PLANKTONIC STUDIES. 613 
B.—TEMPORAL PLANKTONIC DIFFERENCES. 
The first and most remarkable phenomenon, known to every seafaring 
planktologist, is the varying constitution of the plankton and the vari- 
able mingling of its constituents. The remarkable differences of com- 
position apply qualitatively and quantitatively to the oceanic as well as 
to the neritic plankton, They are just as important in the comparison 
of different places during the same time as. at different times in one 
and the same place. We can therefore distinguish local and temporal 
variations, and will first of all consider the latter. 
To obtain a complete and more certain survey of the temporary vari- 
ations of plankton composition, there would be needed especially an 
unbroken series of observations, which had been carried on at one and 
the same place at least for the space of a full year—still better for 
several successive vears—to obtain from the yearly and monthly oseil- 
lations a general average. Such complete series of observations, com- 
parable to the meteorological (with which they stand in direct causal 
connection), have not hitherto been made. They belong to the most im- 
portant tasks of the zoblogical stations now everywhere springing up.* 
Meanwhile, a general conception of the considerable size of the yearly 
and monthly oscillations can be obtained from a comparative summary 
based upon the important series of observations extending over three 
years, which Schmidtlein has given upon the appearance of the larger 
pelagic animals in the Gulf of Naples, during 1875-77 (19, p. 120). 
The contributions of Graeffe upon the occurrence and time of appear- 
ance of marine animals in the Gulf of Trieste are also worthy of notice 
in this connection (20). 
The considerable temporal variations which underlie the appearance 
of the pelagic organisms and which determine such great differences in 
the plankton composition, relative to quality and quantity, may be 
divided into four groups: (1) yearly, (2) monthly, (3) weekly, (4) hourly 
variations. Their causes are manifold, partly meteorological, partly 
biological. They are comparable to corresponding temporal oscillations 
of the terrestrial flora and fauna, on one side depending upon climatic 
conditions and meteorological processes, and on the other upon the 
changing mode of life, especially upon the conditions of reproduction 
and development. As the annual development of most terrestrial 
plants is connected with definite time conditions, as the period of bud- 
ding and leaf development, of their blossoming and fructification, has 
* My own extensive, experience, I am sorry to say, is in this regard very insufficient, 
since I have never worked ata zodlogical station, and since usually I was only so 
fortunate as to go to the seacoast for a few months (or even only for a few weeks) 
during the academic vacation. Only once have I had the opportunity to extend my 
plankton studies at one and the same place to a half year (from October, 1859, to 
April, 1860, at Messina, 3, p. v, 166), and three. times have [I carried them on for 
three months at the same place—in the summer of 1859 at Naples, in the winter of 
1866-67 at Lanzarote, and in the winter of 1881-82 in Ceylon. 
