GENERAL DISCUSSION. 11 



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 vjirious 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 tlie kind of bottom are considerable. At seven stations 

 deeper than 100 fathoms, down to 1132, the bed is marked "rocky;" ten 

 others, down to 782 flithoras, 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 



