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 Liviope cerasiformis, Rhopolonema velatum, and 
Aglaura henustoma. 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 third 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 ableto 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, Liviope cathavinensis, 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 A glauva 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 Meduse 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 Meduse only, and 
I laid stress on the fact that especially the division of the ocean into 
