109 



The causes of migration are intrinsic ones (that is, they 

 depend on something in the organisation of the fish itself), or 

 extrinsic (when they depend on the nature and order of the 

 events that occur in the sea, in the atmosphere, etc.). Intrinsic 

 causes are, for instance, the ripening of the eggs in the ovary 

 or the sperms in the testes. These changes produce " internal 

 secretions," which get into the blood-stream, are carried over 

 the body and stimulate the latter in many ways. Thus the 

 changes in the bodies in boys and girls that occur at the age 

 of puberty are caused by substances that originate in the 

 developing testes and ovaries and are then carried through the 

 body by the blood-stream. Restraint of sexual development 

 is the result of secretions elaborated in the pineal gland (in the 

 brain) ; acceleration of sexual development comes from 

 production of an internal secretion by the brain structure called 

 the pituitary gland ; extreme under-production of the secretion 

 of the thyroid gland leads to dwarfing and a form of idiocy ; 

 over-production of the secretion of the thyroid leads to general 

 excitability and other well-known effects, and so on. Changes 

 in " temperament " and " personality " accompany the above 

 physiological ones. It cannot be doubted that such causes 

 operate also in fishes — thus, an American zoologist stated, long 

 ago, that the internal secretion from the ovaries caused a certain 

 " auto-intoxication," which was the immediate stinmlus to the 

 migratioji of the salmon into fresh water. 



Added to this there is a general " restlessness " in all 

 animals, expressing itself in undirected, random movements. 



The extrinsic causes are chiefly the changes in the tem- 

 perature and salinity of the sea-water. In every animal there 

 are certain " optimum " conditions, that is, certain external 

 circumstances which suit the make-up of the animal " better 

 than any others. Thus a fish, like the mackerel, does best in 

 water of much the range of temperature of that off the S.W. 

 Coasts of Ireland, or outside the entrance to the English 



