61 () REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



Jlouiltf oscillations. — Many pelaj^ic animals ai)peai' at the surface 

 of the sea only at a iJrfinitc hour of the day, some in the moiiiing, 

 others at noou, still others toAvards eveuiiiii'. During the remainder 

 of the day not a single individual of the species is to be found. 

 Agassiz, thirty years ago, brought forward noticeable examples of 

 this from the <'lass of Medusa^ and 1 can froiu my own experience adduce 

 a number of other exami)les. But manj" other i)elagic animals also 

 (<'. (/., Siphonophores, Heteropods) come to the surface only for a few hours. 

 We have long known that the swarms of the uyctii)elagic Preropods, 

 Pijrosoma and many Crustacea, conn} to the surface only during the night 

 and flee the light of day. Other groui)s act just reversely. But the 

 late extensive observations, especially of Murray (6), Chierchia (8), 

 and Chun (15) have taught us how great is the extent and importance 

 of those hourly variations. That these are of great influence upon 

 the composition of the plankton, and that this accordingly is very 

 <liflerent at different times of day, needs n ) repetition. But we must 

 allude once more to how all these temporal oscillations must be taken 

 into consideration, if the equality of plankton distribution is to be 

 proved bj' observation and estimation. In point of fact they all seem 

 to tend to very remarkable inequality. 



C. — Climatic Plaxktox Difkekences. 



The numerous contributions which earlier and later observers have 

 made upon the appearance of the swarms of the pelagic animals in 

 different regions of the ocean, agree iu pointing out the differences 

 among them, corresponding to the climatic zones. Thus the Arctic 

 oceans are characterized by masses of monotonic plankton of Diatom, 

 Beroidw, Copepod, and Pteropod groups, swarms which are often com- 

 posed of milliards of single species. In the oceanic regions of the 

 temperate zone we lueet monotonic plankton of the Fucoid, Koctiluca, 

 Mcdum, Ctenophore, Salpa, ISchizopod, etc., classes, sometimes com- 

 posed of one, sometimes of several species. In the tro})ical ocean im- 

 mense banks of monotonic plankton api)ear, in which the Murracytes, 

 Oscillatoria', Physalia, Pyrosoina, Ostracoda, determine the character of 

 the swimming oceanic population. Although these facts have long 

 been known, up to this time no attempt has been made to arr.ange 

 them chorologically or to define more closely tlie characteristic features 

 of the i)lankt()ii in the climatic zones. Vet I believe, partly upon the 

 ground of the accounts referred to above (particularly of the Challenger 

 and of the Yettor Pis((ni),\yi\\Wy un the gi'ound of my own comparative 

 investigations (of the Challenf/er as well as of tlie Kabbe collections), 

 that even now an important proposition <an be formulated. 



The quantity of the planHon is little dependent upon the elimatie differ- 

 ences of the zones^ the quality very dependent ; especially in this a'((y, that 

 thr number of component species diminishes from the equator toivards 

 both poles. This i)roi)osition corresponds, on the whole, witli the con- 

 ditions which the climatic dilfereiu'es show in the terrestrial fauna and 



