south of the South Tyne the hills soon rise to a considerable height. No portion of the main branch within Northumberland is high enough to reach the Middle zone; but the moor between that and the Allen rises into it within a short distance of Haltwhistle and on the other side of the Allen above Catton. The 300 yards contour-line strikes the Allen about 3 miles south of Allentown, and from Catton Moor curves round the head of the Devil's Water along the edge of Slealey Moor to Shotley. South of the Derwent the watershed ridge is Middle zone as far east as Cold Rowley, a distance of 15 miles from the head of the river, and north of the Wear as far east as Wolsingham. The contour line strikes the Wear 15 miles west of this at St. John's Chapel; but on the south of the stream there is again a continuous high ridge, which is Middle zone as far east as Wolsingham, and a short distance beyond Eglestone; and finally, the contour-line strikes the Tees between Wince Bridge and the High Force. The whole area that reaches into the Middle zone is probably not one-twentieth part of the area of the two counties.
Ascending Limits of Wild Plants.—Of the indigenous plants of Britain, 37 per cent, are, to speak in general terms, distributed throughout the length and breadth of the island, whilst 43 per cent, of the species show a decidedly southern tendency, and 17 per cent, are either northern in their tendencies, or at any rate absent from the south and found northward principally amongst the hills. We may safely connect the 43 per cent, with the warmer temperatures, and the 17 per cent, with colder ones, and, of course, such of the former as come within our bounds, are almost all restricted to the low level country. As we ascend from one level to another the number of species decreases rapidly, partly through climate, and partly because at each successive step there is a smaller extent of surface, and the range of variation in station grows rapidly smaller also. If we attribute the running out to climate entirely we shall make a mistake, as we may see by studying the altitudes reached by the same plants in other parts of the island. For instance, with us there are no trees, either wild or planted, above 600 yards, but in Scotland
