The puzzling distribution of plants and animals in South America and the Galápagos would later make Darwin question how species originated. If each plant and animal was created to match its habitat, why didn't the same species appear in similar environments? Why create the ostrich in Africa and a different running bird, the rhea, in America?
And why, within each continent, were there so many variations? One rhea might have been sufficient in South America--yet there were two distinct species, living in adjacent regions.
As Darwin would later come to realize, if the two species had arisen from a single common ancestor, it would make sense for them to live close together. When Darwin returned to England and began to examine the origin of species in earnest, the distribution of species would provide some of his more persuasive evidence. The pattern of geographic separation he observed was exactly what one would predict if new species evolved from existing ones.
Two of a Kind
While exploring the hot, dry "devil's country" near Bahia Blanca, on the coast of Patagonia, Darwin roamed for days on horseback with local cowboys, or gauchos. With spurs clanking and daggers at their waists, the gauchos smoked, drank, and ate whatever they could shoot. A popular meal was the greater rhea, a large, flightless bird Darwin compared to an ostrich. But the gauchos also told Darwin of a second, smaller rhea, which turned out to be not just a smaller variety, but a distinct species, with different coloring, shorter, more feathery legs and blue-tinted eggs.
Darwin was eager to find this lesser rhea, particularly since a rival French naturalist was searching for it as well. But only after the Beagle sailed several hundred miles down the coast did he finally find one. He later learned that the lesser rhea lived primarily south of the Rio Negro, and the greater rhea to the north. Why, Darwin wondered, did two such similar species live in neighboring regions? Why was there more than one rhea? In the months and years to come, Darwin eventually would ask himself: Could the two rheas perhaps have originated from a common ancestor?
Harder Than It Looks
Darwin was impressed by how the gauchos hunted rhea by throwing bolas--two balls connected by a cord:
"The Gauchos pursued at a reckless pace, twisting their horses about with the most admirable command, and each man whirling the balls round his head. At length the foremost threw them, revolving through the air: in an instant the ostrich rolled over and over, its legs fairly lashed together by the thong."
When Darwin tried to do the same, however, he ended up entangling the legs of his own horse.
"The Gauchos roared with laughter; they cried they had seen every sort of animal caught, but had never before seen a man caught by himself."
A Christmas Surprise
After leaving Bahia Blanca, where Darwin hunted greater rheas with the gauchos, the Beagle sailed south for 17 days. The next stop was Port Desire, about 700 miles down the coast. A few days after Christmas, Conrad Martens, the ship's artist, shot a rhea for dinner. It was rather small, but Darwin assumed it was just a young greater rhea. Halfway through the feast, Darwin realized his mistake: The elusive lesser rhea he had been searching for was sitting half-eaten on his plate.
"It 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."