PLANKTONIC STUDIES. 613 



B.— Temporal Planktonic Diffekknces. 



The first aud most remarkable plieiiomenou, kuowii to every seafaring 

 plauktologist, is the varying constitution of the plankton and the vari- 

 able mingling of its constituents. The remarkable differences of com- 

 position apply quaUtatively and quantitatively to the oecanic 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 temj)oral 

 variations, and will first of all consider the latter. 



To obtain a complete and more certain survey of the temporary vari- 

 ations of i)lankton 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 years — to obtain from the yearly and monthly oscil- 

 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 zoological stations now everywhere sj^ringing 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 l^aples, during 1875-77 (19, p. 120). 

 The contributions of Graeflfe 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 (;oniposition, relative to quality and quantity, may be 

 divided into four groups: (1) yearly, (2) monthly, (3) weekly, (1) hourly 

 variations. Their causes are manifold, partly meteorological, partly 

 biological. They are comparable to corresi^onding temporal oscillations 

 of the terrestrial flora and fauna, on one side depending upon climatic 

 conditions and meteorological processes, and on the other u])on the 

 changing mode of life, esi)ecially upon the conditions of reproduction 

 and development. As the annual developnumt of most terrestrial 

 plants is connected with definite time conditions, as the period of bud- 

 ding aud leaf development, of their blossoming and fructification, has 



* My own extensive cxiiericnoo, I am sorry to say, is in this regard very insufficient, 

 since 1 have never worked at a zoological 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 Lanzurote, and in the winter of 1881-82 in Ceylon. 



