NATURAL HISTORY STUDIES 



besides birds in land-crabs, in fishes like salmon and 

 eel, herring and mackerel, in turtles, in lemmings 

 and field mice, in some deer, in eared seals and in 

 most cetaceans, such as the bottle-nose whale, the 

 right whale, and the white-beaked dolphin. The 

 term migration should not be used, however, without 

 qualification, unless the movement is really periodic 

 a recurrent seasonal movement. Thus we regard 

 the turtles' voyage to the egg-laying beach as 

 migratory, while the lemmings' march is not. 

 Similarly the movements of the salmon and the eel 

 are much more worthy of being ranked as migratory 

 than are those of the mackerel and herring. 



If it be granted that the migratory activity has an 

 inborn instinctive basis, we look none the less for 

 the immediate causes or stimuli which pull the 

 trigger twice a year at the proper time. In the case 

 of the autumnal movement, we think of the increas- 

 ing cold and the decreasing shelter, of stormy 

 weather and the shortening of the daylight hours 

 available for food-collecting, and of the dwindling 

 supply of insects and slugs, fruits and seeds, and so 

 on. But we shall probably go wrong if we regard 

 these unpropitious conditions as more than the 

 trigger-pullers of prepared states of body and mind. 



The stimuli that prompt the northward journey 

 in spring are more difficult to state, especially when 

 we take into account the great diversity of the 

 winter quarters and the fact that a large proportion 

 of the returning migrants are immature. Probably 

 the conditions of temperature, humidity and food- 

 supply are such as to exclude, for many kinds of 



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