570 MIGRATION 
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; yeta 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 Giitke 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 differs from the 
chance occurrence 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, 
1 Manuel d’ Ornithologie, iii. Introd. p. xliii. note. 
