GENERAL DISCUSSION. 13 
Agassiz, which, with perhaps the exception of a limited space at the lower 
edge of the upper and another at the upper edge of the lower belts of light 
serving 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 with light 
organs 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 2252 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 
“ Viringen” secured specimens of MMyctophum Miilleri Gmel. in lat. 71° 
59’ N., long. 11° 40’ W., at a depth of 1110 fathoms in a temperature of 
29.7° F.; the English steamer “ Research ” reported the same species from 
