THE TEST. OF ORGANIC REMAINS. 329 



Yet the improbability of this assumption ma}- be readily 

 shown from Sir R. Murchison's own facts. If, as he points 

 out, the crustacean fossils of the uppermost Silurian rocks 

 in Lanarkshire are, " with one doubtful exception," " all 

 distinct from any of the forms on the same horizon in Eng- 

 land ; " how can it be fairly presumed that the forms exist- 

 ing on the other side of the Earth during the Silurian 

 j^eriod, were nearly allied to those existing here ? Not 

 only, indeed, do Sir R. Murchison's conclusions tadtly as- 

 sume this doctrine of universal distribution, but he distinctly 

 enunciates it. "The mere presence of a gra2:)tolite," he 

 says, " will at once decide that the enclosing rock is Silu- 

 rian ; " and he says this, notwithstanding repeated warnings 

 against such generalizations. During the progress of Geolo- 

 gy, it has over and over again happened that a particular 

 fossil, long considered characteristic of a particular forma- 

 tion, has been afterwards discovered in other formations. 

 Until some twelve years ago, Goniatites had not been found 

 lower than the Devonian rocks; but now, in Bohemia, they 

 have been found in rocks classed as Silurian. Quite re- 

 cently, the Orthoceras, previously supposed to be a type 

 exclusively palaeozoic, has been detected along with meso- 

 zoic Ammonites and Belemnites. Yet hosts of such experi- 

 ences fail to extinguish the assumption, that the age of a 

 stratum may be determined by the occurrence in it of a 

 emgle fossil form. 



Nay, this assumption survives evidence of even a still 

 more destructive kind. Referring to the Silurian system 

 in Western Ireland, Sir R. Murchison says, " in the beds 

 near Maam, Professor Mcol and myself collected remains, 

 some of which would be considered Lower, and others 

 Upper, Silurian ; " and he then names sundry fossils "vvhichj 

 in England, belong to the summit of the Ludlow rocks, or 

 highest Silurian strata ; " some, which elsewhere are known 

 only in rocks of Llandovery age," that is, of middle Silu- 



