616 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 
Hourly oscillations —Many pelagic animals appear at the surface 
of the 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 not a single individual of the species is to be found, 
_Agassiz, thirty years ago, brought forward noticeable examples of 
this from the class of Medusw, and I can from my own experience adduce 
a number of other examples. But many other pelagic animals also 
(e. y., Siphonophores, Heteropods) come to the surface only for a few hours. 
We have long known that the swarms of the nyctipelagie Pteropods, 
Pyrosoma and many Crustacea, come to the surface only during the night 
and flee the light of day. Other groups acfi 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 upou 
the composition of the plankton, and that this accordingly is very 
different at different times of day, needs 1» 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 by observation and estimation. Jn point of fact they all seem 
to tend to very remarkable inequality. 
C.—CLIMATIC PLANKTON DIFFERENCES. 
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 in pointing out the differences 
among them, corresponding to the climatic zones. Thus the Arctic 
oceans are characterized by masses of monotonic plankton of Diatom, 
Beroide, Copepod, and Pteropod groups, swarms which are often com- 
posed of milliards of single species. In the oceanic regions of the 
temperate zone we meet monotonic plankton of the Fucoid, Noctiluca, 
Medusa, Ctenophore, Salpa, Schizopod, 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, 
Oscillatorie, Physalia, Pyrosoma, 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 upon the 
ground of the accounts referred to above (particularly of the Challenger 
and of the Vettor Pisani), partly 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 plankton is little dependent upon the climatic differ- 
ences of the zones, the quality very dependent; especially in this way, that 
the number of component species diminishes from the equator towards 
both poles. This proposition corresponds, on the whole, with the con- 
ditions which the climatic differences show in the terrestrial fauna and 
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