XXXVlll PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



sucli a community of character as to constitute an epoch in time gene- 

 rally, and that it is in this sense we are to understand the pliocene, 

 miocene, and eocene periods respectively. This view may be confirmed 

 by the accumulation of a widely-extended and multifarious body of 

 evidence ; but some of the principal causes of the extinction of exist- 

 ing species, and of the introduction of new sj)ecies, are of a kind that 

 might have come into operation in one portion of the globe, while 

 other parts remained unchanged by similar causes ; therefore the 

 synchronism of formations in distant parts of the globe cannot be con- 

 clusively determined by evidence that is in its nature inconstant. 

 This leads us naturally to inquire, vv'hat the circumstances are on 

 which the distribution and habits of different species of mollusca 

 depend. 



Professor E. Forbes has shown, that the distribution of marine 

 animals is determined by three great primary influences, and is modi- 

 fied by others that may be termed secondary or local. The primary 

 are, climate, composition of the sea, and depth : the secondary are, 

 the nature of the sea-bottom, that is, whether it consist of sand or 

 rock, be gravelly or w^eedy ; tides and currents, and the influx of 

 -fresh water. It is generally admitted by geologists, that at all periods 

 down to our own times, the surface of the earth has been subject to ex- 

 tensive elevations and subsidences ; that plains and lofty mountains 

 have risen where formerly there was sea, and that plains and moun- 

 tains have subsided and been covered with deep water. It is evident 

 that such elevations and depressions, producing variations in the re- 

 lative proportions of sea and land, and not only in the extent but in 

 the elevation of the land, must have caused great changes in atmo- 

 spheric temperature, in the temperature of the sea, in the depths 

 of water, in sea-bottoms, in the direction of currents, and in the 

 influx of fresh water, on different parts of the superficies of our earth, 

 and even on the same parts at different times. But such alterations 

 in the proportions of land and sea could not be sjmchronous over 

 the whole earth, nor is it probable that in two distinct areas they 

 would be alike in amount or in kind. 



Let us, for example, suppose two parts of the ocean (a and b) far 

 distant from each other, under such similarity of condition as to 

 temperature, depths of water, sea-bottom, &c., as to be favourable 

 to the existence of the same or representative species of mollusca ; 

 of littoral species, of those inhabiting zones of moderate depth and 

 of deep sea species ; let us further suppose earthy deposits going on 

 in each part, and inclosing the remains of the dead mollusca that 

 lived on the rocks and sands and amidst the groves of fuci of its 

 bottom. Let us now suppose subterranean action so to raise the 

 bottom of the part a as to cause shallow water above it : immediately, 

 or soon after, the mollusca capable of existing only in deep water 

 would perish and become extinct in that part, others fitted for shallow 

 water would begin to prevail, and newly-directed currents, caused by 

 the altered form of the land, might bring other species, and the remains 

 of these several new species would, in their turn, be inclosed in the 

 deposits going on in the shallow sea. Let us suppose the number of 



