THE DEEP-SEA FAUNA. 167 
as has been suggested by Dawson, that there was a time when 
the arctic regions had a climate similar to that of some of our 
Southern States, like North and South Carolina, and during six 
months were subject to the active light and heat of an arctic 
summer. We may perhaps thus form an idea of the immense 
growth and development which animal life must then have 
known. 
The deep-sea fauna in the track of oceanic currents was 
remarkably uniform, and when they changed, new typés must 
have driven out the old ones. To the action of an equatorial 
current flowing without interruption where the Isthmus of Pan- 
ama now intervenes, we ascribe the marked generic affinities 
found to-day in the deep-sea fauna of both sides of that isth- 
mus; this resemblance of the deep-sea types carries us back to 
the cretaceous period when the equatorial current found its way 
between the islands of the huge archipelago oceupying northern 
South America, and a part of Central America and southern 
Mexico. 
We cannot fail to be struck with the narrow limits of temper- 
ature which characterize most distinct faunal regions, nor can 
we fail, while speculating on the course of oceanic currents in 
former geological periods, to note the simple causes which ex- 
plain satisfactorily the migration of an ancient flora and fauna to 
totally different realms. We need. not seek for cosmic changes, 
when such a simple cause as the formation of a barrier to deflect 
an oceanic current explains the disappearance of climatic con- 
ditions at any given point, and their reappearance in some other 
region of the globe. 
Since geologists have begun to compare the deposits of past 
periods with those of the depths of the sea, as we now know 
them, they have seen how impossible it is to establish from fossils 
alone the synchronisms of distant beds. The nature of the de- 
posits, the distance from shore, in fact, the geographical and 
physical conditions of former times, influenced the marine fauna 
of past ages as much as that of to-day. There is nothing im- 
probable in the synchronism of foraminiferous beds with clay 
and shale deposits in which lived faune having nothing in 
common, while these beds themselves had a radically different 
facies. 
