FACTS AND OBSERVATIONS. 



xlvii 



In Bohemia and some other countries appearance and disappearance of species go on so rapicllj 

 that the life at the top and bottom of a stage would be completely dissimilar but for the recurrents ; 

 and opportunities for vertical range are constantly occurring in the dispersion and reconstruction of 

 societies, a state of things which leads to new abodes, new combinations, and perhaps to increased 

 well-being. 



The Upper- Silurian fossils which people the Prague colonies in fauna D. d, except as they 

 come from another area, are not recurrents, are not the posterity of Bohemian mollusks. They are 

 the precursors of an identical and larger coming fauna. Signs are not wanting that they came from 

 a country where the Silurian epoch was more advanced than in Bohemia ; and they become of great 

 value by indicating local inequality of progress in the act of deposition during this epoch suggest- 

 ing, moreover, that any of the Silurian stages may be in process of formation about the same 

 time with another in different parts of the world. 



Recurrents tolerate many sediments. This has always been a common and useful property of 

 marine life. So it is with the greater portion of our present marine fauna. This they are enabled to 

 do by the fact that different plants are able directly or indirectly to furnish acceptable food for the 

 same animal. The orders which are under disadvantage in this respect are the Echinodermata, 

 Entomostraca, and some Gasteropoda. 



Some recurrent species enjoy enormous longevity, but we may treat on this subject more fully 

 elsewhere. When found in very distant areas they are often ultra-epochal (or serial), and they 

 start up when least expected ; but there are terms of arrest, or horizons of finality, above which 

 neither single nor grouped existence can pass (Bronn, Deshayes, &c.). 



Recurrence varies in its amount with the locality, because no two localities are at all points 

 alike. It is common in Sweden and Canada, and still more abundant in Wales, where the interval 

 between the Lower and Upper Silurian is tolerably well supplied with life. In the Wenlock and 

 Ludlow beds of that district and its vicinity eighty-nine of the fossils are the same (1858) . Vertical 

 range is feeble in Bohemia and Russia, which in the latter country is strange, because no disturbing 

 causes seem to have been present, and none of much power in Bohemia. 



In one region a species may be restricted to a single set of beds, without being so in another. 

 We have this exemplified in the Silurian fauna of Britain and Bohemia, as in the following 

 Table. 



TABLE Y. Some Fossil Species Typical or Recurrent, according to their Basin. 



These restricted or typical fossils, except one, are from the Upper- Silurian stage E, mainly 

 because it is the principal depository of fossils, G and H having comparatively few. A few species, 

 on the contrary, are recurrent in Bohemia, and confined to one stage in Wales. Among others 

 may be mentioned Lepteena euglypha, which is in Caradoc in Wales, and in both E and F in 

 Bohemia. Some are recurrent in both these basins, but not always beginning on the same zone. 



The same genus recurs differently in different countries, and necessarily. Thus of forty 



Llandovery. 



b Middle Silurian. 



