In 1833, Darwin spent a fair amount of time on the East Coast of South America, including in the Pampas, where he had access to abundant fossil material. Here I'd like to examine his writings about some of the megafauna, including Toxodon, Mastodon, and horses, and his further considerations of biogeography and evolution.
In the vicinity of Rio Tercero...
Hearing ... of the remains of one of the old giants, which a man told me he had seen on the banks of the Parana, I procured a canoe, and proceeded to the place. Two groups of immense bones projected in bold relief from the perpendicular cliff [but] I could only bring away small fragments of one of the great molar-teeth ... sufficient to show that the remains belonged to a species of Mastodon. The men who took me in the canoe, said they had long known of them, and had often wondered how they had got there: the necessity of a theory being felt, they came to the conclusion, that ... the mastodon formerly was a burrowing animal!
In remote St. Fe ...
A tooth which I discovered ... interested me much, for I at once perceived that it had belonged to a horse. Feeling much surprise at this, I carefully examined its geological position, and was compelled to come to the conclusion, that a horse, which cannot ... be distinguished from the existing species, lived as a contemporary with the various great monsters that formerly inhabited South America. Mr. Owen and myself, at the College of Surgeons, compared this tooth with a fragment of another, probably belonging to the Toxodon, which was embedded at the distance only of a few yards in the same earthy mass. No sensible difference in their state of decay could be perceived; they were both tender, and partially stained red. ... Certainly it is a marvellous event in the history of animals, that a native kind should have disappeared to be succeeded in after ages by the countless herds introduced with the Spanish colonist! But our surprise should be modified when it is already known, that the remains of the Mastodon angustidens (the tooth formerly alluded to as embedded near that of the horse, probably belonged to this species) have been found both in South America, and in the southern parts of Europe.
Weighty considerations of the distribution of extinct and extant fauna lead Darwin to the neighborhood of modern geological concepts.
Very few species of living quadrupeds, which are altogether terrestrial in their habits, are common to the two continents, and these few are chiefly confined to the extreme frozen regions of the north. The separation, therefore, of the Asiatic and American zoological provinces appears formerly to have been less perfect than at present. The remains of the elephant and of the ox have been found on the banks of the Anadir (long. 175Â° E.), on the extreme part of Siberia, nearest the American coast: and the former remains, according to Chamisso, are common in the peninsula of Kamtschatka. On the opposite shores, likewise, of the narrow strait which divides these two great continents, we know, from the discoveries of Kotzebue and Beechey, that the remains of both animals occur abundantly: and as Dr. Buckland has shown they are associated with the bones of the horse, the teeth of which animal in Europe, according to Cuvier, accompany by thousands the remains of the pachydermata of the later periods. With these facts, we may safely look at this quarter, as the line of communication (now interrupted by the steady progress of geological change) by which the elephant, the ox, and the horse, entered America, and peopled its wide extent.
Now, here we have Darwin on the verge of understanding the rise of the Panama Land bridge (or something like that) based on the biogeography. The above passage, the following passage, and other material is very frustrating. If Darwin was not such a geological gradualist he could have advanced geology to the 1950s with a single fell swoop of reasoning!!!!
The occurrence of the fossil horse and of Mastodon angustidens in South America, is a much more remarkable circumstance than that of the animals mentioned above in the northern half of the continent; for if we divide America, not by the Isthmus of Panama, but by the southern part of Mexico, .. where the great table-land presents an obstacle to the migration of species, ... we shall then have two zoological provinces strongly contrasted with each other. Some few species alone have passed the barrier, and may be considered as wanderers, such as the puma, opossum, kinkajou, and peccari. The mammalogy of South America is characterized by possessing several species of the genera of llama, ..., tapir, peccari, opossum, anteater, sloth, and armadillo. If North America had possessed species of these genera proper to it, the distinction of the two provinces could not have been drawn; but the presence of a few wanderers scarcely affects the case. North America, on the other hand, is characterized by its numerous rodents, and by four genera of solid horned ruminants, of which section the southern half does not possess a single species.
Just so you know, South and North America were separated, and had largely independent mammalian evolution (and migration), until very recently, about five million years ago, when the isthmus of Panama was raised.
Darwin is seeing the very time-deep echo of this event, masked by subsequent migration of North American mammals in to South America, and clouded by the more pressing (to him) question of Old World and New World relationships.
It is interesting that the monkey's (appearing in both the old world and new world tropics) don't freak him out. They freak me out.