Geology and Zoology — A Symbiosis Darwin's Beagle Voyage and Galapagos Experience

Darwin's South American fossil collections yielded many insights. in one section he recorded the gradual vertical change in shell characteristics of fossil oysters. The giant fossil vertebrates brought the realization that several were precursors of living species. Contrary to accepted Catastrophism Theory, he inferred that all taxa found fossilized did not become extinct together since the living agouti, among others, had fossil precursors. He envisioned the way an existing molluscan fauna and its sedimentary matrix could be transformed into fossiliferous rocks reminiscent of the Tertiary, and also speculated about the kinds of fossils that the Galapagos biota would yield in some future rock column.

Volcanic islands (Galapagos Archipelago and others) were linked in Darwin's thought to the appearance of "peculiar" taxa inhabiting these "mere spots" in the sea. Were these migrants from the nearest continents? If so, why were they so distinctive while carrying the imprint of their place of origin? If not, how did these four-legged and other terrestrial creatures get to the islands? Were the archipelagos ever united to the continents? Gradually Darwin was led to a continent-archipelago-continent concept. Continents were elevated archipelagos. (He had the Andean volcanoes in mind.) Archipelagos were submerged remnants of continents. He incorporated into this concept the rise and extinction of species: elevation of archipelagos were the "best nurseries" for appearance of new taxa, while subsidence of continents generally led to extinction of extant forms.