"When Charles Darwin wrote "The Origin of Species " in 1859, the sudden appearance of animal fossils at the beginning of the Cambrian was of particular concern to him. It was at odds with his view that the diversification of life on earth through natural selection had required a long period of time. Darwin's theory predicted that the major groups of animals should gradually diverge during evolution. He knew that the sudden appearance of fossils would be used by his opponents as a powerful argument against his theories of descent with modification and natural selection. Consequently, he argued that a long period of time, unrepresented in the fossil record, must have preceded the Cambrian to allow the various major groups of animals to diverge. At that time the strata that we now regard as Cambrian were subsumed within the concept of the Silurian, so Darwin wrote,
'I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age....Consequently, if my theory be true, it is indisputable that before the lowest Silurian strata was deposited, long periods elapsed, as long as, or probably longer than, the whole interval from the Silurian to the present day.....The case must at present remain inexplicable; and may be truely urged as a valid argument against the views here entertained'
The Origin of Species, 1859, pp. 313 - 314
· Derek E.G. Briggs, Douglas H. Erwin, & Frederick J. Collier
"The Fossils of the Burgess Shale," 1994, Smithsonian Institution, p.39.
http://id-www.ucsb.edu/fscf/library/ori ... brian.html