584 THE AMERICAN NATURALIST. [VoL. XXXIII. 
the discussion, and at present we possess the results of a num- 
ber of investigations in special groups of animals, dealing with 
the relations of the Arctic and Antarctic faunas. All the 
results obtained tend to show that the original contention of 
the writer, derived from a study of the decapods, is fully sup- 
ported by the facts found among other groups, so that the 
theory held by Pfeffer and Murray, that both polar faunas are 
more closely related to each other than to any of the inter- 
mediate ones, is without support. In fact, there have been 
added very few cases of bipolarity of genera to the single case 
established by the writer in 1895,! and one case of a bipolar 
species, discovered by Chun, is to be looked upon with some 
suspicion, for he himself suggests an explanation. All this 
points to the confirmation of the opinion of the writer, that, 
although some cases of bipolarity may exist, such cases are 
extremely rare, and may be explained in one of the ways indi- 
cated in his paper of 1896, while the greater part of each polar 
fauna consists of peculiar forms, which show no closer connec- 
tion with each other than with forms found in tropical latitudes. 
We will now review the chief results of the papers recently 
published, and then draw conclusions as to the theory of bipo- 
larity. However, it is well to observe that some of the authors 
referred to did not give all the necessary data. Especially is 
there a lack of information as to the distribution of genera 
found in both polar seas, outside of their polar range. The 
writer has tried to supply this deficiency by consulting other 
papers, but as he cannot claim to be a specialist in the respec- 
tive groups, he has in some cases failed. Further, a misunder- 
standing seems to exist on the part of some authors as regards 
the term “‘bipolarity’’; they sometimes call a species or genus 
that is found in both polar areas bipolar, although it is also 
present in intermediate localities. But “ bipolarity,” as under- 
stood by Pfeffer and Murray and the present writer, implies 
that a bipolar form is wanting in the intermediate tropical parts 
of the seas, and the chief difficulty in the discussion is the 
explanation of such cases of discontinuity in distribution. 
The first paper to be discussed is an investigation of the 
1 Proc. Acad. Philad. (1895), pp. 189-197. 
