78 THE RIDDLE OF MIGRATION 



This type of thing is matched in certain other 

 migrants. Thus the young of Geocarcinas, the 

 land-crab of the West Indies, are hatched in the sea 

 but when they reach a certain stage of development 

 they automatically take to shore. There is no 

 parental guidance, for the old crabs have returned 

 after spawning. The young desert the scores of 

 other species with which they are then associating 

 and travel inland by themselves to adopt the — for 

 Crustacea — unusual habitat of dry land. Whatever 

 it be, there is undoubtedly something in their 

 hereditary make-up that controls their behaviour. 



In these and all similar cases, whatever the de- 

 tails, intent, and no doubt consciousness as well, 

 cannot be admitted into the argument. Many mi- 

 gration writers assume that birds are ''driven" 

 south by fall indications of coming winter. Such 

 an assumption credits birds with a knowledge of 

 affairs that they cannot possibly possess. It infers, 

 for instance, that they can discriminate between one 

 type of food shortage and another — the irremediable 

 shortage of fall and the temporary shortage oc- 

 casioned by such phenomena as summer snowstorms, 

 protracted droughts, fires, etc., some or all of which 

 must at one time or another have brought them to 

 temporarily straightened circumstances on their 

 breeding grounds but without eliciting a migratory 



