SEA-FISHERIES LABORATORY. 251 



not normally inhabit water of more than a limited depth, 

 nor do they ascend rivers as a flounder does. Experi- 

 ments carried out on a sufficiently big scale with the latter 

 fish would probably show that its migrations are more 

 clearly adaptations to changes of temperature than in the 

 case of the plaice, since it is apparently able to inhabit 

 water of a similar range of depths, and is, besides, able 

 to live in water of a much greater range of salinity 

 variation. Its possible change of habitat is, therefore, 

 greater than that of the plaice. 



The latter fish being unable, then, to remain in water 

 of the same temperature (that is, being limited in its 

 power of adaptation) must react to the change in this 

 factor, and this it does by an obligatory change in its rate 

 of metabolism. At a higher temperature, there must 

 be greater tissue waste — we have experimental evidence 

 that the respiratory movements, and the excretion of 

 C0 2 are increased — and consequently greater tissue 

 repair and assimilation. It is true that the intake and 

 output of material might be equal, but it is hardly likely 

 that this condition of nitrogenous equilibrium should 

 obtain in the case of a growing plaice. In this fish, 

 unlike the Clupeoids or Salmonids, there is apparently 

 little change in the fat-content of the tissues. There are 

 no actual determinations of this that I know of — an 

 obvious gap in fishery investigation — but no visible 

 change in fat content has been apparent in the numerous 

 samples of plaice, taken from different fishing grounds, 

 and at all times throughout the year. Change of tem- 

 perature, therefore, is probably to be associated with 

 change, in the same direction, of rate of metabolism, and 

 change of increment of tissue formation, that is, with 

 variations in the coefficient k, the latter being greater with 

 temperatures above the mean, and less with temperatures 

 below the mean. 



