Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/194

144 mice depends on the number of cats; hence, if the cats were destroyed the field-mice would increase and destroy the bees, in which case the clover would produce no seed, and the cattle would soon be deprived of a most important article of food. The same author calls attention to the fact of cattle determining the existence of trees: "Here there are extensive heaths, with a few clumps of old Scotch firs, on the distant hill-tops: within the last ten years large spaces have been inclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. When I ascertained that these young trees had not been sown or planted, I was so much surprised at their numbers that I went to several points of view whence I could examine hundreds of acres of the uninclosed heath, and, literally, I could not see a single Scotch fir, except the old planted clumps. But on looking closely between the stems of the . heath, I found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one of the old clumps, I counted thirty-two little trees; and one of them, with twenty-six rings of growth, had, during many years, tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land was inclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren, and so extensive, that no one would ever have imagined that cattle would have so closely and effectually searched it for food. Here we see that cattle absolutely determine the existence of Scotch fir. But in several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they" swarm southward and northward in a feral state; and Azara and Rengger have shown that this is