This intrusion of organisms on one another's modes of life goes to the extent of intruding on one another's media. The great mass of flowering plants are terrestrial, and (aside from other needs) are required to be so by their process of fructification. But there are some which live in the water, and protrude their flowers above the surface. Nay, there is a still more striking instance. At the sea-side may be found an alga a hundred yards inland, and a phænogam rooted in salt water. Among animals these interchanges of media are numerous. Nearly all coleopterous insects are terrestrial; but the water-beetle, which like the rest of its order is an air-breather, has aquatic habits. Water appears to be an extremely unfit medium for a fly; and yet Mr. [now Sir John] Lubbock has discovered more than one species of fly living beneath the surface of the water and coming up occasionally for air. Birds, as a class, are specially fitted for an aerial existence; but certain tribes of them have taken to an aquatic existence--swimming on the surface of the water and making continual incursions beneath it, and some kinds have wholly lost the power of flight. Among mammals, too, which have limbs and lungs implying an organization for terrestrial life, may be named kinds living more or less in the water and are more or less adapted to it. We have water-rats and otters which unite the two kinds of life, and show but little modification; hippopotami passing the greater part of their time in the water, and somewhat more fitted to it; seals living almost exclusively in the sea, and having the mammalian form greatly obscured; whales wholly confined to the sea, and having so little the aspect of mammals as to be mistaken for fish. Conversely, sundry inhabitants of the water make excursions on the land. Eels migrate at night from one pool to another. There are fish with specially-modified gills and fin-rays serving as stilts, which, when the rivers they inhabit are partially dried-up, travel in search of better quarters. And while some kinds of crabs do not make land-excursions beyond high-water mark, other kinds pursue lives almost wholly terrestrial. Guided by these two classes of facts, we must regard the bounds to each species' sphere of existence as determined by the balancing of two antagonist sets of forces. The tendency which every species has to intrude on other areas, other modes of life, and other media, is restrained by the direct and indirect resistance of conditions, organic and inorganic. And these expansive and repressive energies, varying continually in their respective intensities, rhythmically equilibrate each other--maintain a limit that perpetually oscillates from side to side of a certain mean.