BIRDS AND LIGHTHOUSES 273 



to prove by a very good test.^ They are thus 

 enabled to perceive landmarks at a distance which 

 to us would be invisible. Practice, or the experience 

 gained by individual birds which have previously 

 made the same journey, must be taken into account 

 when considering how the migrating flocks are led. 

 No one of these causes by itself seems sufficient 

 to explain how migratory birds are guided in their 

 journeys, but, taken collectively, it may be fairly 

 assumed that heredity of habit, combined with 

 acute vision, memory, and experience, explain what 

 otherwise seems so great a mystery. 



In the case of adult birds only can we fairly 

 allege individual experience and memory as the 

 cause. This cannot apply to young birds, which 

 make their journey for the first time; or to those 

 which in the autumnal migration precede their 

 parents (as it is well-known many species do) by 

 intervals of some days or even weeks. This, how- 

 ever, only strengthens the view that the instinct of 

 migration is really transmitted habit, and that the 

 knowledge of routes depends more upon inherited 

 than individual experience. When we consider 

 that this heredity has been in operation for 

 thousands of generations of migratory birds, it 

 may well be supposed to have acquired a force 

 sufficient to preclude the necessity for a 

 knowledge of routes acquired through individual 

 experience. 



There is one more point in connection with the 

 general subject of migration upon which a brief 

 1 See Essays on Sport and Natural History, 1883, p. 126. 



