176 THE WONDER OF LIFE 



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 move- 

 ments of the salmon and the eel are much more worthy of 

 being ranked as migratory than are those of the mackerel 

 and herring. 



The movements of whales are believed to depend in great 

 part on the distribution of the organisms on which they 

 feed, and perhaps in part on ocean currents. But accord- 

 ing to Guldberg, there is also a reproductive factor. Gravid 

 females seek calm and shallow waters. It must be remem- 

 bered, however, that distance does not count much with 

 these powerful swimmers, and just as a gull may cross the 

 Atlantic (Germany to Barbadoes) without the fact mean- 

 ing very much, so a whale's movements may be much 

 less significant than those of a salmon. 



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 increasing 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 liberating stimuli, which act on a 

 prepared state of 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, 



