GENERAL DISCUSSION. Ii 
differentiation from identical species on the two sides, there was a strait of 
moderate depth across the isthmus that favored the passage from the Carib- 
bean to the Pacific, with the current, of species living near the surface. 
This conclusion is reached from the collection, and independently of nearly a 
hundred species asserted with more or less confidence by various authors 
to be identical in Pacific and Atlantic. Subsequent examination of these 
so-called identical species shows that many of them are yet to be compared 
and accurately determined, and that those which probably are identical are 
pelagic and errant types, at home in all the oceans. 
The portion of the Pacific to which this report is confined lies within that 
designated by Agassiz as the Panamic region. Lying between the equator 
and the tropic of Cancer it receives the greatest amount of sunlight and 
heat. It is traversed by the Mexican coast current; it includes the eastern 
extremities of the north equatorial and the north equatorial counter currents, 
and also the northern extremity or efflux of the great Peruvian current. 
The meeting place of all these currents, over a diversified bottom, these 
waters swarm with living organisms and form an ideal locality for the 
ichthyologist. 
The variations in the kind of bottom are considerable. At seven stations 
deeper than 100 fathoms, down to 1152, the bed is marked “rocky;” ten 
others, down to 782 fathoms, are “sandy ;” “hard” bottom (Rhabdamina) 
occurred at several stations with depths ranging from 385 to 918 fathoms; 
at numerous points, with depths from 238 fathoms to 1879, “ Globigerina 
Ooze” was found ; for four locations, in depths of 1471 to 1823 fathoms, 
“Green Ooze” was recorded; and “Green Mud” formed the bottom at 
many places in depths of 85 to 2232 fathoms. Below a thousand fathoms 
of depth the ooze and mud prevail, and rocky and sandy bottoms are 
exceptional. 
The sunlight, striking the surface more directly, penetrates deeper in the 
“Panamic region” than in higher latitudes where it meets the water more 
obliquely. Judging from the fishes, the light must reach depths of nearly or 
quite 200 fathoms. Below these, at the bottom, in the greater depths, there 
is another light, the so-called phosphorescent, due in part to the organic life 
and probably in part to chemical action and reaction, which latter may aid 
the low temperature and the enormous pressure in retarding the decay and 
destruction of organic tissues, whether living or dead, and which possibly 
to some extent may do away with the necessity of so much oxygen, even 
