METABOLISM IN THE SEA 187 



of tidal streams and minor currents, great varia- 

 tions in the abundance of the plankton and the 

 sequence of the various forms occur. Speaking 

 generally, in shallow water the plankton is more 

 abundant and more variable in nature and amount 

 from place to place. It varies in the same place 

 with the time of day ; catches made in the dark 

 usually present some differences from those made 

 in the daylight. It varies also with the depth. 

 Fish eggs, for instance, are only found near the 

 surface. Every zone of depth in the sea in any 

 one place has a fauna and flora which are slightly 

 different from those frequenting any other zone. 

 Far out at sea, however, though this irregular 

 distribution with the depth still holds good, the 

 general nature and amount of the plankton is 

 much more uniform, a result which is to be 

 expected when the disturbing influences of the 

 land cease to be apparent. Then, contrary to what 

 might be expected on a first consideration, the 

 colder seas in polar and sub-polar regions are 

 richer in plankton than tropical and sub-tropical 

 seas. This wealth of life in the colder waters 

 of the globe is due almost entirely to the diatoms, 

 and for a reason to which I shall refer later on. 



All these results and many others have been 

 obtained by the ordinary methods of plankton 

 fishing. But when it became possible, by means 

 of the Hensen methods, to enumerate the in- 

 dividuals of the plankton, many striking and 



