Not much, anyway. However, biology did not stop with Darwin. Much has
been learned about the subject since.
Darwin explained at length how a species can change its form
gradually but, over long periods of time, drastically. Even if new
species did not branch off from such a lineage, many people would still
consider that process an origin of new species.
Darwin proposed that natural selection had a fundamental role in
speciation, but did not elaborate much on the mechanism. It is now
believed that much speciation is due not to natural selection, but to
geographical isolation and genetic drift (allopatric speciation).
However, natural selection is still seen to play a role in other
speciation, such as speciation due to specialization on different hosts
(Filchak et al. 2000), and natural selection drives incipient species
to greater diversity (Presgraves et al. 2003).
Darwin wrote about speciation, including the role of geographic
isolation, in other works (Kottler 1978).
References:
Filchak, Kenneth E., Joseph B. Roethele and Jeffrey
L. Feder. 2000. Natural selection and sympatric divergence in the
apple maggot Rhagoletis pomonella. Nature 407: 739-742.
Kottler, Malcolm J. 1978. Charles Darwin's biological species concept
and theory of geographic speciation: the Transmutation Notebooks.
Annals of Science 35: 275-297.
Presgraves, D. C., L. Balagopalan, S. M. Abmayr and H. A. Orr. 2003.
Adaptive evolution drives divergence of a hybrid inviability gene
between two species of Drosophila. Nature 423: 715-719. See
also Holding, C., 2003. Driving Drosophila diversity. The
Scientist, http://www.biomedcentral.com/news/20030612/02/
Further Reading:
Otte, D. and J. A. Endler. 1989. Speciation and Its Consequences.
Sunderland, MA: Sinauer Assoc.
