GEOGRAPHICAL DISTRIBUTION. 379 



Diphyes ardica has a considerable vertical range, and as already pointed 

 out (p. 369), it follows down the isothermobath most favorable to it in regions 

 where the surface waters are too warm. But there seems to be a limit to this 

 vertical extension, because the species has never been taken at any depth south 

 of 57° N. latitude, in spite of the many deep hauls which have now been made in 

 various parts of the Atlantic. Were it independent of every physical factor 

 except temperature, it would find a favorable environment at some depth in all 

 latitudes. But since it is not found in tropical regions at any depth, there must 

 be some other factor limiting its dispersal. I have elsewhere (Bigelow, : 09a) 

 suggested the possibihty that the absence or presence of light may be a factor 

 in determining the vertical range of holoplanktonic coelenterates, and Hjort 

 (:11) has demonstrated that light is undoubtedly the factor governing the 

 vertical range of the fishes of intermediate waters. Thus there is a group limited 

 to darkness, and one occurring in the zone of very faint light, which show charac- 

 teristic color differences. And, as I have pointed out (:lla), there is some 

 evidence that the Medusae of intermediate waters can be divided into two 

 corresponding groups, the iridescent — slightly pigmented forms occurring 

 above the deeply pigmented red or brown species. Now that light has been 

 shown to be so important in relation to the vertical distribution of other groups 

 of pelagic animals, it would not be surprising to find it occupying a similar role 

 in the case of the Siphonophores. We might suppose that D. ardica is positively 

 phototropic to light above a certain intensity." Were this the case, being other- 

 wise limited in its dispersal by heat, it would occupy the zone of most favorable 

 temperature, i. e. 35°-45°, from the Arctic regions southward to the point where 

 the latter sank below the level of appreciable 'light. But it would not extend 

 further toward the tropics, because to follow its temperature downward would 

 lead into regions when there is too little light for its ecologic needs. We could 

 thus account for its occupancy of a zone from the surface downward for a con- 

 siderable depth in the Greenland Sea; its absence from the surface further to 

 the south, and its entire disappearance south of 57° N. It is true that Damas 

 and Koefoed's ( : 07) records if taken at their face value, indicate a habitat below 

 400 fathoms. But as I have pointed out (p. 377) it is doubtful whether any of 

 the specimens credited by them to 800 fathoms actually came from that depth. 

 Whether or not light is the active factor can be determined only by actual 

 experiment on the light reactions of D. ardica, which should not be a difficult 

 task. Perhaps my suggestion may lead some Norwegian student to under- 

 take it. 



