The chief cause of our natural unwillingness to admit that one species has given birth to other and distinct species, is that we are always slow in admitting any great changes of which we do not see the steps. The difficulty is the same as that felt by so many geologists, when Lyell first insisted that long lines of inland cliffs had been formed, and great valleys excavated, by the agencies which we still see at work. The mind cannot possibly grasp the full meaning of the term of even a million years; it cannot add up and perceive the full effects of many slight variations, accumulated during an almost infinite number of generations.
The survey continued to the south in Patagonia . They stopped at Bahía Blanca , and in cliffs near Punta Alta Darwin made a major find of fossil bones of huge extinct mammals beside modern seashells, indicating recent extinction with no signs of change in climate or catastrophe. He identified the little-known Megatherium by a tooth and its association with bony armour, which had at first seemed to him to be like a giant version of the armour on local armadillos . The finds brought great interest when they reached England.