This was a pivotal moment, an intellectual light-bulb, if you will, in Darwin’s progress of a refined theory as to how species could, in his terms, “undermine stability”, disrupting the common Victorian understanding that species do not change, but remain stable and unchanging (i.e. do not transmute).
And it all boiled down to beak variation; as Darwin wrote in The Voyage of the Beagle, “the most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza, from one as large as the Hawfinch to that of a Chaffinch.” It was a simple intellectual step for Darwin, and he asked if these thirteen birds evolved from one single ancestor. The evidence most certainly pointed to this fact, that they did indeed share a single common ancestor, a ground-feeding, seed-eating Finch. All thirteen present Finches had subsequently adapted their beaks to suit the conditions of each different island: it turns out that specific Galápagos finches have different dietary requirements, some preferring seeds and insects, while others prefer leaves and flowers – their individual beaks suitably shaped for consuming these different foods.
And this, of course, was a breakthrough in scientific understanding; an intellectual leap, one could argue, as profound as Newton’s Universal Law of Gravitation, or Michael Faraday’s research into electricity. It has reshaped the way many of us view life on earth, and it all came from a group of humble Finches.
We urge you to take a look outside and ponder the significance, enjoy the thought that many of the garden birds you observe have derived from a long and complex evolutionary line of common ancestors, dating back to the dinosaurs, and more, into the furthest depths of the past.