234 THE MEDUSAE. 



more than 250 fathoms; and that light below 100 fathoms must be very 

 faint indeed, — a result in entire accordance with the vertical range of 

 littoral Algae. 



Since the intermediate Medusae do not normally come to the surface in 

 cold any more than in warm regions, it is probable that it is not the in- 

 crease in temperature, but light, which is the more important factor of the 

 two in limiting their dispersal. Whether, however, the region most favorable 

 for them is that of absolute darkness, or whether they, like some copepods, 

 are positively phototropic to light of faint intensity, while negatively so to 

 darkness on the one hand and brilliant light on the other, cannot be answered 

 until experiments have been made with this end in view. But in considera- 

 tion of the fact that captures in closing nets from greater depths than 300 

 fathoms have several times been recorded, I believe that the former sug- 

 gestion is the correct one, and that the normal upper limit to the vertical 

 range of these species will eventually be found to coincide approximately 

 with the lower limit to whicli the sun's rays can penetrate with appreciable 

 strength. This, however, by no means indicates that their upper limit is 

 necessarily as deep as 250 fathoms. On the contrary, it is well known that 

 the penetration of light depends very largely on the clearness of the water; 

 and in a region with such a rich fauna and flora as the Humboldt Current the 

 lower limit of appreciable light may be expected to lie at a much smaller 

 depth than in the clearer waters where it has been tested. Judging from 

 the fluctuations in the penetration of light, the upper limit of the intermedi- 

 ate Medusae probably differs greatly in different regions, being nearest the 

 surface in the courses of the densely inhabited ocean currents, and deeper in 

 regions where pelagic life is less dense and the water consequently clearer. 



Besides the actual evidence afforded by the " Albatross " captures, there 

 is strong theoretic ground, judging from the nature of their food supply, for 

 expecting this group of organisms to be most abundant near the upper limit 

 of their vertical range. In the last resort, sustenance, for the intermediate 

 fauna as well as for the abyssal bottom fauna, must be derived indirectly or 

 directly from the pelagic flora of the surface and of slight depths ; interme- 

 diate as well as bottom animals must largely depend for their food on sinking 

 detritus. It would then be only natural to expect them to congregate and 

 multiply in greatest abundance in close proximity to this supply ; that is, 

 near or at the upper limit of their range. This consideration seems to me 

 of very great significance. 



