1898.] Pii-UfKroN OF the paeeoe chakitel. 575 



descends to a comparatively small depth, i. e. about 200 fathoms, 

 the limits of the depth at which light and heat produce any con- 

 siderable variation in the physical conditions of the water. The 

 marked diminution in the number of species below 200 fathoms 

 agrees fairly with the results of the ' National ' Expedition." 



The other experiments with the Tanner net, made in an oceanic 

 basin on the way to Acapulco from the Galapagos, and to the 

 Galapagos from Cape San JFrancisco, " seem to prove conclusively 

 that in the open sea, even when close to the land, the surface 

 pelagic fauna does not descend far beyond a depth of 200 fathoms, 

 and that there is no intermediate pelagic fauna living between 

 that depth and the bottom, and that even the free-swimming 

 bottom-species do not rise to any great distance, as we found no 

 trace of anything within 60 fathoms from the bottom, where it 

 had been fairly populated." 



Prof. Agassiz therefore admits the existence of a deep Meso- 

 plankton near land, but does not state how far from land and in 

 what depth of water his generalization of an Azoic zone begins to 

 hold good. I do not know of any later pronouncement by this 

 eminent oceanographer on the question. Since theu, Captain 

 Tanner has improved his original pattern in detail \ but the prin- 

 ciple of his net remains the same. It is rash, and perhaps a little 

 ungracious, to criticize the working of a net which one has never 

 seen ; but I venture to suggest, on the basis of the drawings and 

 description of the Tanner nets, that a weak point in them is the 

 way in which the tripping lines are suspended ; it seems that it 

 would be so very easy for them to slip oS from the tumbler and 

 close the net before they were intended to do so, under the alter- 

 nate strain and slackening of the warp as the ship rolls ; it also 

 seems likely, and indeed Captain Tanner himself admits, that the 

 angle made sometimes by the net-frame iu turning would practically 

 close the net's mouth. As regards the Sigsbee gravitating trap, 

 there can, I think, be little doubt that it was too small and too 

 violent to throw much light on the question of an Azoic zone. 



Conclusions of the ' Clialhnger^ and other Naturalists: 

 the MesoplanTcton. 



Prof. Agassiz may be regarded as the chief representative of the 

 school of nafcuralists which refuses to accept the alleged existence 

 of a Mesoplankton. The chief supporters of the opposite "saew 

 are the ' Challenger ' naturalists (a distinguished band, of whom 

 Sir John Murray is alone left). Prof. Chun, and Profs. Hensen 

 and Brandt of the ' jS"ational ' staff. 



The ' Challenger ' naturalists arrived at their beUef from a com- 

 parison of serial tow-nets, stopped at intervals along the dredge- 

 rope. As all the tow-nets were open throughout their course, the 

 presence of particular species in the deap nets only seemed to 

 indicate that these species occurred in the deep water only. The 



* Z. L. Tanner, Bull. U.S. Fish Commission, xiv. p. 148. 



