Charles Darwin certainly did not consider it a sin to kill a mockingbird. The 19th-century English biologist killed many of the birds whilst on board the HMS Beagle survey ship as he traveled around the world. The specimens are more important than you might think and you can see a number of them at "Treasures of the Natural World," a new exhibition at the National Museum of Nature and Science, Tokyo.

Before he became a world-renowned scientist, Darwin was in training to join the clergy before accepting a position on the Beagle at the age of 22. Darwin collected thousands of specimens of plants and animals during his voyage, including mockingbirds. When he started examining mockingbird specimens from the islands of the Galapagos, he noticed that the birds were different, depending on which island they'd come from.

It's easy to take evolution for granted these days but, back in the 19th century, most people in the West believed that the Earth was 6,000 years old and that all life on the planet had been created by God. As such, it was sacrilege to imagine that species could change over time. However, Darwin's observations of mockingbirds — and not so much the finches that later became associated with his theory of natural selection — furnished him with some of the earliest evidence that evolution occurred. He developed his theory of natural selection and, in doing so, built one of the most influential scientific theories of all time.