744 



SCIENCE 



[N. S. Vol. XXX. No. 778 



desmo-plankton (tropical) depends upon 

 the existence of such vast swarms of par- 

 ticular organisms in masses of water coming 

 into the North Atlantic from different 

 sources. 



It is possible that in some parts of the 

 ocean, far from land, the plankton may be 

 distributed with the uniformity supposed 

 by Hensen. It is important to recognize 

 that at least three classes of locality exist 

 in the sea in relation to distribution of 

 plankton : 



1. There are estuaries and coastal waters 

 where there are usually strong tidal and 

 other local currents, with rapid changes 

 of conditions, and where the plankton is 

 largely influenced by its proximity to land. 



2. There are considerable sea areas, such 

 as the center of the North Sea and the 

 center of the Irish Sea, where the plankton 

 is removed from coastal conditions, but is 

 influenced by various factors which cause 

 great irregularity in its distribution. These 

 are the localities* of the greatest economic 

 importance to man, and to which attention 

 should especially be directed. 



3. There are large oceanic areas in which 

 there may be uniformity of conditions, but 

 it ought to be recognized that such regions 

 are not those in which the plankton is of 

 most importance to men. The great fish- 

 eries of the world, such as those of the 

 North Sea, the cod fishery in Norway and 

 those on the Newfoundland Banks, are not 

 in mid-ocean, but are in areas round the 

 continents, where the plankton is irregular 

 in its distribution. 



As an example of a locality of the second 

 type, showing seasonal, horizontal and ver- 

 tical differences in the distribution of the 

 plankton, we may take the center of the 

 Irish Sea, off the south end of the Isle of 

 Man. Here, as in other localities which 



' See Dakin, Trans. Biol. Soc. Liverpool, XXII., 

 p. 544. 



have been investigated, the phyto-plankton 

 is found to increase greatly about the time 

 of the vernal equinox, so as to cause a 

 maximum, largely composed of diatoms, at 

 a period ranging from the end of March to 

 some time in May— this yehv to May 28, in 

 the Irish Sea. Towards the end of this 

 period the eggs of most of the edible fishes 

 are hatching as larvse. 



[Statistics and diagrams showing this 

 maximum for the last three years were ex- 

 hibited.] 



This diatom maximum is followed by an 

 increase in the Copepoda (minute Crus- 

 tacea), which lasts for a considerable time 

 during the early summer; and as the fish 

 larvae and the Copepoda increase there is a 

 rapid falling off in diatoms. Less marked 

 maxima of both diatoms and Copepoda may 

 occur again about the time of the autumnal 

 equinox. These two groups— the diatoms 

 and the Copepoda— are the most impor- 

 tant economic constituents in the plankton. 

 A few examples showing their importance 

 to man may be given : Man eats the oyster 

 and the American clam, and these shell-fish 

 feed upon diatoms. Man feeds upon the 

 cod, which in its turn may feed on the 

 whiting, and that on the sprat, and the 

 sprat on Copepoda, while the Copepoda 

 feed upon peridinians and diatoms; or the 

 cod may feed upon crabs, which in turn 

 eat "worms," and these feed upon smaller 

 forms which are nourished by the diatoms. 

 Or, again, man eats the mackerel, which 

 may feed upon young herring, and these 

 upon Copepoda, and the Copepoda again 

 upon diatoms. All such chains of food 

 matters from the sea seem to bring one 

 through the Copepoda to the diatoms, 

 which may be regarded as the ultimate 

 "producers" of food in the ocean. Thus 

 our living food from the waters of the 

 globe may be said to be the diatoms and 



