64 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



conditions are no weaklings bnt are perfectly capable of growing 

 up to the adult state and of producing young in their turn. 

 Later on, in the year 1917, we recorded that " we now have 

 fish producing fertile spawn, which were themselves hatched 

 and reared in our Institution three years ago. There are about 

 20 of these belonging to the hatching season of 1914, and, there- 

 fore, now just three years old, and measuring on the average 

 10| inches in length, which have been kept separate since the 

 summer of 1914, and this is probably a record both as to the 

 .small size and the young age at which the plaice can spawn ; 

 and it is also probably the first time that the second generation 

 of this fish has been produced and reared in captivity." Three 

 generations of the fish were living in our hatching tanks in 

 1918, under Mr. Cregeen's careful management. 



During these same years, from 1907 onwards, we had one 

 or other of the yachts, " Ladybird " and " Kuna," constantly 

 at work during the Easter and summer vacations in the seas 

 around Port Erin, and each of the Annual Keports for this 

 period will be found to give details of the various kinds of 

 experimental nets used and the results obtained in the intensive 

 study of the plankton. The following passage may be quoted 

 from the Report for 1908 as giving some idea of the complexity 

 of the causes of the very marked variation in the abundance 

 and nature of the plankton from time to time : — 



" It is clear that some of the great seasonal variations in 

 the plankton are not due to changes in the sea-water such as 

 are recognised in hydrographic observations, but are caused 

 simply by the normal sequence of stages in the life-histories 

 of organisms throughout the year. No amount of ' hydro- 

 graphic ' change in the water will determine the presence of 

 Echinoderm larvae at a time of year when they are not produced, 

 nor of Crab Megalopas when they do not naturally occur. 



" Three factors, at least, contribute to the constitution 

 of the plankton from day to day throughout the year ;— 



