[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. [12] References on this subject will be found in _Brit. Mus. Cat. Birds_, XVII, p. 13.