Besides the two forms that we have hitherto considered, several other local types exist, and these throw some further light on the problem. Of these the most important in this connexion is chrysoides, which inhabits the whole of southern California and the mainland opposite. This remarkable form is as Allen says, very different from auratus except that it has the quills yellow like auratus, not red like cafer. So that we find here in the extreme west of the whole distribution a type agreeing in one of its chief features with the eastern type. Between this and cafer intergrades have, according to Allen, not been found. The relations of this chrysoides are, Allen thinks, rather with mexicanoides, a southern, smaller race with colours more intense, which inhabits Guatemala, but however that may be, it must be regarded as a cafer which has lost its red quills. The island of Guadeloupe off Lower California has an island form. Beyond the other side of the continent there is also an island form of auratus, inhabiting Cuba, so that clearly the yellow quills can extend into the tropics.
The above account is in many respects incomplete, but it suffices to give an outline of the chief facts. The whole problem is complicated by the undoubted effects of an uncertain amount of migration, and in many, perhaps all, districts, the winter population differs from the summer population of the same localities. The existence of these seasonal ebbs and flows is now well known to ornithologists, and most of the bird species of temperate regions are subject to them.
Difficult as it may be to conceive the actual process of origin of the two types auratus and cafer, it is I think still harder to suggest any possible circumstance which can have determined their development as distinct races, or which can maintain that distinctness when created. Some will no doubt be disposed to appeal once more to our ignorance and suggest that if we only knew more we should see that the yellow quills, the black "moustache" and the red crescent, specially qualify auratus for the north and eastern region, and the red quills, red "moustache" and absence of crescent fit cafer to the conditions of its homes. Each can judge for himself, but my own view is that this is a vain delusion, and that to cherish it merely blunts the receptivity of the mind, which if unoccupied with such fancies would be more ready to perceive the truth when at last it shall appear. Think of the range of conditions prevailing in the country occupied by auratus—a triangle with its apex in Florida and its base the whole Arctic region of North America. Is it seriously suggested that there is some element common to the "conditions" of such an area which demands a nuchal crescent in the Flickers, though the birds of the cafer area, almost equally varied, can dispense with the same character? Curiously enough, the geographical variation of Sphyropicus varius, another though a very different Woodpecker[6] shows that conversely the nuchal crescent can be dispensed with in the Eastern form though it is assumed by the Western.[7]
Allen points out the interesting additional fact that superposed upon each of the two distinct forms, auratus and cafer, are many geographical variations which can very naturally be regarded as climatic. Each decreases in size from the North southward, as so many species do.[8] They become paler in the arid plains, and show the ordinary phases which are seen in other birds having the same distribution. Such differences we may well suppose to be determined directly or indirectly, by environment, and we may anticipate with fuller knowledge it will be possible to distinguish variations of this nature as in the broad sense environmental, from the larger differences separating the two main types of Colaptes, which I surmise are altogether independent of such influences.
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.
In these two States, where the two types overlap, certain forms have been repeatedly found which have been described as two distinct species, Lawrencei and leucobronchialis. Dr. L. B. Bishop and Mr. Brewster showed me two long series of Helminthophila containing various intergrades between the four named kinds, and details regarding these may be found in Chapman's North American Warblers and in Dr. Bishop's paper in Auk, 1905, XXII. Though the characters evidently break up to some extent, the series can be represented as due to recombinations of definite factors more easily than the others which I have described. The differentiating characters are:
| Pinus | Chrysoptera | ||
| 1. | Mantle and lower parts yellow (Y1). | 1. | Mantle and lower parts grey (y1). |
| 2. | Wing-bars white (y2). | 2. | Wing-bars yellow (Y2). |
| 3. | Cheek and throat not black (b). | 3. | Cheek and throat black (B). |
The grey pigment of the mantle is common to both, but is masked by the yellow in pinus, the net result being an olive-green.[10]
I am much indebted to Dr. F. M. Chapman for the loan of the coloured plate in which these distinctions are shown. It first appeared in his book, North American Warblers.