506 
SILUKIA. 
[Chap. XX. 
ring a long life, amid the youngest as well as the most ancient deposits ; 
and if the theoretical considerations in the few preceding pages be objected 
to, they can easily be separated from those historical facts which are esta- 
blished by positive observation, and in the truth of which all geologists are 
agreed. 
The leading object of this volume, I therefore repeat, is to call atten- 
tion to the earliest vestiges of life which have been discovered in the crust 
of the globe, and accurately to chronicle the order in which other and 
more highly organized races followed them. 
From the effects produced upon my own mind through the study of these 
imperishable records, I hope my readers will adhere to the views which 
I entertain of the succession of life from lower to higher classes, always 
bearing in mind that the first living animal of each class was as perfect 
and composite in structure as any of its congeners in after-times. I 
therefore cannot but believe that he who, looking to the earliest visible 
signs of life, traces thenceforward a rise in the scale of beings until Man 
appeared upon the earth, must acknowledge in these successive works 
continuous manifestations of the Design of a Ceeatoe. 
P.S. The last published volume illustrative of the palaeontological researches 
of M. Barrande * has been transmitted to me by the author whilst these pages are 
going through the press ; and in it I find the clearest elaboration of the proofs 
that (like the primeval Crustaceans, the Trilobites) the Cephalopods (of the 
Nautilide family) attained by far their greatest development in the Silurian 
period, their numbers lessening by a very rapid decrement in each of the suc- 
ceeding great Palaeozoic formations. Now this datum, the result of long labour 
and perspicuous discrimination, is strikingly confirmatory of my established geo- 
logical postulate, that, with the exception of its youngest member, the Silurian 
system was an 1 Invertebrate Period ' of immensely long duration. I have sug- 
gested (p. 485) that the natural function of Fishes as destroyers of the lower 
marine animals was exercised in Silurian times by numerous Cephalopods ; and 
it gratifies me to find that the numbers of the latter in those early days infinitely 
exceeded that of which we have hitherto had any knowledge. Again, the same 
view of very long Invertebrate Periods has been sustained by the authority of 
Professor Dana, who has termed the Silurian era the " Age of Mollusks " f. 
* ' Systeme Silurien du Centre de la Boheme,' Ce'phalopodes (Teste), 4to, 1867. 
Partie I. ' Recherches pale"ontologiques,' vol. ii. t 4 Manual of Geology,' 1863, p. 128. 
