of the, Arctic and (he Antarctic Faunas. 317 



■ue CflTUiot say ^vllc■tll('r it has nii<j;rntcd from tlic north or from 

 the soutli ; but as ovcry migration demands time, we can assume 

 with some proLability tliat those occurring in the temperate 

 zone of the northern licmisphcre have come from the nortIi,iind 

 those in the soutliern from the south. But if it were tlic case 

 that the time which a species requires to migrate over the 

 whole deep sea from one ])oh\r zone to the other were trifling 

 in comparison with the length of its sojourn in the deep sea, 

 one could no longer say that an example found near Scotland 

 came from the north, and one found near South Georgia came 

 from the south. But this is not at all how matters stand. 

 ]\Iurray has compiled exact statistics of distribution for the 

 Kerguelen region ; I myself have extended these for the 

 whole earth, though they are still far from being complete. 

 But one thing seems fairly well established, that practically 

 all the unipolar surface and subsurface animals of the higher 

 north and south, which descended into the deep sea, have 

 penetrated to the borders of the tro))ics or into the tropical 

 zone, but not beyond it into the opposite hemisphere. An 

 example known to most zoologists is furnished by the genus 

 Serolis, of which many species are developed in the notal 

 surface-water, and a still greater number in the deep sea, yet 

 its range, apparently, does not extend beyond the equator. 



It would seem, therefore, that the time which has elapsed 

 since the present surface-water species of the higher north 

 and south descended to the depths has not sufficed for a 

 migration beyond the equator to the opposite hemisphere; 

 the excej)tions to this rule disappear almost wholly, if not 

 wholly, on closer consideration, although for certain species 

 of Sponges, Worms, and Bryozoa we must assume an age 

 extending beyond the middle Tertiary period — and this is in 

 no way at variance with the facts. 



Subsurface- Fauna. 



We have now to deal in a few words with the subsurface, in 

 the same way as we have dealt with the deeper water. We 

 know that in Early Tertiary times a universally homogeneous 

 fauna extended over the tropics and the temperate zones. 

 Thus the similar species of north and south had a continuous 

 connexion through the tropical zone. This continuity through 

 the tro])ical zone was probably kept up in part through the 

 deep water. As within the tropical zone at the present day, the 

 fauna of northern habit is found exclusively in the subsurface- 

 water (we shall have to consider later the peculiar conditions 

 of Western America), nothing is more obvious than that there 

 Ann. ct- il%. N. Hist. Ser. 7. Vol. vii. 22 



