Dr. J. J. Bigsby on the ' Thesaurus Siluricus.^ 293 



Australia by Mr. Selwyn, the chief Geological Surveyor of that 

 colony ; and Professor M^C'oy has met with in that country a Sipho- 

 notreta, a Phacops, and eighteen species of Graptolites absolutely 

 identical with those of North America and of Europe. The Pro- 

 fessor loudly expresses his surprise and delight. According to M. 

 Barrande, Orthoceras bullatum (Sowerby) is at Melbourne (Aus- 

 tralia) and in Ireland, Bohemia, Germany, and Russia. Conocoryphe 

 depressa is both in Wales and Texas, one of the American States. 

 Western Tasmania, the Himalayas, Russia, North and South America, 

 and many other regions oiFer ample fossil evidence of the general 

 presence of the constituents of this period. 



The Silurian beds, it must be borne in mind, are usually visible 

 in mere shreds and remainders, met with in any one place only as 

 a stage or a part of a stage, the other portion being covered for 

 perhaps thousands of square miles by more recent deposits, or re- 

 moved by denudation ; or it may be that certain stages have never 

 existed, as we see in Arctic America with respect to the Lower 

 Stage ; while in the South, as in Sardinia, France, and Spain, it 

 is the Upper Stage that is wanting, or very nearly so. 



But the visible geographical spread of these strata is often very 

 great. So extensive are the Silurian areas of North America (2000 

 miles across) that it only needs a short and easy step to induce a 

 belief in a former universal prevalence and domination of this 

 system. 



Sufficient territory resting on Silurian rocks has been spared 

 from oscillatory action to enable us to trace it in one or other of its 

 parts over a large part of the earth. We follow it circuitously 

 from England to Australia, or to America — the interspaces being 

 filled up either by sea, by newer rocks, or by kindred palaeozoic 

 strata, which themselves irresistibly bespeak its frequent continuous 

 existence near at hand. 



This is only a fragment of the argument in favour of the doctrine 

 of Universality of epochs, as just defined. 



Locality. 



The * Thesaurus ' brings conspicuously into view the great influ- 

 ence of locality on the nature and amount of life, in the same way 

 as we observe at the present time. As each region yields up its 

 faima to the collector, much of that fauna is found to be new, the 

 bond of connexion with other Silurian districts being in great mea- 

 sure generic. 



The physical conditions of sea and land being necessarily local, 

 produced as they are from time to time by agencies limited in 

 space, the dwellers among these conditions must in a certain mea- 

 sure be local too, and typical — subject at any moment to removal. 

 The first occupants of any spot who shall point out ? 



The maximum of life, meaning by that expression the largest 

 combination of abundance, variety, and rank, is local. It may take 

 place at the beginning of a stage, or of an epoch, in the middle, 



Ann. ^ Mag. N. Hist, Ser,3. Fo/.xix. 21 



