PACTS AND OBSERVATIONS. 



XXXV 



has proved the richer by about 1200 species. This is to be attributed partly to the nature of the 

 two regions *, and partly to the successful labours of M. Barrande f. The Table (R) placed below is 

 intended to show this. 



TABLE R. The Silurian Fauna and Flora of North America and Europe, as known in 1866. 



Although this Table contains 1500 species less than the number with which we are now (1868) 

 acquainted, it is believed that the reader will be led into no serious error by adopting it. 



We see from it that the Cephalopoda, Crustacea, Brachiopoda, and Annelida of Europe largely 

 exceed in number of species those of North America, while in nine orders the two hemispheres are 

 about equally provided. America surpasses Europe in the number and variety of its Echinodermata 

 and Gasteropoda, as well as, to a smaller extent, in the Polyzoa and Coelenterata. There are also 

 here other particulars worthy of note. 



I am not prepared with any inferences from these facts. We know, however, that the mineral 

 constitution and the external influences of these several parts of the earth were different not that 

 the first is of so much importance as was supposed. 



Many species are marked as undefined in the ' Thesaurus,' because they are often only known 

 by fragments. 



Out of 9030 species of marine creatures now (1868) registered as belonging to the Silurian 

 period, 4628 are only set down as met with in one locality of a certain radius. This has been 

 ascertained by careful search into the writings of the most accredited palaeontologists ; and it is 

 applicable to the works of Barrande, Billings, Hall, M'Coy, Sowerby, Salter, and many others, 

 especially to those of the very able explorers of the United States (Conrad, Shumard, Meek, &c.). 



There has been no further inquiry into the number of places (ranging from one to twenty-five 

 or more) inhabited by the same species, excepting among the Cephalopoda of Bohemia ; of these, 190 

 more appear, each only in two places. 



From our total flora and fauna, therefore, there remain 4402 species to people the Silurian 

 strata, each in two and many more places ; this they do amply. In 4628 species typical of one 

 place, we hear little about varieties or transitional forms, although the former of these are common. 

 Neither can it be safely said that natural selection in these cases has perfected its work ; for these 

 species usually belong to communities consisting of several genera. 



Such a very great number of species being each restricted to a single locality, is an important fact. 

 They are so many specific centres, and probably will never be much curtailed. It indicates that 



* Some countries yield a smaller harvest than others because the rocks are accessible with difficulty, as in all 

 forest-lands, plains, and all hills buried in sand (Africa) where the coasts are flat, the rivers few, and their banks low 

 where metamorphism has been active, as in the interior of Newfoundland (Murray) where no rock-sections are made 

 for public purposes. 



t Large additions to the fauna of Bohemia have been most kindly sent to me by M. Barrande since Table R was 

 constructed. 



