96 NATURAL SCIENCE. Feb., 



of which never occur in any succeeding part of the course. To the 

 south of the Gulf Stream the composition of the Medusa Plankton 

 changes at once, and we meet with another fauna, which remains the 

 same in its chief components till we pass into the North Equatorial 

 Current, which the expedition passed near the Cape Verde Islands to 

 the west of Africa, so that we might speak of a second district. The 

 leading forms in it are Liriope cerasiformis, Rhopolonema velatnm, and 

 Aglaura hemistoma. However, this district is not quite uniform ; in its 

 western part, from the Gulf Stream to the Bermudas, and to the 

 Sargasso Sea, we find, besides the three forms mentioned above, 

 several other characteristic species, which occur also in the Guinea, 

 and even in the South Equatorial Current, and which can only have 

 come there by the connection of the " circle currents"; while another 

 part of this second district, from the Sargasso Sea to the North 

 Equatorial Current, shows only the three leading forms in astonishing 

 equality. In a thivd district, namely, from the North Equatorial 

 Current through the Guinea Stream to the South Equatorial Current 

 (to the north of Ascension) the fauna is not such a definite one, 

 as we have forms there which occur in the second district 

 also, together with forms which we find only in these southern 

 currents. 



From Ascension, Hensen's expedition crossed the Atlantic to 

 Brazil (mouth of the river Amazon), and it is remarkable that in the 

 eastern part of this course some species appear which were not to 

 be found in the western part. Probably they are inhabitants of a colder 

 current coming from, and turning to, the southern temperate zone, and 

 are not able to flourish in the warm water of the equatorial currents. On 

 the other hand, we find in the western half some characteristic forms, 

 as, for example, Liriope catharinensis, which occur only in this limited 

 district. From Brazil homewards the ship crossed the third and 

 second districts again, and the same species were found at the corres- 

 ponding places. Thus we see positively that certain species are 

 confined to certain districts, and we are entitled to speak of the 

 existence of faunistic regions in the ocean. The temperature seems to 

 be an important factor for producing such limits, but what is of still 

 greater influence in determining them is the course of the great oceanic 

 currents, as is quite evident from a comparison of the map with the 

 facts mentioned above. By their effect, also, the wide distribution of 

 other forms, as Aglaura hemistoma, which occurs in the whole Atlantic 

 to the north of the Gulf Stream, can be explained. A further interes- 

 ting fact is the identity of some species of the middle (second) district 

 with the hypogenetic Medusae of the Mediterranean, and I can state 

 this identity with the greater certainty, since I have studied the 

 Mediterranean fauna throughout a whole year. 



In a former publication I made all these statements with great 

 reserve, as being of value in the first place for the Medusae only, and 

 I laid stress on the fact that especially the division of the ocean into 



