OF THE KERGUELEN REGION OF THE GREAT SOUTHERN OCEAN. 493 



of the tropical belt, and then to other Fucacece again in the south temperate and 

 Antarctic seas." ^ In the paper the}' give a list of fifty-four species common to the 

 Northern and Southern Oceans but not occurring within the tropics. It has sometimes 

 been supposed that the shallow-water marine animals may pass by way of the deep sea 

 from high northern to high southern latitudes, but this explanation would in no way 

 apply to sea-weeds which can live only in the shallow-waters of the sea. Mr George 

 Murray informs me that the abundance of calcareous incrustation on marine Algse 

 follows the same distribution as in the case of the lime-secreting animals, these incrusta- 

 tions being much more abundant within the tropics ; he says : — " No Siphonece with 

 encrustation of calcium carbonate occur in the Arctic or Antarctic Seas, or for the 

 matter of that in the colder temperate seas. The Corallincco, which are massively 

 encrusted and occur in great abundance of individuals in the tropics, exhibit a pro- 

 gressive diminution in both mass and number towards the colder areas of the sea. 

 While several genera found in the tropics and temperate seas are absent from the polar 

 seas, the Corallinece are yet represented by four genera in the Arctic Sea. The ten 

 species of Lithothamnion recorded from the Arctic Sea would indicate at first sight a 

 high degree of representation of the most massively encrusted genus. But these ten 

 species are notoriously of insecure foundation, and really only form one species in the 

 opinion of some phycologists. In addition to the four Arctic genera of Corallinece 

 two others are recorded (six in all) from the Antarctic region as delimited in the 

 paper quoted. But this again is accounted for l)v the line of delimitation being too 

 far north and including an area properly soutli temperate in character. IMost other 

 encrusted Floridece are confined to tropical or warm seas. Taking marine Algae with 

 calcium carbonate encrustation as a whole, it is undoubtedh- the case that they diminish 

 both in numbers of species and individuals, and in massiveness of encrustation towards 

 the polar seas." 



Whoever may have read the foot-notes appended to the foregoing lists in this paper 

 must have been struck with the numerous instances in which an author was in doubt as 

 to whether a certain specimen should be described as a new species or referred to a known 

 species. The great geographical distance separating the spots where specimens were col- 

 lected is sometimes considered a sufficient reason for laying great stress on some slight 

 variation, and creating a new species ; this is especially the case with specimens from 

 high northern and high southern latitudes. Again, an author has often had a difficulty 

 in deciding whether to include a species in a known genus or to create a new genus for 

 its reception. The descriptions of the several authors very much impress one with the 

 great want of equivalence in the features of an organism which serve as specific and 

 generic characters in the diflerent groups of invertebrates. But for these considerations 

 the resemblances between the marine faunas of high northern and southern latitudes 

 would be much more evident than appears from the statistics in the foregoing pages with 

 reference to the species which have hitherto been recorded from the two polar areas. 



^ Phycological Memoirs of the British Museum, part iii. p. 8S, London, 1895. 



