i 28 Reviews — Bigshys Thesaurus Siluricus — 



to place under physical changes and vital laws, has ever been a 

 great fact. The fauna of every great geological epoch (and the 

 flora in many cases also), has, through all its changes, whether slowly 

 or suddenly, been subject to the vicissitudes of life through the long 

 voyage of time. The colonization of unoccupied spots from crowded 

 areas, changes of place through reproduction and individual wants, 

 the great influence of "natural selection, and the struggle for 

 existence," are inducements to, and causes of migration. The group- 

 ing then, or communities of organic remains, the colonies of olden 

 times that lie entombed in the shales, limestones, and sandstones of 

 the Silurian rocks, are to us but expressions and faithful witnesses of 

 the distribution of life through that long epoch ; sea-deserts of sand, 

 and barren wastes, unsuited to and impassable by many of the mollusca, 

 and other creatures, occurred then as now to arrest migration, or con- 

 centrate a peculiar fauna in one spot. Such patches of fossil life in the 

 midst of old untenanted wastes in the Silurian rocks are well-known 

 to all investigators. Some beds of Trilobita which (like an oasis to 

 the collector) really do occur amid such wastes in the Bohemian basin, 

 and research tells us that whole communities have returned to the 

 country from which they had migrated, or had long abandoned. One 

 amongst others of this kind of repossession of an abandoned area is 

 illustrated by Mr. God win- Austen, as occurring in the Palaeozoic 

 rocks of the Boulonnais, where alterations or alternations of level 

 have introduced into the same area, successively, distinct assemblages 

 of suitable marine life, one or two of these accomplishing a repe- 

 tition of occupancy (Quart. Journ. Geol. Soc. Lend. ix. p. 244). 



The migratory power of the mollusca is undeniable, but it varies 

 in certain groups. The Pelagic Cephalopoda, Pteropoda, etc., being 

 free dwellers in the open seas, drop on, and into, all grounds on the 

 ocean floor, and live independently of all sediment, whilst currents 

 sweep away the Bjssoid £rachiopoda and fixed Coslenterata (corals). 



Bathymetric conditions also govern the distribution of life — 

 and time being granted — with shore-lines, and coasts, and some 

 constancy of ocean depth ; and the distribution of the ocean fauna at 

 the present day, may be applied to the faunse of every past epoch. 

 Centres of distribution or migration may be determined by the use of 

 the Thesaurus for any place so well worked and understood as Great 

 Britain, Bohemia, Esthonia, etc., and much of America. The author 

 at p. xliii. of his introduction gives a table (table U) showing the 

 direction pursued by 210 species which are common to North-East 

 America and Europe (Western chiefly), or the direction of species in 

 transitu between North America and Europe, together with the 

 " Isozonals " of both hemispheres. He enumerates 15 groups, and 

 shows by means of the Tabular Thesaurus that 35 species migrated 

 West from Europe to America, and 30 East from America to Europe ; 

 and also shows that 145 species, or double the number of migrants, 

 are " isozonal," i.e. are common to the two hemispheres in the same 

 rock, or to the Silurian system ; and six out of the fifteen groups 

 exhibit no trace of travel or migration, although they are large and 

 important, namely, the Echinodermaia 294 species; the Lamelli- 



