8o LIFE : OUTLINES OF GENERAL BIOLOGY 



the tadpoles have become small toads, there is a journey in the 

 opposite direction, the parents having returned inland some time 

 previously. The same is true of the C rested Newt in Britain, but the 

 distances covered are not so great as in the case of the toad. Simi- 

 larly in the common Grass Frog there is a summer movement of 

 the young ones from the water to the fields, whither the parents 

 have preceded them some time before. The return to the water, 

 or to the vicinity of the water, is autumnal, not vernal as in toads, 

 and it is not so well-defined. 



FiSHKS. — The term migration has Ix'en mistakenly applied to 

 many mass-mo venunts of fishes, e.g. herring and mackerel, which 

 have no connection with a return to a particular spawning-ground 

 or tyjK' of spawning-ground. These non-migrational movements are 

 largely e.xplicable in terms of changes in the distribution of the 

 planktonic and other organisms on which the fishes feed, or, what 

 may come to the .same thing, in terms of changes in temperature, 

 salinity, oxygenation, carbon dioxide tension, and so forth. No 

 doubt such changes may sometimes serve as external stimuU to 

 true migrational movements, but no mass-movement of fishes 

 should be ranked as migrational, unless it is directly concerned with 

 approaching or leaN-ing a spawning area, as with the salmon. 



True migration is familiarly illustrated in the salmon {Salmo 

 salar). The eggs are liberated, often in midwinter, on suitable 

 gravelly stretches of the river-bed. There are successive stages of 

 alevins. fry, parr, and smolts. The last, when over two years old, 

 pass down the rivers to the sea, usually in the early summer months. 

 A vigorous nutritive life is sjxjnt in the si^-a, where the food consists 

 largely of young herrings and mackerel; and the salmon may 

 remain there for .several years. Adolescent .salmon, which have not 

 quite put on the adult characters, are called grilse, and are normally 

 three and a half years old, having descended to tlie sea as smolts 

 the previous year. These may ascend the rivers as grilse and may 

 even spawn as grilse- ; but the grilse stage is often passed through in 

 the s<^a, so that the maiden fish which are entering the fresh water 

 for the first time are often, in the strict .stMise, .salmon. The adult 

 salmon eat very little, if at all, in fresh water; they return to the 

 sea. if thry can. after spawning. In some ca.ses it has Ix-en proved 

 by marking that .salmon return from the .sia to tlieir own particular 

 native river. 



W hen a migratory fish comes inshore or ascends the rivers to 

 spawn, the term anadromous is used, with its counterpart cata- 

 dromous when the spawning occurs in deepish salt water; and other 

 useful terms for different tyjxs of movement have Ixen defined by 

 Meek. In contrast to the salmon, the casi* of the flounder {Pleiiro- 

 nectus Jlesus) may be mentioned. In many places it has Ix^come a 

 freshwater fish, and may be found flourishing in stretches of river 



