Fig. 5

The three patterns of relationship between fossil and living species at Bahia Blanca, Patagonia as Darwin explicitly saw them in “February 1835”—and as early as September/October 1832 when he was collecting the fossils and observing the local biota. Lower horizontal line indicates the fossil beds; upper line the recent. Times of origins and extinctions hypothetical but staggered to reflect Darwin’s assertion that the births and deaths of species are “gradual.” A Two examples of extinct large edentate mammals (ground sloth and glyptodont) “allied with” modern sloths and armadillos, respectively. The dissimilarities in size and morphology between fossil and recent species are too great to suggest anything more than replacement of extinct species by other species in the same natural group. C Darwin considered the fossilized shells of mollusks (and other invertebrates) in the Punta Alta beds to belong to the same species still extant in Bahia Blanca. In this instance, it is persistence, but not replacement, of fossil species into the modern biota. B The intermediate case: Darwin considered his fossil cavy to be an extinct species closely allied to (“congeneric” with) the modern cavy which takes the place of the extinct relative. The birth of the modern species follows closely the extinction of the fossil cavy species—illustrating the general pattern that Darwin focused on in “February 1835”