PLANKTONIC STUDIES. 613 



B. — Temporal Planktoxic Differences. 



The first and most remarkable pheiiomenou, known to every seafaring 

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

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

 position apply qualitatively and quanUtaUvely to the oceanic as well as 

 to the neriUc plankton. They are just as im])<)rtant in the comparison 

 of different places during the same time as at different times in one 

 and the same ])lace. We can therefore distinguish h)cal anil temporal 

 variations, and will first of all consider the latter. 



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

 ations of plfinktoii comi)ositiou, 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 ohservations, com- 

 parable to the meteorological (with which they stand in direct causal 

 connection), have not hitherto been made. Tliey belong to the most im- 

 portant tasks ofthe zoological 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, whi(;h Schmidtlein has given u{)on the api)earance of the larger 

 l^elagic animals in the (iulf of Naples, during 1875-77 (19, p. 120). 

 The contributions of Graeffe upon the occurrence and time of appear- 

 ance of marine animals in tlie Gulf of Trieste are also worthy of notice 

 in this connection (20). 



The considerable temporal variations which underlie the appearance 

 of the i^elagic 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 corresiionding 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 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 weelis) 

 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. 



