No. 391.] BIPOLARITY OF MARINE FAUNAS. 589 



along the western coast of America to 22 o N. L., but there it 

 has been dredged at greater depths (1 200-1400 m.). Although 

 no species of this group has yet been found in the northern 

 Pacific, it is very probable, nevertheless, that such species do 

 exist. In that case we would here have again an instance of a 

 connection, along the western coast of America, of an apparently 

 bipolar group of marine animals. Examples of this kind have 

 been pointed out, by the present writer, among the decapods. 



Of the 6 ophiuroid genera common to both the polar seas, 4 

 (Ophioglypha, Ophiactis, Amphiura, Ophiacantha)are also repre- 

 sented in the littoral of the tropics. The 2 remaining genera 

 (Ophiocten, Gorgonocephalus) are chiefly abyssal genera, and, 

 as far as the writer could determine, Ludwig does not give any 

 information — at least, Ophiocten is also found in tropical 

 latitudes. 



Thus in these three groups we have again the same result : 

 that bipolarity, if present at all, is extremely rare, and that the 

 most prominent feature of the respective faunas of the polar 

 seas consists in their dissimilarity. 



Yet Ludwig calls attention to a certain general likeness of 

 both faunas, expressed by the mutual prevalence of certain 

 genera and the mutual lack of others as compared with the 

 tropic faunas. This is not to be regarded at all as a remark- 

 able fact, and has no connection with the question under dis- 

 cussion ; indeed, it would be very strange if other conditions 

 prevailed. When, out of a number of genera present in the 

 tropics, a certain number disappears as we approach either 

 pole, while a certain number does not, this shows only that 

 the latter are not affected by the change of conditions, — chiefly 

 climatic, — while the former are, and of course by the disappear- 

 ance of a number of types the percentage of the remaining must 

 increase, if the deficiency is not made up by other genera 

 making their appearance in the colder regions. This is again 

 an instance where statistics give a wrong idea of the true con- 

 ditions; the increase of the percentage of certain genera in the 

 polar seas is not due to an actual increase of species and a more 

 vigorous development, but only to the lack of species of other 

 genera. 



