m 



to direct the movements of the larvae and young to dilute water, 

 and while it has been found that the migrations each season 

 are closely related to temperature an additional impulse to the 

 shore region may be provided. It is not universal in its influence, 

 for at the period that plaice and flounder in the immature state 

 are found in brackish water, the same species of about the same 

 size may be procured beyond territorial waters. We can only 

 say then that during the winter some of the immature plaice 

 and flounders migrate offshore, and that both may migrate into 

 estuaries at the same period of the year. 



In complete contrast to the seasonal migrations of the immature, 

 are the migrations of these species when they receive the call of 

 maturity. Then the migration is to a great distance, and it 

 appears universally to be a contranatant migration. The flat fish 

 of the Northumberland coast, so far as our information extends, 

 migrate with approaching maturity far to the north to join spawn- 

 ing assemblages off the east coast of Scotland, and the Scottish 

 marking experiments, so fully reviewed by Fulton, plainly indicate 

 that the flat fish of the east coast of Scotland similarly migrate 

 to the north and contranatantly, and may, many of them, reach 

 the north of Scotland, even the Atlantic. In this case it is evident 

 that the migration is performed in response to an impulse brought 

 about by the change in the gonad. During its development the 

 gonad gives rise to an internal secretion, which carried by the 

 blood has special effect upon the nervous system. 



That this is the fact is at once clear when we recall the migra- 

 tion of the eel at maturity to the western side of the North Atlantic, 

 and the many anadromous migrants in and into fresh water. In 

 these cases the blood by the internal secretion is able to control 

 the nervous system, and a striking migration results. But it is 

 evident that more or less all fish are similarly affected. In certain 

 cases the internal secretion is able to produce somatic effects as 

 well, as in the dragonet, elasmobranchs and the salmon in which 

 secondary sexual characters are developed. It is worth recalling 

 also that the migratory effects are not confined to fishes, for the 

 common crab behaves with oncoming maturity almost exactly 

 as the plaice 'and flounder. As every inshore fisherman knows, 

 the crab migrates each season with the greatest regularity inshore 

 in summer and offshore in winter, and the fact has been proved 



