168 
THE GEOLOGICAL MIDDLE AGE. 
within distinct zoological provinces, as at the 
present time. The Fishes of Massachusetts Bay 
are not the same as those of Chesapeake Bay, nor 
those of Chesapeake Bay the same as those of 
Pamlico Sound, nor those of Pamlico Sound the 
same as those of the Florida coast. This division 
of the surface of the earth into given areas within 
which certain combinations of animals and plants 
are confined is not peculiar to the present crea- 
tion, but has prevailed in all times, though with 
ever-increasing diversity, as the surface of the 
earth itself assumed a greater variety of climatic 
conditions. D'Orbigny and others were mistaken 
in assuming that faunal differences have been in- 
troduced only in the last geological epochs. Be- 
sides these adjoining zoological faunas, each epoch 
is divided, as we have seen, into a number of 
periods, occupying successive levels one above an- 
other, and differing specifically from each other 
in time as zoological provinces differ from each 
other in space. In short, every epoch is to be 
looked upon from two points of view : as a unit, 
complete in itself, having one character through- 
out, and as a stage in the progressive history of 
the world, forming part of an organic whole. 
As the Jurassic epoch was ushered in by the 
upheaval of the Jura, so its close was marked by 
the upheaval of that system of mountains called 
