I lO DARWINISM AND THE PROBLEMS OF LIFE 



winter scarcity of food, and stores up grain in its den in 

 the autumn. 



Thirdly, the bird's sense of direction had to be 

 improved. Otherwise how could they find their way on 

 the long journey ? 



There is, however, another theory. Remember how 

 we conceived the origin of the migration. Among the 

 birds that penetrated further north only those survived 

 that flew south at the beginning of winter. May not in 

 some of the birds an instinct have arisen in the course 

 of thousands of years, during which natural selection 

 was at work amongst them, to fly straight to the south 

 when the cold weather set in ? These birds would thus 

 be preserved, and might transmit this instinct — which 

 does not differ essentially from the instinct that forces 

 the salmon up the river — in increasing power to their 

 offspring, And may not an instinct have arisen by 

 selection to keep the route unerringly during the migra- 

 tion so that the animals will reach the warm countries 

 in safety? Certainly the formation of a "magnetic 

 sense " of this kind by natural selection is quite possible, 

 and the Siberian traveller, Middendorf, believes it is 

 present in migratory birds.^ Such an instinct is not 

 more wonderful in principle than the marmot's instinct 

 to store up grain, or the bees' instinct to build their 

 ingenious cells. 



But we know from observed facts that the migratory 

 birds do not regulate their flight by a magnetic sense. 

 They do not fly straight, but follow certain paths which 



^ There are many animals with senses that cannot be reduced to 

 any of our familiar five senses. 



