WHAT GUIDES BIRDS DURING THEIR MIGRATIONS 1 137 



way to lead him on the right path ; nevertheless, undismayed, he 

 once more spreads his wings, until at last he finds a haven of rest 

 amidst the palms of glowing Africa. 



And all this wondrous journey, the first in its life, the little 

 bird is assumed to have been enabled to accomplish solely in virtue 

 of such a knowledge of the features of its feeding-places as it may 

 have acquired in the neighbourhood of its nesting station ? The 

 mere fact, already indicated above, that these feeding-grounds not 

 only extend to the south of the nest, but also to the east and west 

 of it, renders such an assumption absolutely untenable, for what 

 should induce the bird to select the first of these directions in pre- 

 ference to the others ? Apart from this, the vast majority of birds, 

 like the Warbler above instanced, perform their migratory journeys 

 at night, and at such an elevation as to make it quite impossible 

 for them to distinguish the nature of the ground of the stretches of 

 country which lie far beneath them enshrouded in darkness. 

 Under such conditions the most highly developed local sense 

 would be of no avail, and hence all theories of this kind, however 

 ingenious and plausible, fail to advance by one step this enigmati- 

 cal factor of the migratory phenomenon. 



Besides this theory of the inheritance of collected migratory 

 experiences, the view has been put forward that one generation of 

 birds is in the habit of handing down the sum of such experiences 

 to its successors ; I have however shown, and it is now generaUy 

 acknowledged, that the young birds perform their first migratory 

 journey alone, and independently, one or two months before their 

 parents, so that this Theory of Tradition, as it may be termed, must 

 likewise fall to the ground ; for how could the old birds possibly 

 impart their own migratory experiences to their offspring except by 

 practical instruction and guidance while travelling in their com- 

 pany ? These old birds, however, are in many cases proceeding 

 towards raising a second brood, or are still engaged in their autumn 

 moult long after their offspring have reached their winter quarters. 

 Further, both the Theory of Inheritance as well as that of Tradition 

 are also quite untenable in regard to the periodical migratory 

 phenomena of other animals endowed with the power of flight, such 

 as beetles and nocturnal Lepidoptera. Among the latter the migra- 

 tions of Plusia gamma furnish in Heligoland excellent material 

 for observation. During their autumn migrations these small 

 creatures travel from Slesvick-Holstein to England, across the 

 North Sea, a breadth of water of four hundred miles in extent. 

 They pass this island in enormous swarms, resembling, as seen from 

 the lighthouse, a dense snowstorm driven by a light breeze. Thus, 

 according to an entry in my journal for 1882, on the night from the 



