358 GEOGRAPHICAL DISTRIBUTION. 



a surface temperature of 65° (A. Agassiz, :06), and off Aguja Point, the lowest 

 surface temperature of the Cruise was noted, 64°. North of Aguja Point the 

 Humboldt Current is no longer traced as a definite current and the temperature 

 rapidly rises to that of the Panamic region. Westward and southwestward 

 from the axis of the stream the surface temperature rises steadily, though with 

 various anomalies, until 74° is reached at Easter Island, and 80°-81° north- 

 eastward of Manga Reva. 



Like most other ocean streams the Humboldt Current bears a very rich 

 pelagic fauna and flora with it (A. Agassiz, :06, p. 15; Bigelow, :09a, p. 222). 

 Indeed never, even in the Gulf Stream, have I seen a greater density of organic 

 life than daily met our notice while our work lay within the Humboldt Current. 

 This richness extends also to its westward flow, and to the general south-equa- 

 torial drift into which it merges. On the other hand southwest of the Current 

 there is an area as poor in all forms of life, bottom as well as pelagic, as the 

 Current is rich. The older voyagers, especially New England whale fishermen, 

 long ago observed the faunistic contrast here outlined (Maury, '55, pi. 9). 

 But it remained for the "Challenger," and especially for Alexander Agassiz to 

 bring it to the notice of zoologists, and to give it scientific standing. 



In plotting the distribution of the Siphonophores in the region explored 

 by the "Albatross" I was struck by the fact that several of the species which 

 were taken often enough to afford a fair estimate of their occurrence were taken 

 only sporadically, if at all, in the colder axial region of the Humboldt Current, 

 although they occurred with some regularity along our lines in the warmer 

 regions to the west.' The members of this group are Diphyopsis dispar, Galea- 

 laria monoica, G. australis, and Agalma akeni. These were taken respectively 

 at Stations 32, 13, 29, and 25. On the accompanying chart (Plate 30), on 

 which the occurrence of these four species is shown, it is seen that only one 

 record of any species was made to the eastward of the curve of 68° surface tem- 

 perature, and that all the captures lie considerably to the westward of the 

 region where the temperature at twenty-five fathoms is 67° or less." 



Most species of Siphonophores taken during the Expedition were captured 

 both in the cold waters of the Current, and in warmer waters. The records of 

 five of the commoner of these, Abylopsis tetragana, Bassia bassensis, Diphyes 

 appendiculata, Diphyes bojani, and Diphyapsis mitra, are shown on Plate 31. 



' For these curves, see explanation of the chart. 



^ I should point out that the small number of records on t lie passage Manga R(>va-Acapulco is due 

 to the fact that unfavorable weather interrupted our work (Agassiz, : 06, p. X). 



