Varieties and species – the Thencas

 Darwin's ornithological notes from the Galápagos Islands. CUL DAR 29.2, f. 74

Even before he stepped foot on the Galápagos Islands, Darwin had been pursuing the implications of the relationship between living and extinct creatures in the same location, and exploring the question – ‘Are these Species or Varieties?’ His collection of four specimens of ‘Thencas’, or mocking birds, from as many islands, advanced his ideas: later reviewing his notes, he realised that there was something ‘singular’ in this case, comparable to the eyewitness reports that distinct Tortoises also came from the different islands in the group. Eight months later and half a world away in the Atlantic, when he was writing up his ornithological notes, Darwin amplified his analysis of the Galápagos mocking birds, concluding: ‘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.’