THE MACKEREL FAMILY. 161 



and is descended from shore-living forms in the past, and 

 although the adult mackerel is perfectly suited to a pelagic 

 existence, yet the eggs and young stages for their successful 

 development still require the proximity to land, hence the 

 periodic migration of the adults landwards and that of the 

 adolescent forms seawards. The life history of the frog with its 

 familiar aquatic tadpole-stages has usually a similar interpreta- 

 tion put upon it. Granted, however, that the primary cause of 

 the migration of these fishes is due to demands made by the 

 needs of the young there is no doubt that perhaps in a lesser 

 degree, the direction of migration is determined by the sources 

 of food ; thus shoals of mackerel are frequently found in keen 

 pursuit of millions of young clupeoids and ' mackerel-midges ' 

 (see Rockling). This pursuit of food has been assumed by some 

 to be the primary cause of the migration. 



We may approach the question from another point of view. 

 Along with, and contemporaneous with the migration is the 

 crowding together, in a close mass, of myriads of individuals : 

 this massing is directly favourable to the fertilisation of the 

 eggs, it being obvious that the greater the number of fishes 

 that shed their sexual elements in a given area, the greater the 

 probability of fertilisation ; on the other hand it is directly 

 unfavourable to the nutrition of each fish, for the supply of food 

 in a given area is limited. Hence the fulfilment of the repro- 

 ductive function furnishes a factor tending to shoaling whilst 

 that of the nutritive function supplies, on the other hand, a 

 factor tending to keep the individuals apart. If it were not 

 that the latter function is largely suspended in most fishes at 

 the reproductive period shoaling would be almost impossible to 

 the vast extent that occurs in many clupeoids. 



The periodic appearance and disappearance of the mackerel, 

 as in the case of the herring, has formed the basis for a good 

 deal of speculation and for the initiation of many curious 

 legends and superstitions. The two most generally prevalent 

 theories in the past may here be mentioned. 



It was supposed by many that the Arctic regions were the 

 home of the mackerel (cf. Herring) and that from the Polar seas 

 emanated the shoals to our shores. Lacepede and others relate 



M. F. 11 



