348 Proceedings of Royal Society of Edinburgh. [sess. 
terisation (as in the case of the minute Ostracod and Molluscan 
shells). 
Of the remaining forms, about a dozen find their northern 
representatives in the Japanese seas, where they form part of a 
fauna predominantly southern in its relations, and where at least 
the occurrence of any particular form cannot be taken, ipso facto, 
as evidence of a boreal centre of distribution. 
Both these last forms and the remnant of equal number that 
are quoted as occurring in the North Atlantic as well as in or near 
the Southern Ocean, are for the most part deep-water species, and 
have in a large proportion of cases peculiar characters of their own. 
We cannot say at present that they are forms characteristic of any 
particular geographical province, and their specific area of distribu- 
tion has in some cases been greatly extended since the date of their 
original discovery. 
Turning, in conclusion, to particular groups, we find the bipolar 
hypothesis specifically rejected by Prof. Herdman in the case of 
the Tunicata, by Prof. Ludwig in that of the Holothurians, and by 
Dr Ortmann in that of the Crustacea ; and (to limit ourselves to 
the groups that we have particularly discussed) we have found no 
single species of fish, of Decapod, of Isopod, no certain one out of 
a large fauna of Amphipods, to inhabit at once the Arctic and 
Antarctic Oceans, or the regions adjacent thereto. 
Before leaving this subject in the meantime, it is proper to admit 
that the question of specific identity between Arctic or sub- Arctic 
and Antarctic or sub- Antarctic forms is not the only evidence, and 
not necessarily the most important evidence, by which to decide 
the truth of the “bipolar hypothesis.” In the distribution of the 
land mammalia, the existence of a northern circumpolar or circum- 
terrestrial region is unaffected by questions of the specific identity 
of, among many other examples, the European and American 
bison, heaver, elk, or reindeer. And though, on the one hand, the 
separate existence in the Arctic and Antarctic of one or more well- 
marked types of fish or higher invertebrates, of whose specific 
identity there could be no question, would be the simplest proof of 
a sound basis in the bipolar theory, yet, without any such cases of 
specific identity, the theory might still find firm support if it could 
be shown that similar and truly allied forms gave to the two regions 
