In the mid-19th century, paleontologists scrambling for fossils focused more on the prehistoric ancestors of contemporary creatures than on dinosaurs. But after Charles Darwin published his On the Origin of Species in 1859, the theory of evolution became one of the most controversial topics of the time.

When American paleontologist O.C. Marsh identified the Equus parvulus (now Protohippus), many biologists considered the skeleton to be validation of Darwin's theory. In this brief letter from Charles Darwin to O.C. Marsh, Darwin expresses his thanks for Cope's work in the field.

Aug 31 1880
My dear Prof. Marsh
I received some time ago your very kind note of July 28th, and yesterday the magnificent volume. I have looked with renewed admiration at the plates, and will soon read the text. Your work on these old birds on the many fossil animals of N. America has afforded the best support to the theory of evolution, which has appeared within the last 20 years. The general appearance of the copy which you have sent me is worthy of its contents, and I can say nothing stronger than this.
With cordial thanks, believe me yours very sincerely
Charles Darwin

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