I have encountered so often the story that Darwin used the Galapagos islands in the Origin of Species as a a mechanism to show diversity and natural selection within a rather secluded part of the word that it was quite a standard for me. I can even remember reading in biology books that Darwin found species totally different from island to island thus demonstrating evolution in action.

The fact is that Darwin never mentioned such an argument in the Origin of Species.

The amazing thing is that Darwin used the Galapagos in entirely different – and more cunning – argument that has to do with the fact that the species found in the Galapagos show a definite relation and affinity with the species found in South America. So far this sounds natural, one would expect to see related species in close geographical areas. The crux is though that although this affinity and resemblance is present the climate, conditions and state of the Galapagos islands is totally different from that found in South America.

In other words the argument does not have to do with diversity but it is an example of affinity which demonstrates common ancestors which have led via the evolution to different (morphological and actual) species due to the very different conditions in South America and the Galapagos.

(page 115)