the environment, and if this shall go on changing in the direction of these peculiarities, they will increase. Suppose that the climate should gradually grow colder; the result might be a denser growth of hair. Those which, by strength or otherwise, chanced to have denser hair than others, would more readily endure the change, and would transmit the habit to their young; others less fitted to endure the change would die out. In a short time the young would be born with the dense hair, as it is well known that any new habit assumed by animals is exhibited earlier and earlier in the young, as long as it is a necessity of life. These variations are never due to one single cause. Organized life is wrought upon by a wonderfully complex web of influences. Darwin has the following passage touching the action of vegetable and animal life upon each other.
"In Staffordshire, on the estate of a relation where I had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously, and planted with Scotch fir. The change in the native vegetation of the planted part of the heath was most remarkable,—more than is generally seen in passing from one quite different soil to another; not only the proportional numbers of the heath plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath. The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations which were not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds. Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception that the land had been enclosed, so that cattle could not enter. But how important an element enclosure is, I plainly saw near Farnham in Surrey. 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 enclosed, 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 unenclosed 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 multitudes 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, judging from the rings of growth, had during twenty-six years tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so 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 the 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 caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by birds. Hence if certain insectivorous birds (whose numbers are probably regulated by hawks or beasts
