MIGRATION AND HYBERNATION. 139 



tion, but whilst in the former it seems to be due 

 to the need of securing a certain and suitable 

 food supply, in the latter it appears to be rather 

 the need of securing a larger amount of protec- 

 tion for the offspring. In this solicitude, if we 

 may call it so, for the preservation of the species, 

 many fishes have succeeded in passing what 

 proves an insuperable barrier to most — to wit, 

 the passage from salt water to fresh, and vice 

 versa. Surface temperature, however, and climate 

 present an additional barrier to many fresh water 

 fishes, preventing their further movement even 

 if they could survive the transition into salt 

 water. That is to say, a fish which might sur- 

 vive this exchange of medium, would succumb 

 to the effects of changed temperature. Salt 

 water fishes do not appear to be so deeply 

 affected in this matter. 



In addition to this orderly and periodic migra- 

 tion, in which shoals of countless millions are in- 

 volved, we have a form of what we may call sporadic 

 migration — many marine fishes individually as- 

 cending rivers for hundreds of miles of their 

 course, whilst many fresh water fishes similarly 

 descend into the sea, though these are fewer in 

 number. This passage from fresh to salt water is 

 often very gradual, broken by a longer or shorter 

 sojourn in brackish water, but in some cases (as 

 in the common stickle-back) the transition may 

 be quite sudden without producing any injurious 

 results. Migration of this kind is not associated 

 with any known cause. The exchange from a 

 salt to a fresh water habitat may have been 

 to avoid competition in the more crowded sea ; 



