34 8 NOTES AND COMMENTS [may 



The Bipolar Hypothesis. 



Professor D'Arcy W. Thompson has subjected the so-called " bipolar 

 hypothesis " to careful scrutiny and has found it wanting. In the 

 introduction to his paper (Proc. Roy. Soc. Edin. 1898, pp. 311-349) 

 lie points out that Theel was one of the first to suggest that a peculiar 

 likeness exists between the northern and southern extra-tropical faunas, 

 and particularly between those of the arctic and antarctic regions. The 

 suggestion was elaborated by Pfeffer, and has of late been dealt with 

 in great detail, and in relation to the antecedent causes that might 

 have led to such a phenomenon by Sir John Murray. Dr. A. E. 

 Ortmann and Professor C. Chun have both pronounced against it, the 

 latter believing that the occurrence of a small number of forms common 

 to far northern and far southern seas may be sufficiently accounted for 

 by the continuous distribution or gradual intermixture of forms in the 

 depths of the intervening oceans under present conditions, without our 

 needing to have recourse to an explanation of the phenomenon in the 

 different conditions of a former age. 



Professor Thompson's analysis " shows that of some ninety species 

 quoted by Murray as common to northern and southern localities while 

 absent from the intermediate zone, there are more than one-third in 

 which grave doubt as to their identification was expressed by the 

 original describers, or in which the identification has been doubted or 

 denied by later writers. 



" In somewhat more than another third the evidence of identity 

 is inconclusive or even inadmissible by reason of the nature of the 

 examination to which the specimens were subjected (as in the case of 

 the horny and calcareous sponges), or by reason of the small size of 

 the objects and lack of adequate marks of characterisation (as in the 

 case of the minute ostracod and molluscan shells). 



" Of the remaining forms about a dozen find their northern repre- 

 sentatives 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 parti- 

 cular geographical province, and their specific area of distribution has 

 in some cases been greatly extended since the date of their original 

 discovery." 



We have quoted the above paragraphs since the problem is one of 

 great interest and importance, and to show that the indictment of the 

 hypothesis is a heavy one. The author proceeds to point out that the 



