ROUTES 41 



no observers, but Mr Dixon and M. Quinet made 

 their routes follow rivers and coast lines, whether 

 there was evidence to support this idea or not. 



Only to a certain extent can it be safely contended 

 that the present route of a species is an indication 

 of its earlier journeys, or that the direction of 

 original dispersal is recapitulated in the present line 

 of migration. Heredity, experience, and imitation 

 would certainly tend to preserve and confirm the 

 general direction ; the shortest and easiest passage 

 from food - base to food - base would become an 

 hereditary route, unless circumstances arose which 

 caused a change. Mr Cooke shows how there has 

 probably been evolution of the route as well as of 

 everything else concerned with a mutable animal. 

 The fly-line across an arm of the sea may be length- 

 ened if this lengthening means a corresponding 

 advantage in reaching the desired haven. Thus the 

 birds which now cross the Gulf of Mexico at its widest 

 part, at one time probably coasted round the Gulf, 

 as many do still, by the land-bridge of Mexico and 

 Central America. The gradual straightening of this 

 curve would shorten the journey both in time and 

 distance, though lengthening the actual single flight 

 across a portion of the sea. We can imagine a bird 

 arriving in autumn at the mouth of the Mississippi, 

 at first passing from Louisiana to Mexico, so as to 

 save the time of travel through Texas. Generations 



