570 MIGRA TION 



the undisputed truth of his observation, his assertion seems to be 

 only partially proved. That the birds which lead the flock are the 

 strongest is on all accounts most likely, but what is there to shew 

 that these are also the oldest of the concourse % Beside this, there 

 are many Birds which cannot be said to migrate in flocks. While 

 Swallows, to take a sufficiently evident example, conspicuously con- 

 gregate in vast flocks and so leave our shores in large companies, 

 the majority of our summer- visitors slip away almost unobserved, 

 each apparently without concert with others. It is also pretty 

 nearly certain that the same species of Bird does not migrate in 

 the same manner at all times. When Skylarks arrive on our 

 north-eastern coast in autumn they come flitting over in a constant, 

 but intermittent stream, not in compact flocks ; yet a little later these 

 same birds collect in enormous assemblages which prosecute their 

 voyage in company. It is indeed possible that each bird of the 

 stream intentionally follows that which goes before it, though in a 

 long sea-passage it must be hard to keep the precursor in sight, 

 and it may perhaps be granted that the leader of the whole is a 

 bird of experience. But then we must consider not these cases only, 

 but also those of Birds which do not migrate in company, and we 

 must also have regard to what is implied in the word " experience." 

 Here it can only signify the result of knowledge acquired on former 

 occasions, and obtained by sight. Now it was stated by Tem- 

 minck^ many years ago, and the statement has been abundantly 

 confirmed by Herr Gatke and others, that among migrants the 

 young and the old always journey apart and most generally by 

 different routes. The former can have no " experience," and yet 

 the greater number of them safely arrive at the haven where they 

 would be. The sense of sight, essential to a knowledge of land- 

 marks, as we have above attempted to demonstrate, is utterly 

 insufficient to account for the success that attends Birds which 

 travel by night, or in a single flight span oceans or continents. 

 Yet without it the idea of " experience " cannot be substantiated. 

 We may admit that inherited but unconscious experience is a 

 factor in the whole matter — certainly, as Mr. Wallace seems to 

 have proved, in originating the migratory impulse, but yet every 

 aspect of the question is fraught with difficulty, and we must leave 

 to time the discovery of this mystery of mysteries. 



There yet remain a few words to be said on what may be 

 termed Exceptional Migration, that is when from some cause or 

 other the ordinary practice is broken through. This difters from the 

 chance occuiTence of the waifs and strays with which we began to 

 consider the question in that the Birds subject to it keep in a great 

 measure their customary habit of migrating, and yet are compelled 

 to indulge it in an irregular, or perhaps an altogether novel, 

 ^ Manuel d'Ornithologie, iii. Introd. p. xliii. note. 



