Two mockingbirds, which are said to have helped Charles Darwin develop his theory on evolution, are to go on public display for the first time.
The specimens, gathered by Darwin from the Galapagos, are said to be the "catalyst" for his transmutation theory - how one species changes into another.
A variety of differences between the specimens led to him questioning the "stability of species".
The birds will go on show at London's Natural History Museum next week.
The mockingbirds will feature in an exhibition dedicated to the pioneering work of the naturalist, which is part of Darwin200, a national programme of events running throughout 2009, celebrating the 200th anniversary of his birth.
"What is fantastic about these two birds is that visitors will be able to see for themselves the crucial differences that Darwin saw," said Jo Cooper, the museum's bird curator.
The mockingbirds were collected during Darwin's five-year voyage on board HMS Beagle, which was captained by Robert FitzRoy.
One of the birds was captured on the island of Floreana, while the other was gathered from another Galapagos island, which is now called San Cristobal.
As a result of an earlier visit, Darwin knew that there was only one species of mockingbird in South America, yet he found a different species on each of the islands in the Pacific Ocean archipelago he visited.
From this, he reasoned that all mockingbirds in the world had descended from a common ancestor, because they shared a number of similarities with each other.
This ultimately led Darwin to the conclusion that all organisms on Earth had common ancestors.
Randal Keynes, Darwin's great-great-grandson, said the specimens played an important part in shaping the ideas that led to Darwin's theory of evolution.
"Darwin's first sighting of the Galapagos mockingbirds were to prove historic," he said.
"He later noted in the The Voyages of the Beagle that the small differences between the two birds on the two islands was a 'most remarkable fact in the distribution of organic beings'."
"Darwin had come to understand that species can change and this ultimately led to our present understanding of life on Earth."