[2] For a case in which a red-headed female × a black-headed male gave a black-headed female and a red-headed male, see Avian Mag., N. S., IV, pp. 49 and 329
[3] The other variations of this bird are also interesting and important. The normal male has a red head and a red throat. The female has a red head and a white throat, but varieties of the female are known with a black head, thus again illustrating the change from black to red. It should be noted that this is not a mere retention of a juvenile character, but, as the birds mature, the red feathers come up, or as an exception, the black. There is also a western species, ruber, in which both sexes have a great extension of red, and are alike. The male of nuchalis intergrades with this type, but the female does not.
[4] Dr. W. Brewster, for example, has a remarkable specimen of the Teal (Nettion carolinense) with a white collar strongly developed at the front and sides of the neck, in a place where the normal has no such mark.
[5] This variety is spoken of as the Ringed Guillemot and is sometimes regarded as a distinct species to which the name ringvia was given by Brünnich. In support of this view Dr. William Brewster, to whom I am indebted for much assistance in regard to the variation of birds, called my attention to observations of his own and also of Maynard's, that the ringed birds were sometimes mated together, though in a small minority (see Brewster, Proc. Boston Soc. N. H., XXII, 1883, p. 410). It would however be possible to produce many instances of varieties mated together though surrounded by a typical population (e. g., two varying Blackbirds, Zoologist, p. 2765; two varying Nightjars, ibid., p. 5278). I am inclined to believe that in nature matings between brothers and sisters are frequent in many species of animals, and that the production of sporadically varying colonies is thus greatly assisted.
[6] The Sap-suckers feed on trees and somewhat resemble our Spotted Woodpeckers in general appearance. Colaptes feeds on the ground and corresponds perhaps rather with the European Green Woodpecker.
[7] For an introduction to this example I am indebted to Mr. W. D. Miller of the American Museum of Natural History. Some account of the facts is given by Baird, Brewer, and Ridgway (A Hist. of N. Amer. Birds. 1874, II, pp. 540, 544, etc.). S. varius occupies the whole country in suitable places from the Atlantic to the eastern slopes of the Rockies, and all Mexico to Guatemala. S. nuchalis was first known from the Southern Rockies only, but many were afterwards taken in Utah. S. ruber is restricted to the Pacific coast. In Ridgway's opinion all three are geographical forms of one species. In ruber the sexes are alike having both a great extension of the red in the throat, and a red crescent. The male of nuchalis grades to the ruber form, but the female does not. This female has some red in the throat like the male of varius, whereas the female of varius has a whitish throat.
[8] Not only vertebrates but the marine Crustacea and Mollusca illustrate this curious "principle" of variation, as Canon Norman formerly pointed out to me with abundant illustrations. There are of course cases to the contrary also.
[9] Chapman, F. M., Bull. Amer. Mus., IV, 1892, p. 1; see also Ridgway, Birds of North and Middle America, 1902, Part II, p. 214.
[10] It would aid greatly in factorial analysis if the descriptive term "green" could be avoided in application to cases where the green effect is due only to a mixture of black and yellow pigments. The absence of yellow is the sole difference between the mantle and underparts of pinus and chrysoptera.
[11] Bull. Amer. Mus. Nat. Hist., XXIII, 1907, p. 467.