736 LHE AMERICAN NATURALIST. [Vow XXXII. 
variations are the rule. These variations are conditioned not 
only by temperature, specific gravity, atmospheric pressure, 
and light, but probably by fundamental phenomena of which 
science as yet knows nothing. Certain forms spend the day 
in the depths, appearing at the surface only at night ; for vari- 
ous forms the reverse is true. There are other temporal differ- 
ences, — yearly, monthly, daily, and hourly variations, — whose 
causes are manifold, in part climatic or meteorological, in part 
depending upon the conditions of life, of reproduction, and 
development. Still other variations are brought about by the 
numberless currents great and small, which not only collect 
the organisms into eddies and scatter the “schools,” but trans- 
port organisms characteristic of one region to places far remote 
from their home, e.g., the Gulf Stream carries tropical forms 
far into the cold northern seas. 
All the organisms which are borne about helplessly by cur- 
rents, or whose motions are determined by protoplasmic activi- 
ties (heliotropism, chemiotropism, etc.), as distinguished from 
special and effective locomotory organs, constitute the Plankton 
(a word coined by Professor Hensen from the Greek rAavacOa, 
to wander). The Plankton has attracted naturalists since the 
studies of Johannes Miiller, but Professor Hensen was the first 
to give earnest attention to the economic importance of the 
Plankton, and to the problems of the food supply based upon it. 
He was led to this through his attempt to get an approximate 
idea of the number of fish in corresponding districts. This 
work brought him to the question of the food supply for these 
fishes, and from that to consideration of the general primary 
sources of food and the cycle of matter in the ocean. This has led 
to important results in tracing the cycle of changes through which 
the organic elements, carbon, hydrogen, oxygen, nitrogen, sul- 
phur, phosphorus, iron, and others pass; in showing how they 
either singly, or united in simple combinations, become incor- 
porated into a living (it may be microscopic) plant ; how this 
plant is eaten by a mollusc or a small fish, a prey in turn for 
larger and fiercer fish, which ultimately die and are broken up 
by microscopic plants (bacteria) into the original elements, to 
again nourish plants. The actual cycle is rarely so simple as 
