BIPOLARITY 257 



been adopted by the Challenger Society for biblio- 

 graphical work, and is reprinted here as Chart VIII. 



The theory, or rather dogma, of bipolarity (that the 

 arctic and antarctic faunae are more closely allied to 

 each other than they are to the inhabitants of the more 

 equatorial waters) is based on the fact that identical or 

 closely allied forms have been found in both north and 

 south Polar seas, but have not been reported from the 

 intervening waters. The number of undoubted cases 

 of this is not large, and most of them may perhaps be 

 explained as being instances of species which belong 

 to a widespread deep-water fauna, which have been 

 taken at the Poles at moderate depths, but have escaped 

 notice in the abysses of the warmer regions where our 

 knowledge of the deep-sea fauna is very limited. 



The local distribution of the fauna within the 

 narrower limits of each region appears to depend 

 largely on the varying character of the sea bottom. 

 Within the limits of the zone which we are considering 

 (from the lower limit of the Laminarian Zone to the 

 edge of the Continental Shelf) it may be said that the 

 differences of fauna due to depth are insignificant, as 

 compared with those associated with the different kinds 

 of bottom, and therefore it is impossible to attach 

 importance to the subdivisions of this zone which have 

 been proposed. The types of sea bottom usually met 

 with may be arranged in a descending series beginning 

 with rock and passing through stones, gravel, sand and 

 gravel, pure sand and muddy sand, to mud, varied by 

 tho occurrence of beds of dead shells or of the calcareous 

 alga usually known as " coral " (Lithothamnian) . In 

 any given district it is usual to find that each of the 

 above-named types of bottom has its own special 

 fauna, and that many species seem to recognize a more 

 minute subdivision than man, whose observations are 



