LONDON (AP) — Two dead birds, one big idea.

Mockingbirds collected by Charles Darwin on the Galapagos Islands may not be the most visually exciting part of an exhibition that opened Friday at the Natural History Museum, but they stimulated the thinking that led to the theory of evolution. The specimens have never before been on public display.

Darwin found that the mockingbirds he saw in the Galapagos Islands in September and October of 1835 were different from the ones he had seen all over South America. "It struck him immediately that is was a very different bird: it's bigger, it has this dark chest, the bill is quite long," said Jo Cooper, the museum's curator of birds.

Darwin noted greater variations in the birds from different islands in the Galapagos than he had seen on the continent, "and that really made him start thinking," Cooper said.

It set Darwin on a course that challenged the prevailing idea that each species was distinct and unchanged — or stable — since the moment it was created.

"When I see these Islands in sight of each other and possessed of but a scanty stock of animals, tenanted by these birds but slightly different in structure and filling the same place in Nature, I must suspect they are only varieties," he wrote, adding: "If there is the slightest foundation for these remarks, the zoology of archipelagoes will be well worth examining; for such facts would undermine the stability of species."

The museum's exhibition includes live specimens of the green iguana and a horned frog, animals Darwin saw; the first known sketch by Darwin of an evolutionary tree of life; and a recreation of Darwin's study at his home in Kent in southeastern England.

There are also some hairs, believed to be from Darwin's beard, which were kept by his daughter Etty. While serving as a naturalist aboard the Beagle from 1831 to 1836, Darwin collected six mockingbirds in South America and four from the Galapagos, one from each of the largest islands.

The type he found on Floreana island became extinct there later in the 19th century, but a few more than a hundred Floreana mockingbirds now survive on two nearby small islands, Champion and Gardner.

In May, the Floreana mockingbird was listed as critically endangered, the highest threat category of the International Union for the Conservation of Nature. Mockingbirds on two other large islands of the Galapagos are listed as endangered.

The two birds on display, and two others in the museum collection, are doing their bit to save the Floreana mockingbird.

Bits of the footpads snipped from each bird are being analysed to establish a DNA profile to assist in breeding and selecting birds to establish a new population on Floreana. Karen James, a molecular biologist at the museum, said results have been encouraging.

"It's really important when you're examining DNA from old specimens that you don't get contamination from other specimens in the same drawer or laboratory," James said.

"Preliminary results are looking promising. It looks like not only have we got usable DNA from the footpads, but they also share genetic markers with the surviving birds on the satellite islands."

The Charles Darwin Foundation, which maintains a research station in the Galapagos, is heading a ten-year project which includes eradicating non-native species on Floreana, establishing a captive breeding program and finally reintroducing birds to the wild. The Darwin exhibition leads into next year's celebrations of the 200th anniversary of Darwin's birth on February 12, and the 150th anniversary of "On the Origin of Species" on November 24.

Natural History Museum, www.nhm.ac.uk

Darwin Foundation, –www.darwinfoundation.org

Darwin's mockingbirds, http://oikos.villanova.edu/Nesomimus