61 G REPORT (5F commissioner OF FISH AND FISHERIES. 



Hourly osciUailons. — Manj' pelagic auiuials appear at the surface 

 of tlie sea only at a definite hour of the day, some in the morning, 

 others at noon, still others towards evening. During the remainder 

 of the day uot a single individual of the species is to be found, 

 Agassiz, thirty years ago, brouglit forward noticeable examj^les of 

 this from the class of Medime, and I can from my own experience adduce 

 a number of other examples. But many other pelagic animals also 

 {e. g., Sq)honophorc.s', Reteropods) come to the surface only for a few hours. 

 We have long known that the swarms of the nyctipelagic Fteropods, 

 Pyrosoma and many Crustacea, come, to the surface only during the night 

 and flee the light of day. Other groups act Just reversely. But the 

 late extensiye observations, especially of Murray (0), Ohierchia (8), 

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

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

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

 different at different times of day, needs n > repetition. But we must 

 allude once more to liow all these tiimporal oscillations must be taken 

 into consideration, if the equality of planlton distrlhution is to be 

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

 to tend to very remarkable Inequality. 



C. — Climatic Plankton Differences. 



The mimerous contributions which earlier and later observers have 

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

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

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

 oceans are characterized by masses of monotonic plankton of Diatom, 

 .Beroidw, Gopepod, and Fteropod groups, swarms which are often com- 

 l)osed of milliards of single species. In the oceanic regions of the 

 temperate zone we meet monotonic plankton of the Facoid, Kovtiluca, 

 Medusa, Ctenophore, Salpa, Scliizopod, etc., classes, sometimes com- 

 posed of one, sometimes of several species. In the tropical ocean im- 

 mense banks of monotonic plankton appear, in which the Murracytes, 

 OsciUatoriic, FhysaUa, Fyrosoma, 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 arrange 

 them chorologically or to define more closely the characteristic features 

 of the plankton in the climatic zones. Yet I believe, partly ui)on the 

 ground of the ac( ounts referred to above (particularly of the Challenger 

 and of the Vetf or Fisani), imrtly on the ground of my own comparative 

 investigations (of the Challenger as well as of the Rabbe collections), 

 that even now an important proposition can be formulated. 



The quantity of the planldonis little dependent upon the climatic differ- 

 ences of the zones, the quality very dependent; especially in this tcay, that 

 the number of component species diminishes from the equator towards 

 both poles. This i)ropositiou corresponds, on the whole, with the con- 

 ditions which the climatic differences show in the terrestrial fauna and 



