As evening comes on, the doves come in from the desert, flying low along the line of seepage. The vultures return to roost, lazily circling the cottonwood’s crown. While it is still light the butterflies seek cover in the cottonwood leaves. As it gets dark the moths come out, and after them the bats, beep-beeping as they cut erratic patterns through the dusky air. Soon the breeze will die down, and the starlit night will throb with the long drawn trill of tree crickets. In the wee small hours there will be no sound, no breath of air or outward sign of life. Then suddenly along a sandy trail moves a blackness shaped like a high-backed child’s chair. It is a striped skunk, tail-high, come to take its turn at the waterhole. [Illustration: The javelina, or peccary, smells like a skunk. This nighttime wanderer uses the scent for territorial marking, not defense. Curious and shortsighted, javelinas might approach a hiker—not to attack, but to investigate.] Few and far between are the springs with sufficient flow to send a brook singing down a ravine. But such a place is Glenn Spring, the chief spring along a dry draw that starts in the Chisos and cuts deeply through many-colored clays as it crosses the desert. Historic photos show Glenn Spring enclosed within a man-made rock wall. Today you cannot even find the source, so thick is the tangle of tules and cane grown up around it. The flow from Glenn Spring trickles down the draw about 1.5 kilometers (1 mile), collecting in pools and gurgling over rocks before it goes underground. Some of the pools are crystal clear, and some are black with the acids of plant decay. Deeper pools are fern-green with algae. Little black snails harvest algae on the rocks and leopard frogs croak and plop, so quick to hide among the reeds that you can hardly find them. These slim, spotted amphibians, insect feeders, mate in water. Their larval young, free-swimming tadpoles, must live in water, feeding on microscopic organisms until they grow lungs and legs for life ashore. The tadpole itself falls prey to giant water bugs, air-breathing water dwellers that are also strong fliers. The water boatman is a vegetarian who sculls about from one underwater plant to another. You can hardly tell him from the backswimmer except that the latter swings his oars upside down and spends much time on the surface hanging head down, the better to spy the aquatic insects upon which he feeds. The water strider is another hunter, but this spider-legged semi-aquatic skates atop the water, seeking terrestrial insects that have dropped onto the surface. Just as the birds and bats eat different foods at different feeding levels, so do the creatures that inhabit a pool, be it only centimeters deep.