iJ4 The Treasures of Siluria. [Jan., 



59, Trochoceras 36, Phragmoceras 25, and 4 others tLe remain- 

 ing 19. 



What Dr. Bigsby calls " universality " illustrates the other 

 side of the question in an c(|ually forcible manner. His defi- 

 nition is that " a formation luay be considered to be universal 

 v?hen it occupies large and small areas in very many parts of the 

 earth, often remote from, and even antipodal to, each other, 

 when it is always of hke stratigraphical relations, is composed of 

 like materials, and contains numerous genera of existences in 

 common, together with some representative, and some identical 

 species." Of course this definition makes the Silmian a universal 

 formation ; and if its spirit be truly applicable to a formation, 

 it must be equally so to a genus or a species. Our author has 

 therefore constructed a table, showing that 210 species are com- 

 mon to Europe and America, of which 85 travelled from the Old 

 to the New World (that is, appear in more recent deposits in the 

 latter) ; 30 from America to Em'ope ; while 145 are what he calls 

 " Isozonals ;" that is to say, occur on the same horizon in both 

 hemispheres. At first sight it seems as if Dr. Bigsby were of 

 opinion that the last-named species did not migrate at all, but 

 were sown broadcast over the whole area at the time of their 

 creation. We presume, however, that this is not really his 

 meaning. 



Some Silurian species occur almost everywhere where Silurian 

 rocks occur. And it is noteworthy that most of these " universal " 

 species possess also the greatest vertical range, and the greatest 

 variability recognized as such. Take, for instance, the well-known 

 Cali/mene BlumenhacJiii. This species, though not so variable as 

 some others which might be named, occurs in England in all 

 divisions of the Silurian system, from the Llandeilo to the Wenlock 

 inclusive. And many other instances of a like natm-e might be 

 quoted. Indeed, our author is well acquainted with them. 



As to geographical range, this same Calymene Blumeiibacliii, 

 so well known in the United Kingdom, is found abundantly in 

 the U])per Silurian rocks of Northern Europe, in both the Lower 

 and Upper divisions of the French Silurian strata, and in the 

 Lower, Middle, and Upper divisions in those regions of North 

 America where they rcsjwctively occur. 



It certainly does surprise us that Dr. Bigsby, in dealing with 

 a topic of such great interest as that of the duration of i<i)ecies, 

 should not once allude to the law originally laid down by De 

 Verneuil and D'Archiac, advocated by Forbes, Kogers, and others, 

 and adopted by almost every subsequent writer. This law may 

 be enunciated thus : — Species which are most extended in geogra- 

 phical space are generally those which have the greatest vertical 

 range. It refers specially to organisms of the same rank, and 



