IMPERFECTION OF GEOLOGICAL RECORD 275 



group, suddenly appear in the lowest known fossil i- 

 ferous rocks. Most of the arguments which have con- 

 vinced 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 douht that all the Silurian trilobites 

 have descended from some one crustacean, which must 

 have lived long before the Silurian age, and which prob- 

 ably differed greatly from any known animal. Some 

 of the most ancient Silurian animals, as the Nautilus, 

 Lingula, etc., 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 of the 

 orders to which they belong, for they do not present 

 characters in any degree intermediate between them. 

 If, moreover, they had been the progenitors of these 

 orders, they would almost certainly have been long ago 

 supplanted and exterminated by their numerous and 

 improved descendants. 



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 un- 

 known, periods of time, the world swarmed with living 

 creatures. 



To the question why we do not find records of these 

 vast primordial periods, I can give no satisfactory 

 answer. Several of the most eminent geologists, with 

 Sir R. Murchison at their head, are convinced that 

 we see in the organic remains of the lowest Silurian 

 stratum the dawn of life on this planet. Other highly 

 competent judges, as Lyell and the late E. Forbes", 

 dispute this conclusion. We should not forget that 

 only a small portion of the world is known with 

 accuracy. M. Barrande has lately added another and 

 lower stage to the Silurian system, abounding with 

 new and peculiar species. Traces of life have been 

 detected in the Longmynd beds, beneath Barrande's 

 so-called primordial zone. The presence of phosphatic 



