526 Eeviews — Big shy s Thesaurus Siluricus — 



of Asaphus, only four species of which genus occur in Bohemia and 

 ten in Britain ; the local distribution of such genera as Maclurea, the 

 peculiarities of the geographical distribution of the Brachiopoda, etc., 

 can only be arrived at through recasting again the facts of locality 

 and appearance given in the " table." This old geographical distri- 

 bution of certain forms of life can be approximately explained by 

 the peculiarities of the present fauna of Guadaloupe, the Madeiras, 

 St. Helena, Madagascar, Australia, South America,^ etc. etc. Clearly, 

 then, the 9000 Silurian species, here for the first time catalogued, will 

 well repay the student wishing to construct for himself tables of 

 life and localities, or to comprehend tha faunas of the old geogra- 

 phical areas once occupied by the Silurian seas. 



3. First appearance. It is with the earliest-known expression, or 

 dawn of life on the globe, that the compiler of the Thesaurus deals ; 

 but we may ask, who has seen, or knows, where the line has yet to be 

 drawn, by any system, that shall say, "below this there are no remains 

 of a former life. A few years since it was commonly believed that 

 the term " Primordial Zone " (as used by Barrande, Murchison, 

 and others) defined the limit of our knowledge of the downward 

 range of organic life. This view has long since been abandoned, for 

 thousands of feet below the so-called Lower Lingula flags of Great 

 Britain and Bohemia we have life entombed in the metamorphosed 

 Laurentian rocks of Canada, and in the older Cambrian of Wales we 

 are daily disentombing some new forms of life from sedimentary 

 rocks of the highest antiquity, but " first appearance as yet," does 

 not settle the date of the commencement of life ; this we have still to 

 learn. Thus much we do know, that certain groups of organized 

 beings make their appearance much about the same time, viz., the 

 Arliculata, Brachiopoda, and Annelida, which are all associated in the 

 Primordial beds, and also in the underlying and older Cambrian. 



4. TJie life or duration of Species. — This and the determination of 

 a species is one of the vexed questions of the day in Biological 

 science. The statistics of life, as chronicled in the Thesaurus open 

 up this question, and enable us to trace the vertical range of life 

 and development of species through vast lapses of time, and what- 

 ever may be the causes that modify forms {with ivhich the Thesaurus 

 does not attempt to deal), we are nevertheless enabled to see and trace 

 the range of those forms called species, and which, as recognized, 

 aid the Geological student in the determination and fixing of certain 

 definite lines of demarcation, and in the construction of systems. 

 Genera are less afiected than species, and have lived on through 

 a lapse of time beyond our conception ; whereas the life of an indi- 

 vidual is ephemeral, for, says the author of Thesaurus, so "numerous 

 are their generations that it is idle to speak of them as less than 

 hundreoB of thousands" (p. xxxviii). Under this head the author, 

 in his introduction, enumerates and suggests some important facts 

 for our consideration, and clearly sets forth the views of Barrande 

 upon the relative measurement of organic existence and longevity, 



* In fact, they represent "outliers," or remnants of old Pre-Silurian fauniB, not 

 entirely swept away by the stronger influx of Silurian life. 



