It would not be at all strange if your bat, which you have procured, should prove a new one, since five species have been found in a neighbouring kingdom. The great sort that I mentioned is certainly a nondescript: I saw but one this summer, and that I had no opportunity of taking.

Your account of the Indian grass was entertaining. I am no angler myself; but inquiring of those that are, what they supposed that part of their tackle to be made of, they replied," of the intestines of a silkworm.”

Though I must not pretend to great skill in entomology, yet I cannot say that I am ignorant of that kind of knowledge: I may now and then perhaps be able to furnish you with a little information.

The vast rain ceased with us much about the same time as with you, and since we have had delicate weather. Mr. Barker, who has measured the rain for more than thirty years, says, in a late letter, that more rain has fallen this year than in any he ever attended to; though, from July, 1763, to January, 1764, more fell than in any seven months of this year.

LETTER XXIII.

TO THE SAME.

SELBORNE, Feb. 28, 1769. DEAR SIR,—It is not improbable that the Guernsey lizard and our green lizards may be specifically the same; all that I know is, that when, some years ago, many Guernsey lizards were turned loose in Pembroke college garden, in the university of Oxford, they lived a great while, and seemed to enjoy themselves very well; but never bred. Whether this circumstance will prove anything either way, I shall not pretend to say.

I return you thanks for your account of Cressy-hall; but

do so. This is much to be regretted; for it is one of our most interesting birds of passage, and its arrival is hailed with pleasure by those who watch its curious habits and instincts.-ED.

recollect, not without regret, that in June, 1746, I was visiting for a week together at Spalding, without ever being told that such a curiosity was just at hand. Pray send me word in your next what sort of tree it is that contains such a quantity of herons' nests; and whether the heronry consists of a whole grove or wood, or only of a few trees.

It gave me satisfaction to find we accorded so well about the caprimulgus; all I contended for was to prove that it often chatters sitting as well as flying, and therefore the noise was voluntary and from organic impulse, and not from the resistance of the air against the hollow of its mouth and throat.

If ever I saw anything like actual migration, it was last Michaelmas-day.* I was travelling, and out early in the

* The subject of migration appears to have been a very favourite one with our author, occupying the greater part of many of his subsequent letters, and evidently often the subject of his private thoughts. He sometimes seems puzzled with regard to the possibility of many of the migrating species being able to undergo the fatigue of long or continued journeys; and often wishes almost to believe, though contrary to his better judgment, that some of these enter into a regular torpidity. We find torpidity occurring among animals, fishes, the amphibiæ, and reptiles, and among insects; but we have never found any authenticated instance of this provision taking place among birds. Their frames are adapted to a more extensive locomotive power; and the change to climates more congenial to their constitutions, preventing the necessity of any actual change in the system, is supplied to those animals deprived of the power for extensive migration, by a temporary suspension of the most of the faculties which, in other circumstances, would be entirely destroyed. Birds, it is true, are occasionally found in holes, particularly our summer birds of passage, in what has been called a torpid state, and have revived upon being placed in a warmer temperature; but this, I consider, has always been a suspended animation, where all the functions were entirely bound up as in death, and which, by the continuance of a short period, would have caused death itself—not torpidity, where various functions and secretions, capable for a time of sustaining the frame, are still going on.

The possibility of performing long journeys, as we must believe some species are obliged to do before arriving at their destination, at first appears nearly incredible; but, when brought to a matter of plain calculation, the difficulty is much diminished. The flight of birds may be estimated at from 50 to 150 miles an hour; and if we take a medium of this as a rate for the migrating species, we shall have little difficulty in reconciling the possibility of their flights. This, however, can only be applied to such species as, in their migrations, have to cross some vast extent of ocean, without a resting-place. Many that visit this country, particularly those from Africa, merely skirt the coast, crossing at the narrowest parts, and again progressively advancing, until

morning: at first there was a vast fog; but, by the time that I was got seven or eight miles from home towards the coast, the sun broke out into a delicate warm day. We were then on a large heath, or common, and I could discern, as the mist began to break away, great numbers of swallows (hirundines rustica) clustering on the stunted shrubs and bushes, as if they had roosted there all night. As soon as the air became clear and pleasant, they all were on the wing at once; and, by a placid and easy flight, proceeded on southward, towards the sea: after this I did not see any more flocks, only now and then a straggler.

I cannot agree with those persons who assert, that the swallow kind disappear some and some, gradually, as they come; for the bulk of them seem to withdraw at once; only some stragglers stay behind a long while, and do never, there is the greatest reason to believe, leave this island. Swallows seem to lay themselves up, and to come forth in a warm day, as bats do continually of a warm evening, after they have disappeared for weeks. For a very respectable gentleman assured me that, as he was walking with some friends, under Merton-wall on a remarkably hot noon, either in the last week in December, or the first week in January, he espied three or four swallows huddled together on the moulding of one of the windows of that college. I have frequently remarked that swallows are seen later at Oxford than elsewhere: is it owing to the vast massy buildings of that place, to the many waters round it, or to what else?

they reach their final quarters; and during this time having their supply of suitable food daily augmented.

The causes influencing the migration of birds, appear more difficult to solve than the possibility of the execution of it. They seem to be influenced by an innate law, which we do not, and cannot, comprehend, though in some measure dependent on the want of food or climate congenial to the system of each, and which acts almost without the will of the individual. Neither this, however, nor the duties incumbent on incubation, can be the only exciting causes, as we may judge by the partial migrations of some to different parts of the same country, where food and the conveniences for breeding are alike; by the partial migration only, of a species from one country to another, differing decidedly in temperature, and where the visiting species thrives equally with the resident one; and by the males of some species migrating, while the females remain.-W. J.