164
R. K. DELL
successfully float from one island to another. Rare though the dispersal
method must be, the chiton has achieved a very wide distribution.
Perhaps the ability to arrive in a new locality with a number of welldeveloped juvenile specimens is of more importance in achieving dispersal, than a floating larval stage.
It seems extremely unlikely that there is a single factor which will
explain why brooding of young is so common in Antarctic seas. Some
of the reasons may well be found in the past geological history of the
area.
VIII. BIPOLARITY
Hedgpeth (1970) stated, “ This perennial favourite of biogeographers dies hard ”. Unfortunately this will probably not be the
epitaph for the topic of bipolarity, the concept that some species occur
in Arctic and Antarctic waters as a kind of relict of a once widely dispersed fauna, which has been replaced everywhere except in the two
polar regions. The topic has captured the imagination of writers on
semi-popular biology and far too many scientists have devoted their
efforts to listing examples of bipolarity, or to providing explanations to
avoid the “ relict ” hypothesis. Peculiarly enough, now that the hypothesis itself has no adherents, bipolarity seems to have taken on an
importance of its own.
Hedgpeth (196984 reviewed the argument by D’Arcy Thompson
(1898) who had shown that even at that date the number of supposedly
bipolar species was comparatively insignificant in relation to the total
fauna, and had indicated that many of these species would be shown by
critical comparison to be different species. Both lines of argument are
even more apposite today. Groups in which bipolarity is still considered to occur are on the whole “ difficult ” groups systematically.
Thus Koltun (1969, 1970) recognized three bipolar species of sponges
from an Antarctic fauna of 300 Antarctic species. Amongst sipunculids
Edmonds (1969) rather cautiously accepted seven species as bipolar.
The priapulids have been considered to present good examples of bipolarity. Better collections and more critical systematics have shown
that priapulids are much more widely distributed in deep water, and
that most of the other “ species ” concerned differ in each polar area
(Lang, 1951 ; Murina and Starobogatov, 1961 ; van der Land, 1970).
There appear to be no bipolar species of brachiopods (Foster, 1969),
stylasterine corals, scleractinian corals, benthic molluscs, benthic
pycnogonids or benthic fishes. The identification of an anomuran crab
from the Antarctic as Paralomis spectabilis known previously from off
Greenland and Iceland (Birstein and Vinogradov, 1967) is one of the
R. K. DELL
successfully float from one island to another. Rare though the dispersal
method must be, the chiton has achieved a very wide distribution.
Perhaps the ability to arrive in a new locality with a number of welldeveloped juvenile specimens is of more importance in achieving dispersal, than a floating larval stage.
It seems extremely unlikely that there is a single factor which will
explain why brooding of young is so common in Antarctic seas. Some
of the reasons may well be found in the past geological history of the
area.
VIII. BIPOLARITY
Hedgpeth (1970) stated, “ This perennial favourite of biogeographers dies hard ”. Unfortunately this will probably not be the
epitaph for the topic of bipolarity, the concept that some species occur
in Arctic and Antarctic waters as a kind of relict of a once widely dispersed fauna, which has been replaced everywhere except in the two
polar regions. The topic has captured the imagination of writers on
semi-popular biology and far too many scientists have devoted their
efforts to listing examples of bipolarity, or to providing explanations to
avoid the “ relict ” hypothesis. Peculiarly enough, now that the hypothesis itself has no adherents, bipolarity seems to have taken on an
importance of its own.
Hedgpeth (196984 reviewed the argument by D’Arcy Thompson
(1898) who had shown that even at that date the number of supposedly
bipolar species was comparatively insignificant in relation to the total
fauna, and had indicated that many of these species would be shown by
critical comparison to be different species. Both lines of argument are
even more apposite today. Groups in which bipolarity is still considered to occur are on the whole “ difficult ” groups systematically.
Thus Koltun (1969, 1970) recognized three bipolar species of sponges
from an Antarctic fauna of 300 Antarctic species. Amongst sipunculids
Edmonds (1969) rather cautiously accepted seven species as bipolar.
The priapulids have been considered to present good examples of bipolarity. Better collections and more critical systematics have shown
that priapulids are much more widely distributed in deep water, and
that most of the other “ species ” concerned differ in each polar area
(Lang, 1951 ; Murina and Starobogatov, 1961 ; van der Land, 1970).
There appear to be no bipolar species of brachiopods (Foster, 1969),
stylasterine corals, scleractinian corals, benthic molluscs, benthic
pycnogonids or benthic fishes. The identification of an anomuran crab
from the Antarctic as Paralomis spectabilis known previously from off
Greenland and Iceland (Birstein and Vinogradov, 1967) is one of the