FIG. 4. "Brewster's Warbler," male; another of the integrading forms. FIG. 5. _Helminthophila chrysoptera_, male. FIG. 6. _Helminthophila chrysoptera_, female.] Allen[11] has described another excellent American example, the Tits of the group _Baeolophus bicolor-atricristatus_. The form _bicolor_ belongs to the eastern States and ranges from the Atlantic coast to the Great Plains, and _atricristatus_, of east Mexico, extends from Vera Cruz to central Texas. In southern and central Texas the breeding ranges adjoin, and in this country various intermediates occur. The chief types differ in two main points. _B. bicolor_ Forehead varies from deep _black_ to dull black, suffused with rusty brown. Crown and crest _grey_, slightly darker than the back. _B. atricristatus_ Forehead _white_ to buffish white. Crown and crest _black_, abruptly contrasting with the back. The intergrades between the two have, as usual, received specific names. A detailed description is given by Allen, from which it appears that the gradation is very complete. In one case a series of 16 adults were all intermediates. It is not stated whether the collector took these at random, but from the local lists it is clear that the types are found not far away from the place where the intergrades were shot. Another very striking case is that of the Tanagers, of the genus _Rhamphocoelus_. In this group there are several local forms which are related to each other in remarkable ways. The forms known as _passerinii_ and _icteronotus_ exhibit the clearest phenomena of intergradation. The species _passerinii_ has a brilliant scarlet and black male, and it inhabits Honduras and Nicaragua. Proceeding southwards along the isthmus we find next _costaricensis_ which has a male like that of _passerinii_ (but a female with more orange than the olive-grey female of _passerinii_). Next we come to Panama which is occupied by _icteronotus_, sharply distinguished from _passerinii_ by the fact that the _scarlet is replaced by lemon-yellow_. This same _icteronotus_ occurs again as a pure type in Ecuador and many other parts of South America; but Colombia, _between Panama and Ecuador_, contains scarlets like _passerinii_, yellows like _icteronotus_, and various intergrades of several shades of orange. The _passerinii_ males from Nicaragua are indistinguishable from those of Colombia, and the _icteronotus_ of Ecuador are the same as those in Panama. The orange intergrades, doubtless heterozygous forms, though collected at the same locality (Medellin in Colombia) as several pure yellows and pure scarlets, are in the British Museum series sorted out as a separate species under the name _chrysonotus_! Complications are introduced by the relations of these forms to another named type, _flammigerus_, but we may for our purpose leave that out of consideration, and say that the order of geographical sequence from Honduras to Ecuador is (1) scarlet,