"Dappled" is an inherently happy word that comes with favorable connotations. It is perfectly suited to describing a favorite horse or a pleasant summer's day. It conjures up sunlight, shadows that don't threaten, well-behaved foliage (or a mellow horse), and peaceful, indolent days with babbling brooks. "Dappled" is too genteel to ask which ingredient, sun or shadow, is more essential to the condition. It is too pleasant to ask whether sun overlays shadow or vice versa. (Merriam-Webster avoids that issue by giving usage examples of both sun dappling and shadow dappling. Can you have it both ways?) Similarly, "dappled" is too polite to ask whether the condition inherently raises the zebra-like question whether it is dark on light or light on dark. (It is now usually thought that zebras are dark, with white stripes.) Which comes first on a sunny day? Do sun and shadow reach the ground simultaneously? Light travels; does dark? The brightest light stops traveling when it strikes leaves.

"Freckles" and "speckles" seem wholesome, the sort of thing Doris Day and Van Johnson had. "Mottled" has less favorable implications, especially in a medical context. But in the context of nature's graphics, all of these terms seem valid, appropriate, and neutral enough.

Thus, dappled birches.

And moose.

And calves (more patterned than dappled).

And dogs. And snow.

And Majorca walks and walls and lemons.

And oceans.

And yards. And dogs.

Does dark travel? Trick question. Absent sunlight, everything is already dark. The dark is already "there." When sunlight travels from the sun and stops at the intervening foliage, the pre-existing dark remains because the foliage prevents the sunlight from reaching whatever is in line with the sun and the foliage. If the sun were suddenly extinguished (Thought Experiment), the dark would not travel from the sun.