THE ZooLocist— DECEMBER, 1875. 4711 
On the Migration of Birds. By Capt. Henry Haprtexp. 
Proressor NEwToN, in his interesting paper read at a meeting 
of the British Association (Zool. 8.8. 4637) says, “We can only 
guess at the cause of migration, and not even guess at the faculty 
by which it is performed.” But many birds, we know, are com- 
pelled at the approach of winter to migrate south or they would 
perish—some of cold, others for want of food. The spring northern 
migration, though seemingly not so imperative, is nevertheless a 
necessity for some species, both for suitable breeding-stations and 
food. 
As to Dr. von Middendorff’s theory that “birds know where 
the magnetic pole is situated, and steer their course accordingly,” 
I shall not attempt to controvert it, there being more on “earth 
than is dreamt of in our philosophy ;” but it may be as well to 
consider whether birds do not “steer” their course by other means. 
Being so sensitively and delicately organized, any change of tem- 
perature would be readily felt and acted on, so that no great 
deviation from the true course—that is, north or south—could be 
made. That birds are also endowed with a mysterious instinctive 
faculty, allied to reason, there can be no doubt, but how to define 
it we know not, and never shall know. 
Then there is Herr Palmen’s somewhat less wonderful theory 
that “experience” is the key—flocks of migrants being always led 
by birds which have made the journey before. But how, may 
I ask, does this Finnish naturalist account for the migratory flight 
of the first or original flock? How was that “ steered,” or rather 
piloted? 
Another writer supposes that “want of light” determines the 
autumnal migration of the swallow and martin; but that cannot be, 
seeing that many of both species remain for weeks after the main 
body has left. I have paid some attention to the migration of the 
Hirundines, but, few breeding in this neighbourhood, I would 
suggest to ornithologists more favourably situated, that they mark 
young birds; we might then hear of them, and know more of their 
migratory course. Though we have still much to learn, I cannot 
but think we have made some progress “since Gilbert White’s 
time.” 
I find, on referring to the number of the ‘ Zoologist’ for January, 
1874 (S. 8. 3834), some remarks by Mr. Stephen Clogg on the late 
