71 



the Atlantic, and both may be said therefore to be related, but 

 the contranatant powers of the adults tend to correct the early 

 distribution. With developing powers of contranation and with 

 increasing individuality the older fish press beyond the realms of the 

 two currents. . The two races may be said therefore to be more 

 liable to mixing in the neighbourhool of the Dogger Bank than 

 elsewhere, even than in the Atlantic. 



The young plaice of the year so familiar in shore pools in 

 the summer arrive at a size of a little over 1 cm. They are 

 carried into shallow water, to the shore and into estuaries. During 

 the summer they grow rapidly, and in October attain a size of 

 5 to 10 cm., the range depending on the above circumstances. 

 The inshore members of the recruits then migrate from the shore, 

 and the winter is spent in shallow waters and in estuaries. During 

 the second summer they still occupy much the same ground and 

 progress in size, 11 to IS cm. At the end of the summer a partial 

 migration takes place into slightly deeper water— partial in the 

 sense that those outside migrate a short distance into deeper 

 water, their places being taken by those inside and so on. The 

 shore waters at this period are almost completely deserted. A 

 return migration of a similar nature takes place in the spring. 

 They are now in Group II., and the procedure is again very 

 similar to that of the previous summer. Groups II. and III., 

 plaice which have had two and three winters, form in numbers 

 the main bulk of the plaice captured in shallow waters, and our 

 trawling records show that they are accompanied by the succeed- 

 ing immature groups, together with spent fish. Our experiments 

 have been conducted in shallow depths of about 3 fathoms or even 

 less, and they therefore demonstrate with great clearness how 

 closely the seasonal migrations follow the annual trend of the 

 temperature of the sea. The experiments have shown also that 

 the migration is out and in with reference to the coast, and that 

 at the same time the species does not merely tend to maintain 

 its position to the coast but actually during the migrations makes 

 some degree of progress contranatantly. This has been demon- 

 strated with reference to both the Forth and the Northumberland 

 schools. It might be urged that the original distribution of the 

 young would explain the fact that the distribution affects the 

 district in each case as a whole and the sub-divisions thereof, 



