The infectious, or contagious nature of this virulent malady is incontestibly substantiated by an overwhelming amount of evidence, which cannot be adduced at full length here, but which may be classified under the following heads: _first_, the constant spreading of the disease from countries in which it rages to others which, previously to the importation of diseased animals, had been perfectly free from it. This may be proved in the case of England, into which country it was carried in 1842, by affected animals from Holland. Twelve months after, it spread from England to Scotland, by means of some cattle sold at All-Hallow Fair, and it was only twelve months afterward that cattle imported as far north as Inverness took the disease there. Lately, a cow taken from England to Australia was observed to be diseased upon landing, and the evil results were limited to her owner's stock, who gave the alarm, and ensured an effectual remedy against a wider spread. Besides, the recent importation of pleuro-pneumonia into the United States from Holland appears to have awakened our agricultural press generally, and to have convinced them of the stubborn fact that our cattle have been decimated by a fearfully infectious, through probably preventable, plague. A letter from this country to an English author says: "Its (pleuro-pneumonia's) contagious character seems to be settled beyond a doubt, though some of the V.S. practitioners deny it, which is almost as reasonable as it would be to deny any other well-authenticated historic fact. Every case of the disease is traceable to one of two sources; either to Mr. Chenery's stock in Belmont (near Boston, Massachusetts), into which the disease was introduced by his importation of four Dutch cows from Holland, which arrived here the 23d of last May; or else to one of the three calves which he sold to a farmer in North Brookfield, Massachusetts, last June." _2dly._ Apart from the importation into countries, we have this certain proof--to which special attention was drawn several years ago--that cattle-dealers' farms, and public markets, constitute the busy centres of infection. Most anxious and careful inquiries have established the proposition that in breeding-districts, where the proprietors of extensive dairies--as in Dumfries, Scotland, and other places--abstain from buying, except from their neighbors, who have never had diseases of the lungs amongst their stock, pleuro-pneumonia has not been seen. There is a wide district in the Vicinity of Abington, England, and in the parish of Crawford, which has not been visited _by_ this plague, with the exception of two farms, into which market-cattle had been imported and thus brought the disease.