Page:Popular Science Monthly Volume 68.djvu/496

492 insects to artificial heat and cold. The butterflies of the glacial regions and those developed in an ice-chest have a pale coloration, and a warm environment deepens the pigment.

It has not been shown that any of these effects are hereditary, or that they constitute a factor in the formation of species, although climatic effects may enter into the process of natural selection. I have before me a series of woodpeckers selected by a student (Mr. Hubert Jenkins), which illustrates at once climatic and other subspecific variation. The collection represents the species known as hairy woodpecker (Dryobates villosus). Taking the typical form villosus, from the eastern United States, we note that specimens from further south (auduboni) are smaller in every way, but otherwise similar. To the northward, in Canada and on the Arctic Sea, the birds are much larger, (leucomelas) ten to eleven inches in length instead of eight to nine, while the feet are scarcely if at all enlarged. In all these the space before the eye is black, and the belly is darker in specimens from the region having the most rainfall. In the Bahamas is a form still smaller, seven to eight inches long (maynardi), with the space behind the bill white. Further westward, in all woodpeckers of this species, the white spots on the wing coverts, characteristic of the eastern forms, nearly or quite disappear, leaving the feathers plain black. Here again the northernmost forms are largest, harrisi of the Californian-Alaskan region being nine to ten inches long, those from California being nearly white below, those from the Vancouver region smoky gray, darkest when the rainfall is greatest. The Mexican form (jardini) is seven to eight inches long, but in the moist regions of Central America it too becomes deep smoky brown. Of these traits, those relating to the size of the bird and the smoky coloration of its lower parts may probably be regarded as climatic. Whether a bird born of northern parents would reach its full stature in the south, or whether it would grow up with white belly plumage in a rainy district, are both open to question. The experiment can hardly be tested with woodpeckers, but some other group may offer conditions more favorable to artificial breeding.

On the other hand, the loss in the western birds of the white wing spots characteristic of villosus and its subspecies, leucomelas, auduboni, and maynardi, can have apparently no climatic cause, but is one of the results of the primitive separation of the forms on the two sides of the Rocky Mountains, or more properly of the treeless plains where woodpeckers of this type are not found.

It may be noticed that in the related but much smaller American species known as the downy woodpecker (Dryobates pubescens), the eastern forms (pubescens) have also the wing coverts profusely spotted with white, while in the western form (gairdneri) the wing coverts, as in the western forms of Dryobates villosus, are nearly or quite plain