94 NATURAL SCIENCE. Feb.. 



Dana was one of the first to distinguish such geographical 

 faunistic provinces in the open sea by drawing thermic (or, as he 

 called them "isocrymal") lines on the map, which did not quite 

 correspond to the lines of geographical latitude. The observations 

 of temperature were made on shore, and the empirical data were not 

 very large, but he has taken into consideration the " modifying prin- 

 ciples," currents, &c., and has deduced some good general concep- 

 tions for his work on Crustaceans, the distribution of which had led 

 him to his ideas. 



As another interesting attempt in this regard, we may mention 

 the account given by Brandt in his monograph of the Radiolarians 

 of the great oceanic currents in the Atlantic, which he supposes 

 to be the most important factors in the distribution of animal 

 life in the ocean. It is remarkable that he considers them as 

 " circle streams," by the direction of which a large amount of 

 pelagic animals are always kept within certain limits, and only very 

 few of them can be carried away by the side branches of the currents 

 to other currents, temperatures, &c., where they probably perish, 

 while the majority remain under their regular life-conditions. But 

 the attempts to divide the ocean into faunistic districts are very few, 

 whether it is that no such limits have been believed to exist at all, 

 or that our empirical knowledge has not been large enough till now 

 to allow conclusions ; and we have to wait for the results of many 

 explorations till we can obtain an idea about these complicated 

 relations. 



The Plankton Expedition, limited as it was in extent, has given 

 us some insight at least into a certain part of the ocean, and we may 

 draw conclusions from it with comparative security, since the fishing- 

 stations were fairly close together, and since improved methods were 

 employed, such as, for example, the ordinary vertical net going down 

 always to a depth of 400 m. in order to catch the animals which rise 

 and sink to avoid the changing influences of the surface at certain 

 times. The results which I have obtained in the Medusas have con- 

 firmed my conviction that we can distinguish in the ocean certain 

 districts of horizontal distribution. 



The Craspedote Medusae can be divided, as is well known, into 

 Leptolina or metagenetic forms, wliich are derived from a polypoid 

 stage, and Tvachylina, or forms with a direct (hypogenetic) develop- 

 ment through a free swimming planula and actmula stage. As might 

 be expected from the places of capture, chiefiy lying in the open sea, 

 where polyps have scarcely any chance of flourishing, the Trachylina 

 form by far the majority of the Plankton Medusae, and the few 

 Leptolina which have been discovered always show a relation to 

 the coast. It might be interesting to control this advance, as the 

 result of life in the open sea, by a comparison with other groups, 

 which also have a sessile stage in their life-history. The Narco- 

 medusae among the Trachylina being acknowledged by all compe- 



