SEA-FISHERIES LABORATORY. 299 



changes in these conditions, that is, rise or fall of tem- 

 perature, as the case may be, are the proximate causes of 

 the migrations of plaice which are to be mentioned. It 

 may safely be assumed that these periodic migrations are 

 really adaptations. The fish is living under conditions 

 of a certain kind, and its functioning is of a certain 

 nature. When the conditions change the fish responds, 

 not by a change of structure, or functioning, in the 

 special sense, but by a migration into another area where 

 the conditions are similar to those of the area in which it 

 was previously living. In general, it will be the case 

 that the adaptation brought about by such a migration 

 will be incomplete, that is, the fish will not be able to 

 remain within optimal conditions by making a migration, 

 but the object of the latter movement is, no doubt, one 

 of this kind. 



This is probably, to some extent at least, the meaning 

 of many kinds of fish migration ; that of the Mackerel, 

 Bass, Garfish, and some other species into the Irish Sea. 

 In these cases the fish, experiencing a rise of tempera- 

 ture in the seas to the South and West, in the mouth of 

 the Channel, endeavours to correct this by migrating 

 to the North. But it may also be the case that the 

 meaning of such migrations is also the effort of the fish 

 to enlarge its area of distribution : as the temperature 

 rises in adjacent areas it moves into those regions. This 

 may be the nature of the migrations of the Cod and 

 Whiting into the Irish Sea during the early months of 

 the year. These fishes, living in colder waters to the 

 North, migrate to the South as the temperature there 

 falls and approximates to that of the sea-area in which 

 they find their optimal conditions. If this hypothesis is 

 sound, it ought to be possible to predict the time of maxi- 

 mum appearance of Cod in the northern part of the Irish 



