DEEPER PROBLEMS OF MIGRATION 291 



Our view, then, is this, that an original instinctive 

 Mutation must be postulated, which amounted to " a new 

 idea," but was not an idea, which found expression in 

 restlessness, in sensory alertness, in adventurous experi- 

 ment, in a " Wander-trieb." Perhaps we see something 

 like the beginning of it to-day in animals which seem to 

 be sensitive to impending storm, and act accordingly. But 

 given some sort of definite beginning of a migratory in- 

 stinct, as a germinal mutation, we would account for the 

 diffusion and augmentation and specialisation of this by 

 the well-known Darwinian interpretation, especially elabo- 

 rated by Dr. Alfred Russel Wallace. Those individual 

 birds who were too dull, or too wilful, or too unplastic, 

 to take the hints and warnings of the increasing scarcity, 

 the cold winds, or it might be the dry heat, would be 

 eliminated. This elimination, still a dreadfully real pro- 

 cess, would, being discriminate, gradually raise the 

 standard of migratory capacity century after century, 

 millennium after millennium, for we must bear in mind 

 that the great climatic changes must have come about 

 with extreme slowness. 



Another general theory of the conditions which led to 

 the establishment of a migratory instinct, lays the emphasis 

 on the food-supply. Many birds are prolific, and over- 

 crowding is apt to occur. They have to extend their range, 

 and they take the lines of least resistance as regards food. 

 They push northward in spring, exploring new grounds, 

 staying as long as they can, and retreating before the 

 Winter to the original home. Instead of crowding in one 

 area all the year, and involving themselves in want, they 

 exploit two areas, each for about half of the year. We 

 see the same sort of thing among men, for instance, in the 

 Summer migration to the high " alps," and the return in 

 Winter to the village in the valley. It has often been 



