The Fauna and Flora of the Silurian Period. 527 



taking the Bohemian Cephalopoda, BracJiiopoda, Trilohita, Gastero- 

 poda, etc., as groups whereon to establish his views of duration, 

 succession, and measurement of stages, periods, or eras. 



Extinction of Species, etc. — If any branch of geological enquiry 

 interests the palaeontologist more than another it is the beginning and 

 ew(?m^ of forms called species, which includes also their range. We 

 are enabled to arrive at a solution to this important question through 

 the Thesaurus. The great subdivisions of the geological series are 

 based upon the first appearance and persistence in time of a certain 

 community of species, like that marking the Silurian, Devonian, 

 Carboniferous, Oolitic, Cretaceous, and Tertiary periods exhibit ; the 

 close of each epoch being marked by the extinction, modification, or 

 migration, of the life of the period to a greater extent than at other 

 horizons. And thus we find Silurian life was greatly modified, though 

 of course not discontinued, everywhere on the same horizon. In 

 Britain tico species only can be said to be common to the Silurian 

 and Devonian periods. 



One of the causes of extinction is, and ever has been, in universal 

 operation, more or less everywhere ; this is oscillation of level, 

 which constantly alters the form and proportions of land and sea ; 

 the temperature and all the physical conditions attendant upon them, 

 and consequently also the life thereon and therein, the peopling of 

 the many stages of the Silurian system can, both in their advent and 

 extinction, be arrived at through the details set forth in the first 

 column, entitled '' Subdivision," and the three following headed 

 stages in the Thesaurus. To our minds the philosophy of the 

 whole question of locality, duration in time, extinction, migration, 

 recurrence, and divergence in species is based upon and governed by 

 the same laws which regulate the relative amount of sea and land, 

 both vertical and horizontal, and with it the life subjected to those 

 vicissitudes. Habit is changed, locality shifted, and all must either 

 accommodate themselves to new feeding-grounds and homes, or perish. 

 Extinction or migration, accommodation or change, must follow, 

 whether the time involved in effecting the change be long or short. 

 The author of the Thesaurus happily and briefly explains in the intro- 

 duction many of the phenomena attendant upon such ever-recurring 

 conditions of the sea and land. The table states this, but registers only 

 the fact of occurrence ; and when we know that the whole of the 

 9000 species analysed therein seem to have passed away at the close 

 of the Silurian epoch, we may well endeavour to account for, or 

 propound a doctrine which shall satisfactorily explain away this 

 apparent extinction, without having recourse to the old notion of 

 cataclysms. 



5. Migration. — Transport and migration are two distinct aats, one 

 passive and dependant, the other an individual power, governed by 

 will or instinct. Marine and fresh-water forms, when life has ceased 

 to perform its functions, are transported by currents, waves, or 

 streams. The remains of older formations, through the influence of 

 denudation, are occasionally re-deposited in conglomerate-beds, or 

 transported to other sediments, and migration or removal from place 



