250 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



low on fishing grounds where the plaice were very- 

 abundant, that is, " overcrowded." I am not convinced 

 now that there is any evidence in favour of the hypothesis 

 that the condition, or rate of growth of plaice varies 

 inversely with some function of their degree of aggrega- 

 tion on the sea bottom. The view depends on the analogy 

 with human populations, where "overcrowding" is 

 generally concomitant with poverty, and insufficient 

 means of nutrition ; or perhaps with the conditions which 

 are said to obtain in a river " overstocked " with trout 

 artificially hatched and reared. Before it can be applied 

 to a population of fish in an open sea, or an estuarine 

 area, some estimate of the degree of aggregation of the 

 fish on the ground must be obtained ; as well as estimates 

 of the amount of food actually present. What we do 

 know points to so great an abundance of the latter on the 

 grounds much frequented by small plaice that it is diffi- 

 cult to imagine that food is deficient on these areas. In 

 the few cases known to me where the sea bottom as well 

 as the plaice living there have been sampled, the former 

 reveals an extraordinary abundance of invertebrate life, 

 being sometimes literally carpeted by small mussels, or 

 Scrohicularia or Pectinaria or Cardium, 



It is far more probable that variations in the value 

 of the coefficient k are due to purely physical conditions. 

 I have discussed this question, and the evidence bearing 

 upon it in another paper in this report, and have suggested 

 that the main cause of the periodic migrations made by 

 plaice is change of temperature. The migration is of the 

 nature of an adaptation to a change in the environment of 

 the animal, and the latter responds by so moving that the 

 temperature change becomes minimal. But the range of 

 temperature, at the same time, on the grounds available 

 to plaice is probably limited, since obviously plaice do 



