MUSEUM OF COMPARATIVE ZOOLOGY. 27 
proof, The later results are of course, as Haeckel says, in absolute 
contradiction to former experiences. Hence their great value, and the 
burden of proof is not upon the recent explorers, but upon those who 
build castles in the air based on the incorrect data obtained by the 
earlier expeditions. They have got to show that what they obtained 
from these great depths did not come from the upper belt of the pelagic 
fauna, from 250 or 300 fathoms in depth, up to the surface. 
Tun Prracıo Fauna AND THE “ NATIONAL" EXPEDITION. 
It is a great merit of Hensen’s ! explorations, that he was the first to 
see that the quantitative measurements of the pelagic fauna could only 
be reached by the examination of the contents of a vertical column of 
water, This, if examined from the point of its minimum density up- 
wards to the surface, will very nearly represent the total amount of life 
which lives in any given locality. We may by thus fishing upwards get 
at any locality practically all that is characteristic of it, whether the 
fauna has congregated at the surface or near it, or at any other part of 
the pelagic faunal belt, owing to the peculiar conditions of light, heat, 
wind, and weather. 
In the “National” expedition in charge of Hensen,? the Plankton 
fishing hauls were limited to depths of 200 to 400 meters, and, if I un- 
derstand the statements of Dr. Brandt® correctly, several hauls were 
made at the same spots to determine the vertical distribution of the 
pelagic organisms. A modification of the Petersen-Chun net was also 
used in the “ National” expedition, and according to Hensen and Brandt 
there are still found in very considerable depths between the surface and 
the bottom “noch Organismen leben allerdings sehr viel weniger als 
in den oberen, von Licht durchstrahlten Wasserschichten.” The num- 
ber of indviduals decreased very materially the deeper the net was sunk, 
and there was also a rapid decrease in the number of species. Only 
Copepods and certain Radiolarians (Phaeodari®) were found living at 
depths of 3,500 meters. Five casts from 3,500 to 2,000 meters contained 
nothing except Copepods and Phaeodarie. Nearer the surface, fron 
2,000 to 1,000 metres, Sagittæ were added to the above groups, besides 
1 Hensen, V., Ueber die Bestimmung des Plankton’s. Fünfter Bericht der Kom- 
mission zur wiss. Untersuchung der deutschen Meere in Kiel. 1887. 
2 Die Plankton Expedition von Victor Hensen, 1891. 
3 K. Brandt, Verhandl. d. Gesell. f. Erdkunde, Berlin, 1889, p. 615. Ueber die 
biologischen Untersuchungen der Plankton-Expedition. 
