Plate X shows portions of the canyons of the Yellowstone and the Colorado rivers respectively. In the first the contour interval is 100 feet, and in the second, 250 feet. The horizontal scale is ¹⁄₁₂₅₀₀₀ (about 2 miles to the inch) in the first, and ¹⁄₂₅₀₀₀₀ in the second. These scales should be borne in mind in interpreting the map. Falls, rapids, narrows, and other peculiar features, due primarily to inequalities in the hardness of the rock affected by erosion, will be considered later. [Illustration: +Fig.+ 84.—A surface illustrating the struggle for existence among gullies. Most of the smaller gullies shown on the slope can have but little growth before being absorbed by their larger neighbors. A type of erosion surface common in the Bad Lands. Scott’s Bluff, Neb. (Darton, U. S. Geol. Surv.)] THE STRUGGLE FOR EXISTENCE AMONG VALLEYS AND STREAMS. It is not to be inferred that every gully becomes a valley, nor that every small valley becomes a large one. Among valleys, as among living things, there is a struggle for existence, and fitness determines growth and survival. At an early stage of its erosion history the number of small valleys in a given area is often great, while at a later stage the number is less and the size of the survivors greater. [Illustration: PLATE IX. NIAGARA FALLS. U. S. Geol. Surv.] [Illustration: PLATE X. Fig. 1. YELLOWSTONE PARK. U. S. Geol. Surv. Fig. 2. ARIZONA. U. S. Geol. Surv.] [Illustration: +Fig.+ 85.—Diagram illustrating the absorption of one gully by another by lateral erosion. The successive lines represent successive cross-sections.] One phase of the struggle for existence is often well illustrated on a freshly exposed slope of clay. The number of miniature gullies which develop on such a slope, even in a single shower, may be very large (Fig. 84); but the history of many of them is ephemeral. If two adjacent ones are of unequal depth the widening of the deeper narrows and finally eliminates the divide between them, and the two become one (Fig. 85). Another phase of the struggle for existence is shown in other situations. Examination of a good map of the north shore of Lake Superior or the west shore of Lake Michigan shows a large number of small streams and gullies (Fig. 1, Pl. IV). The valleys are short and narrow, and between and beyond them are considerable areas untouched by erosion. The drainage near the lake is therefore young, and each of the small valleys is growing. This condition of things is perhaps typical of that which has been, is, or will be along the average coast at a certain stage in its erosion history. No equal stretch of coast-line where erosion is far advanced can boast of a number of large rivers comparable to that of the many small ones along the coasts mentioned. It therefore seems evident that of these many small streams a few only will attain considerable size.