Near the end of his voyage, Darwin pored over his notes and began to compile them. In a set of notes on birds, written while still on the Beagle, Darwin first began speculating about evolution in writing.

Darwin wondered: Were the different mockingbirds he had seen in the Galpagos just varieties of one species, or were they distinct species? Or might they even be varieties on the way to becoming different species? It would take years of thinking about the idea of evolution, and a great deal more evidence, for Darwin to convince himself his suspicions were correct. But once raised, the question would not go away. Could species evolve?

First Notes on Evolution

Document: Charles Darwin's "Ornithological Notes"
 Date: June 1836; Page: 74 of 85

Pondering the Galpagos mockingbirds raised troubling questions for Darwin about the very nature of species. At first Darwin suspected that all the different mockingbirds were just varieties of a single species. Later he speculated on whether these varieties might represent the early stages of the appearance of new species. In the Ornithological Notes, Darwin writes,

"When I see these islands in sight of each other...tenanted by these birds, but slightly differing in structure & filling the same place in Nature, I must suspect they are only varieties."

But what if some time ago a single mockingbird species had migrated from the mainland, spread through the archipelago, and then begun to vary on the different islands? What if these varieties grew to be so different that they actually became separate species-that is, they could no longer breed together? The implications were enormous. As Darwin wrote,

"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."

If species were not stable, but could give rise to new species, everything in natural history would be seen in a new light.

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