ulations of the creature's form which might otherwise be apparent; while each 

 of these stripes looks in itself like a bright stick, twig, grass-blade, leaf-edge, 

 or weed stem standing out above a shadow — either its own or a more general 

 background-shadow, which shows on either side of it. Or again, this mark- 

 ing suggests a shiny, cylindrical stick or steni, with its central streak of high- 

 light, and the bordering shadows on itself, caused by its own curvature. Still 

 another detail of ground-scene which these chipmunk-stripes suggest is the 

 pattern made by sunlight falling between parallel twigs or stems — narrow, 

 shadow-bordered sun-streaks on the earth or leaves or stones. They are, in 

 short, generalized but most efficient picture-patterns. The western Four- 

 striped Ground Squirrel (Tamias quadrivittatus) is colored and marked much 

 like the eastern Chipmunk, but its stripes, besides being more in number, are 

 narrower, and, as picture-patterns, perhaps even more generalized. Notable 

 among the several other modifications of this pattern to be found on North 

 American mammals is that worn by the beautiful Thirteen-lined Ground 

 Squirrel (Spermophilus tridecimlineatus). The black intervals between its 

 many light-colored secant stripes are traversed by lengthwise chains of bright 

 spots, like slender flower-spikes or little leaves over shadow. On some sper- 

 mophiles (as 5. grammurus douglassii), the secant pattern is reduced to a 

 single broad black stripe along the middle of the back — an exaggeration of 

 the dusky^' ridgepole' mentioned at the beginning of Chapter XX. 



An illustration of the obscurer type of generalized flecky background pat- 

 tern on mammals was given in Plate VII. It is a type extremely prevalent 

 among terrestrial mammals. Indeed, if we include all its oflEshoots, such as 

 the system of regular, scale-like bright flecks on a dark ground, worn for in- 

 stance by certain spermophiles, it is the commonest as it is the obscurest 

 kind of mammalian pattern. It is worn by many terrestrial rodents, etc., 

 of the open ground — e. g., hares, gophers, lemmings, spermophiles — and, 

 variously modified, by many terrestrial forest mammals. Simplified to a 

 close, bark-like grizzling, it is characteristic of many arboreal beasts, such as 

 squirrels and some of the marmosets and lemurs. With very few exceptions, 



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