It is generally supposed that phenomena like those now so well established in the case of _Colaptes_ are very exceptional, and as has already been stated a number of circumstances must combine in order that they may be produced. I suspect however that the examples are more numerous than is commonly thought. In all likelihood the three forms _Sphyropicus varius_, _nuchalis_ and _ruber_ are in a very similar condition though the details have not, so far as I know, been worked out. A complex example which is closely parallel to the case of _Colaptes_ was described by F. M. Chapman[9] at the same date as Allen's work. This is the case of _Quiscalus_, the Grackles, which in the North American Continent have three fairly distinct forms which Chapman speaks of as _Q. aeneus_, _Q. quiscula_, and _Q. quiscula aglaeus_. The birds are all, so far as pigment is concerned, dark blackish brown, but the head and mantle have superposed a metallic sheen of interference-colours which in the various forms take different tints, bluish green, bronze green, or bronze purple. The details are complicated and difficult to appreciate without actual specimens, but the two common types are sufficiently distinct. The birds inhabit the whole area east of the Rockies, _quiscula aglaeus_ occupying Florida and the Southern States southwest of a band of country about a hundred miles broad extending roughly from Connecticut to the mouth of the Mississippi; and _aeneus_ taking the area north and west of this band. In discussing this case Chapman expresses the same view as Allen does in the _Colaptes_ case, that there are two distinct populations, substantially fixed, and that the band of country in which they meet each other has a mongrel population, with no consistent type, but showing miscellaneous combinations of the character of the two chief types. The warblers of the genus _Helminthophila_ provide another illustration which has points of special interest. The two chief species are _H. pinus_, which has a yellow mantle and lower parts, white bars on the wings, a black patch behind the eyes and a broad black mark on the throat; and _H. chrysoptera_ with dark grey mantle and pale whitish grey lower parts, yellow bars on the wings, and grey marks on cheeks and throat where _pinus_ has black. These two birds are exceeding distinct, and in addition their songs are quite unlike. _H. pinus_ ranges through the eastern United States up to Connecticut and Iowa. _H. chrysoptera_ is a northern form extending down to Connecticut and New Jersey. Both are migrants.