FOSSIL EVIDENCE OF EVOLUTION

Once the HMS Beagle reached South America, Darwin spent time
digging in the dry riverbeds of the pampas (grassy plains) of Argentina.
He found the fossil remains of an extinct hippopotamuslike
animal, now called Toxodon, and fossils of a horselike animal,
Thoantherium. Both of these fossils were from animals that
were clearly different from any other animal living in the region.
Modern horses were in South America, of course, but Spanish
explorers had brought these horses to the Americas in the 1500s.
The fossils suggested that horses had been present and had become
extinct long before the 1500s. Darwin also found fossils of giant armadillos
and giant sloths. Except for their large size, these fossils
were very similar to forms Darwin found living in the region.
Fossils were not new to Darwin. They were popularly believed
to be the remains of animals that perished in catastrophic
events, such as Noah’s flood. In South America, however, Darwin
understood them to be evidence that the species composition of
the earth had changed. Some species became extinct without
leaving any descendants. Others became extinct, but not before
giving rise to new species.