RECONSIDERATIONS 381 



afterwards proved to have been adapted to their environment, have at 

 first failed, it has been due to liberation in too scanty numbers ; that 

 aliens freed under these circumstances have scattered so widely and 

 wandered so far apart from one another that in some cases they have 

 lacked mates and in others have fallen victims to vermin ; that individuals 

 thus strayed associate with creatures in no way akin to them ; that 

 species successfully acclimatised increase circlewise until containing 

 barriers are reached; that a migration once begun follows the line of 

 least resistance, as may be determined by the leaders ; that the line 

 of trek having been decided upon, it is followed by all members of the 

 horde, young or old ; that broods or litters born by the way also pass 

 forward ; that rapidity of migration is determined by the speed of the 

 leaders ; that a check in front is communicated with remarkable celerity 

 to the entire migrant chain ; that species on the march retain the special 

 habits and idiosyncrasies which mark them in quiescence ; that the 

 leaders are probably males ; that certain alien species have travelled in 

 company with one another; lastly, that possibly the single goldfinch 

 and single minah cited may have been scouts that, at any rate, they 

 were not leaders of a trek already in motion. 



