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_ 1. Mantle and lower parts _yellow_ (Y^1). 2. Wing-bars _white_ (y^2). 3. Cheek and throat _not black_ (b). _Chrysoptera_ 1. Mantle and lower parts _grey_ (y^1). 2. Wing-bars _yellow_ (Y^2). 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_. We cannot tell whether _yellow_ or _not-yellow_ is due to the presence of a factor, but we may suppose that one or other gives the special colour to the parts. The black of character 3 is no doubt a dominant. Thus _pinus_ becomes Y^{1}y^{2}b and _chrysoptera_ in y^{1}Y^{2}B. The _Lawrencei_ which has the underparts _yellow_, wing-bars _white_, and _black_ patches is Y^{1}y^{2}B and _leucobronchialis_ which has mantle and underparts _not-yellow_, wing-bars _yellow_ and _no black patches_ is y^{1}Y^{2}b. This representation, it should be clearly understood, is tentative and approximate only. The characters are not really sharp, for there is much grading; but allowing for the effects of heterozygosis and for some actual breaking-up of factors I believe it gives a fairly correct view of the case. In particular we can see how it meets the difficulty which Chapman felt in accepting _leucobronchialis_ as in any sense derived from _pinus_ which has a yellow breast, and _chrysoptera_ which has a black throat, seeing that _leucobronchialis_ has neither. We now recognize at once that this form could be produced by ordinary re-combination of the absence of Y^{1} with the absence of B. I note also with great interest that the modern observers agree that the so-called hybrids may have the song either of the one species, or of the other, or a song intermediate between the two. It may also be added that these two types have several times been seen, in the breeding season, paired with each other or with one of the other combinations. [Illustration: FIG. 1. _Helminthophila pinus_, male. FIG. 2. _Helminthophila pinus_, female. FIG. 3. "Lawrence's Warbler," male; one of the integrading forms.