GEOGRAPHICAL DISTRIBUTION. 225 



only .6° (A. Agassiz, : 06, p. 24). Furthermore, even these slight varia- 

 tions were fluctuating, and not in any way connected with the line of faunal 

 diminution. Another factor which must be considered is salinity. But it 

 is unlikely that this is any more important than temperature in the present 

 connection ; for although the density decreases with the increase in surface 

 temperature from the South American coast westward, this decrease is 

 extremely gradual and without any sudden changes. And it is well known 

 that many Medusae, for example Aurelia and Gonioneinus, can undergo much 

 greater fluctuations in salinity without suffering the slightest ill effects. 



The question of food supply is no doubt effective in checking the 

 increase of intermediate organisms in the barren area ; for the surface 

 fauna and flora are quantitatively much less abundant here than within the 

 Humboldt Current. But since the diminution in the food supply is gradual, 

 it cannot explain the much more sudden and extreme diminution of the 

 intermediate fauna. On the contrary, the abundance of dead animal and 

 plant detritus (belonging for the most part to surface forms) taken in the 

 intermediate nets far beyond the limit of abundance of the intermediate 

 fauna, compared with the absence of such material within the Current where 

 the latter is abundant, suggests in a striking way that it is not scarcity of 

 food that limits the effective barrier, but that the food supply outside the 

 Current is more than sufficient for the few intermediate organisms which 

 occur there. 



It has been suggested to me that variations in the degree of oxygenation 

 of the sea-water might be a more important factor in the ecology of the in- 

 termediate fauna than has generally been supposed. Our actual knowledge 

 of the gaseous content of the deeper waters of truly oceanic areas is, on 

 account of the difficulty of working on shipboard, still so limited that the 

 suggestion may be worth consideration. On theoretic grounds it would seem 

 not unlikely that vertical circulation must be much more active in regions 

 where the water is in constant horizontal motion, on account of the much 

 greater disturbance of the surface layer caused by storms in such regions ; 

 and that such overturning must favor the absorption of oxygen in oceanic 

 regions just as it does in fresh-water rivers. It is most unlikely, however, 

 that the slight fluctuations which might thus be produced could influence the 

 distribution of Medusae when we consider what extensive changes in oxy- 

 genation several genera of Medusae undergo in their normal habitat. 



One of the more important features of the collection is the support which 



15 



