XIV MIGRATION 375 
ing to understand it, we are like a colour-blind man 
who tries to see 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 larvee 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!! 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. 
