Origin, Chapter 9, part III

Darwin knew he had to address as a potential flaw in his theory the apparently sudden appearance of whole groups of species, which would argue for special creation and against his theory of descent through gradual modification through natural selection. “But”, Darwin wrote,

“we continually overrate the perfection of the geological record, …We continually forget how large the world is, compared with the area over which our geological formations have been carefully examined; we forget that groups of species may elsewhere have long existed and have slowly multiplied before the invaded the ancient archipelagos of Europe and of the United States…”

Darwin’s explanations have been largely borne out by paleontological research in the 150 years since publication of the Origin, and the sudden appearance of most groups or organisms is another artifact of an incomplete, and incompletely known, fossil record.

Precursors of the "Cambrian explosion" (an artist's impressionistic interpretation shown here) are now known in the Precambrian. Image of the "Cambrian Explosion" is from http://www.futurehi.net/images/cambrian.jpg