While island-hopping in the Galapagos, he noticed that the finches on each island had beaks of different shape and size, longer or broader, depending on the type of indigenous seeds available to them on their respective islands. The tortoises had shells of different shape and configuration; their necks were longer or shorter, and Charles Darwin surmised that this was so because of the type of foliage and calories available to them in their respective environments.
And so he starts coming up with this odd theory called “natural selection.” It may be the hugest and most cataclysmic scientific idea of all time, but it is ridiculously simple. Darwin surmised that all forms of life on earth descended from a common ancestor, and the stunning variety of living things as we know them results from five simple facts that are represented by the mnemonic acronym VISTA.