Origin of Species Variorum

On the sudden Appearance of Groups of allied Species in the lowest known Fossiliferous Strata .

There is another and allied difficulty, which is much more serious. I allude to the manner in which numbers of species of the same group suddenly appear in the lowest known fossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the same group have descended from one progenitor, apply with nearly equal force to the earliest known species. For instance, I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age, and which probably differed greatly from any known animal. Some of the most ancient Silurian animals, as the Nautilus, Lingula, &c., do not differ much from living species; and it cannot on my theory be supposed, that these old species were the progenitors of all the species belonging to the same group which have subsequently appeared, for they do not present characters in any degree intermediate between them. ↑

Consequently, if my theory be true, it is indisputable that, before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.

To the question why we do not find richly fossiliferous records of these vast primordial periods, I can give no satisfactory answer. Several ... eminent geologists, with Sir R. Murchison at their head, were until recently convinced that we beheld in the organic remains of the lowest Silurian stratum the first dawn of life on this planet. Other highly competent judges, as Lyell and