I20 THE MIGRATION OF BIRDS. 



the distance ! But the fact is, these Httle emigrant 

 birds only know the way to that winter home, which 

 has been their winter home from the remotest times, 

 perhaps as long as their species has had existence; 

 and they follow routes towards it along which their 

 emigrations have extended. Their present fly-lines 

 of migration then are inseparably connected with 

 the direction of their past emigration, and indicate 

 unerringly the road the colonists have followed from 

 the central area of distribution, in opening out new 

 and ultimately wide tracts of country for their 

 surplus population. 



But although we are able to trace with exactness 

 the routes of recent emigration, the more ancient 

 tracks that many species followed from common 

 centres of dispersal have long been utterly obliter- 

 ated. The present dispersal of obviously allied 

 species, however, enables us to trace some of them 

 almost with equal precision. The least used routes 

 of Migration are the ones that best indicate the 

 direction of ancient Emigration. They are routes, 

 once easily and extensively followed, which from 

 physical causes (especially submergence) have either 

 been discarded altogether, or only followed by a 

 few species. Most of these old routes would be 

 entirely lost to us were it not for these lingering 

 migrants across them, or the very obvious near 

 alliance of the species along the route, where it still 

 remains partially continuous, or the more distant 

 yet equally certain relationship of forms at either 

 end of that broken route. 



