And now a word as to the food of the otter. That it destroys fish we are not about to deny. But this liking for fish has become such a stereotyped fact (?) in natural history that it is glibly repeated, parrot-like, and so continues until most readers have come to accept it. The otter destroys but few fish, using the word in its popular acceptation. What it destroys are for food, and not out of love of killing. The greater part of its diet consists of fresh-water crayfish, thousands of which it destroys, and it is for these that long journeys are so frequently made. This does not apply to the pairing season; the wanderings have then another end. Many miles in a night are traversed for these crustaceans, the beds of mountain and moorland streams being tracked to their source, almost every stone on the way being examined. At least upon two occasions have we found the remains of the moor-hen after an otter's meal. CHAPTER IV. COURIERS OF THE AIR. The power of flight being almost exclusively the characteristic attribute of birds, it is somewhat strange that even the most eminent naturalists should be silent upon it. And yet this is almost universally so. Those who mention the speed of flight do so upon the most insufficient evidence, as witness Michelet's statement that the swallow flies at the rate of eighty leagues an hour. Roughly this gives us a thousand miles in four hours; but assuredly, even in its dashes, the swallow does not attain to anything like this speed. The Duke of Argyll is rather under than over the mark when he computes the speed at more than a hundred miles an hour. Here, however, the mechanism of flight in the swallows is carried through an ascending scale, until in the swift it reaches its highest degree of power both in endurance and facility of evolution. Although there are birds which may, and probably do, attain to a speed of one hundred and fifty miles an hour, this remarkable rate is not to be looked for in any of the birds of the swallow kind. There is something fascinating in the idea of eliminating time and space, and with this attribute popular fancy has in some measure clothed the swallows. At the greater rate of speed indicated above the swallow might, as has been stated, breakfast round the Barbican, and take its mid-day _siesta_ in Algiers. This, however, is a popular myth. In their migrations swallows stick close to land, and never leave it unless compelled; they cross straits at the narrowest part, and are among the most fatiguable of birds. From this it will be seen that although swallows may possess considerable speed, they have no great powers of sustained flight or endurance. These attributes belong, in the most marked degree, to several ocean birds.