THE DEEP-SEA FAUNA. 167 



as has been suggested by Dawson, that there was a time when 

 the arctic regions had a cUmate similar to that of some of our 

 Southern States, Hke North and South CaroHna, and during six 

 months were subject to the active Hght 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 types 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 occupying 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 faunae having nothing in 

 common, while these beds themselves had a radically different 

 facies. 



