248 Mr. J. W. Fewkes on Deep-sea Medusce. 



same as that found at the surface, or is it characteristic ? 

 Can the animals which compose it be circumscribed in bathy- 

 metrical zones, out of which they cannot pass with impunity ? 

 Do we, in short, have in the nomadic oceanic life a cliange of 

 fauna as we sink below the surface '? 



Naturalists have been led to suppose that since we find 

 peculiar modifications in animals living upon the sea-bottom 

 at great depths we should necessarily look for the same 

 variation among nomadic animals at intermediate depths. It 

 would then seem probable that there are bathy metrical zones 

 for free-swimming animals, and that these animals are cha- 

 racteristic as compared with others which live at tlie surface. 

 An investigation of the character of this fauna, if such there 

 be, has an interest to the evolutionist, for it might be sup- 

 posed to acquaint him with facts bearing on the general 

 characters of the ancestors of certain genera of surface- life. 



I can imagine few places on the earth's surface where the 

 uniformity of physical conditions is greater than in the depths 

 of the sea. I do not mean, as might be supposed, necessarily 

 on the floor of the ocean, but at the depth of say 1000 fathoms 

 separated from the ocean-bed by a wall of water of the same 

 depth. Here, if anywhere, we may look for uniformity of 

 conditions, and if environment has anything to do with modi- 

 fications in the generic forms of animal life, here we may 

 expect to discover animals which preserve ancestral features. 

 On the surface of the ocean there are changes of temperature 

 and of light and climatic variations ; at the floor of the ocean 

 there may be reactions of the interior of the earth upon its 

 crust, perhaps lava-flows or geological oscillations * ; but 

 midway between these two places, equally removed from 

 both, disturbing causes only rarely penetrate, and conditions 

 remain more constant year by year. May we not expect to 

 find here a corresponding uniformity in the fauna as compared 

 either with the highly organized animals of the surface or 

 with those of the depths of the ocean ? Is that fauna more 

 uniform than any other in the ocean ? 



No group of animals is better suited for a study of the 

 questions which suggest themselves concerning the bathy- 

 metrical zones of characteristic animals, free-swimming at 

 different depths in the ocean, than the Medusas. The group 

 is a large and very variable one. It is confined, with but 

 few exceptions, to the ocean. Moreover, it is probable that 

 its ancestors were oceanic animals. No group of marine 



* Sucb changes might take place even if the oceaus have practically 

 been the same in past geologic times as at present. 



