202 MIGRATION 



water and that the salmon, shad, etc., belong to a fresh-water group, 

 the usual procedure in describing the life history should be reversed, 

 and the migration of the young fishes to the sea regarded as an 

 excursion to an area of rich food supply, much prolonged over the 

 initial visits when the habit began (cf. Rich, 1920). The return may be 

 to the stream in wliich they hatched from the egg, a fact that has 

 not quite been demonstrated (O'Malley and Rich, 1920; F. A. David- 

 son, 1934). 



Such anadromous fishes as the salmon and shad are characterized 

 by a return migration from the rivers to the sea. It is the return up 

 the rivers at maturity that has attracted attention. The individuals 

 of many species die after spawning. During the long journey up- 

 stream, the salmon do not feed and have few important relations 

 with other animals. However, in the lakes in which they breed, the 

 dead bodies of the adults foul the shores and waters, though the effects 

 of this have not yet been investigated. By virtue of a longer life 

 span, the shad and alewife return to the sea after spawning, and hence 

 may repeat the visit to the parent stream several times. 



Catadromous fishes are marked by the reverse habit, but they are 

 relatively few in number. The most remarkable example is the eel, 

 the breeding grounds of which lie between the Bermudas and the West 

 Indies. The young migrate from these breeding grounds in the spring, 

 entering the estuaries and rivers of western Europe and eastern Amer- 

 ica, where they remain for five to twenty years or more. When 

 mature, they set out upon the return journey of three thousand miles 

 and upon arriving they spawn and die, as it seems (Schmidt, 1922- 

 1924) . Heape concludes that the entire procedure is incomprehensible, 

 but while the medium renders a solution much more difficult, it appears 

 highly probable that it will be compounded of metabolic conditions 

 and external factors (cf. Ward, 1921). The behavior of fresh-water 

 fish is like that of marine species to the extent that many of them 

 se^ the shallower water of rivers or their tributaries, while a few 

 travel in the opposite direction. 



Insects. In the insect group there are examples of emigration and 

 return migration, but the tracing of individuals, as has been done in 

 birds by banding and in ungulates by deformed antlers and other 

 recognition marks, is all but impossible. Return migration is less 

 common among insects than in other groups and when it occurs in 

 grasshoppers, the returning individuals are the offspring of the emi- 

 grants. The migrations of insects have been summarized in consider- 

 able detail by Felt (1928) and by Heape (1931) ; those of butterflies 

 in particular have been treated in a comprehensive fashion by Williams 



