GENERAL DISCUSSION. 13 



Ao"assiz, which, with perhaps the exception of a hinited space at the lower 

 edge of the upper and another at the upper edge of the lower belts of light 

 servino- as retreats and hiding-places, is comparatively uninhabited and 

 deserted, except as crossed to and fro by bathybial species of vertical 

 derivation, from a pelagic ancestry, Scopeloid or other, many of them 

 provided with lanterns, flash lights, or other luminous organs to prevent 

 mates or individuals of a school from losing one another, or witli light 

 oro-ans to lure the prey. The size and development of the visual organs 

 and the rarity of blind forms among deep sea fishes are further evidences 

 in favor of the existence of a bathybial light. 



As the surface waters of the Panamic area are warmer than those of 

 higher latitudes so the waters of the bottom in the tropics are higher in 

 temperature than those to the north or to the southward. From the sur- 

 face downward the temperature lowers with tolerable regularity as the 

 depth increases. Approximately the warmth at 100 fathoms is 56' Fahren- 

 heit, that at 300 fathoms is 46', that at 500 fathoms is 41°, that at 1000 

 fathoms is 37°, and that at 1800 fathoms is 36°. The lowest temperature 

 for the collection, 35.8 F., was noted at 1772 and at 2232 fathoms; 36° was 

 found at depths of 1322 to 1879 fathoms ; 36.2° to 36.8° were recorded for 

 depths of 1020 to 1823 fathoms ; and 37° was taken at various depths from 

 919 to 1588 fathoms. At the same depth in different stations the tempera- 

 tures are higher or lower, conditions that would cause bathybial currents 

 and which must be regarded as proof of their existence. No doubt there 

 is an annual rise and fall of temperature at the bottom, however deep, but 

 that it is sufficient to hold the fishes to the annual .spawning periods of 

 their ancestors in the long continued presence of the retarding effects of 

 very low temperatures is somewhat doubtful. Farther from the tropics the 

 annual variation of temperature at the bottom is much greater, but in those 

 localities the greater retardation effected by still lower temperatures must 

 also be considered. Fishes abound in the lowest temperatures taken in 

 the Panamic region; no less than thirty genera were found in temperatures 

 of 36° F. or lower. The lowest temperatures that can be endured by 

 fishes is not yet definitely determined, but there is no doubt of their 

 ability to exist in those of less than 32° F. The Norwegian steamer 

 '•■ Yuringen " secured specimens of Mydoplmm Mi'dleri Gme]. in lat. 71° 

 59' N., long, ir 40' W., at a depth of 1110 fathoms in a temperature of 

 29.7° F. ; the English steamer " Research " reported the same species from 



