Page:Popular Science Monthly Volume 76.djvu/331

Rh with questions of their plumage, and especially with the coloration of the same. A good summary of this knowledge is contained in Vernon.

More remarkable than these observations are the facts ascertained by A. E. Wallace, and communicated by him to Darwin. Thus he states that

Artificially produced alterations in the pigmentation of American birds are shown by the experiments of C. W. Beebe. These experiments demonstrate that the effect of a very humid atmosphere is to increase the dark pigment in the three species studied, namely, the wood thrush, the white-throated sparrow and the inca dove. Beebe mentions that in a state of nature, where the dark forms have been isolated by geographical barriers (and where, of course, natural selection, or other adaptive forces, have been at work for generations), other structural differences are to be found. "With this darkening of the skin structure is frequently correlated a distinction in point of size, either of the body and skeleton as a whole or superficially, as of larger or shorter feathers of the wings or tail." Since Beebe mentions no structural changes of the body as a result of his artificially produced humidity, one infers that the changes were confined to the pigmentations.

In the early stages of embryogeny, heat and light, especially heat, affect the rate of development, but there is nothing, as far as I know,