vintage 1920s Little Nature Library, natural history / botanical color plates

>Four volume set of the Little Nature Library series books, vintage 1920s. Titles here are Butterflies, Birds, Trees, and Wildflowers, all with lovely color plates.

The books measure about 8 1/4" x 5 1/2". All have some wear and spotting from age and use, though it's a fairly nice older set overall.