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, 1 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. 



