xiv MIGRATION 375 



ing to understand it, we arc like a colour-blind man 

 who tries to sec a colour to which his eye is not sensi- 

 tive. But this is not the only fact in zoology that is 

 beyond our comprehension. How do certain wasps 

 know the exact point at which to sting a spider, whom 

 they wish to paralyse without killing, so that he may 

 not decay before their larvae emerge hungry from the 



egg? 



There has been great dispute as to the routes 

 followed by migrants, some authorities maintaining 

 that in almost all cases sea-coasts or river valleys form 

 the lines along which they steer. This view may 

 possibly have arisen from the fact that so much of 

 our knowledge of migration has been obtained from 

 lighthouses, and from the fact that waterbirds often 

 make for river valleys because they can find food there. 

 Herr Gatke thinks that a great deal too much has 

 been made of migration routes, and he very per- 

 tinently quotes the case of Richard's Pipit, which in 

 its journey westward from Lake Baikal crosses a 

 number of streams flowing north and south, and the 

 Ural Mountains into the bargain ! x On the other hand 

 the flocks of birds that alight on Heligoland seem 

 to show that that small island comes in a well-defined 

 migration track. And if there are tracks over sea, 

 they may well exist over land. On this subject we 

 must wait for further knowledge. 



1 The most elaborate attempt to trace the routes followed is 

 to be found in Palmen's Zugstrassen der Vogel. 



