, which, when thrown, entangle the target’s legs), and manages to accidentally catch his own horse with the bolas (the horse, unlike Darwin’s pride, was not injured).
The party encounter large numbers of tame partridge-like birds (Nothura major). Nothura are Tinamous, and are closely related to ratites (but can fly). He notes that in very flat countries, there are rarely very many trees, and speculates that this may be caused by wind, or by the properties of drainage. Darwin collects many specimens – “several quadrupeds, eighty kinds of birds, and many reptiles, including nine species of snakes”, as well as several tame deer (Cervus campestris) – which are tame except for a fear of horsemen bearing bolas. He also finds eight species of mice and a capybara.
He finds and keeps several tuco-tucos (Ctenomys brasiliensis) – a mole-like creature. These are, he reports, incapable of jumping due to a missing hind-leg ligament. They are very commonly blind, but are able to find their way around despite this. Darwin writes:
Considering the strictly subterranean habits of the tucotuco, the blindness, though so common, cannot be a very serious evil; yet it appears strange that any animal should possess an organ frequently subject to be injured. Lamarck would have been delighted with this fact, had he known it, when speculating (probably with more truth than usual with him) on the generally acquired blindness of the Asphalax, a Gnawer [rodent] living underground, and of the Proteus, a reptile living in dark caverns filled with water; in both of which the eye is an almost rudimentary state, and is covered by a tendinous membrane and skin. In the common mole the eye is extraordinarily small but perfect, though many anatomists doubt whether it is connect with the true optic nerve; its vision must certainly be imperfect, though probably useful to the animal when it leaves its burrow. In the tucotuco, which I believe never comes to the surface of the ground, the eye is rather larger, but often rendered blind and useless, though without apparently causing any inconvenience to the animal; no doubt Lamarck would have said that the tutotuco is now passing into the state of the Asphalax and the Proteus.
Birds are abundant on the plain. One (Molothrus niger) perches on all manner of things – fences, cows, horses – and tries to sing (badly) from this vantage point. It is suspected of behaving much like a cuckoo, and overly-sized and oddly coloured eggs are sometimes found in the nests of a local sparrow. Darwin tells us that there is another Molothrus in North America (M. pecoris), closely allied with this one. He wonders why two Molothrus would share this habit with cuckoos, and why any of them would behave like that anyway. M. Prévost found that cuckoos lay 4-6 eggs, but in overlapping clutches of 1-2, which may involve different mates. As such, raising all of them would either mean sitting on the eggs too long, or else hatching them in separate batches. Darwin suspects that he is probably right, and wonders if this is also why female rheas share nests with other females (which the males then incubate). The modern genus Molothrus (Cowbirds) are brood parasitic New World brids.
A tyrant flycatcher, now called Pitangus sulphuratus can hover and stoop like a hawk whilst searching for prey, though it is slower and weaker. It can hover over watch and catch fish similar to a kingfisher, as well. It perches on bushes and gives a “shrill but agreeable” call.
Mimus saturninus (a mockingbird – Darwin calls it a M. orpheus) has the best song that Darwin hears in South America, comparable to a sedge warbler. They are tame but bold – often coming into houses and eating the meat. A closely allied species, M. patagonica has a slightly different song, by which Darwin figured that it was probably a different species. However, when he collects one, he figures from its appearance that it is the same species as M. saturninus. However, he sends this to Mr. Gould, who, from the appearance alone (with no knowledge of the song), classifies it as a different species.
Darwin expresses disgust at the habits of the numerous carrion-feeding birds – the caracara, turkey buzzard, gallinazo and condor. He suggests that if you fall asleep on the plain you would likely wake up to find one of them staring at you with an “evil eye”, and I am driven to wonder if he knows this by direct experience. He describes each of these species. Darwin rounds out the chapter with a description of siliceous tubes found in the nearby sand-hillocks, formed by lightning strikes on loose sand.
With this chapter, as with the last, I am struck by the importance of the voyage to Darwin’s thinking. Not just that he finds similar but nonetheless distinct species in different areas, but that his observations of convergent evolution, vestigial organs, tameness and adaption by observing various species were critical, I think, to formulating his theory.
Next time: Rio Negro to Bahia Blanca.