OVERLAPPING FORMS 159 



times been seen, in the breeding season, paired with each other 

 or with one of the other combinations. 



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, ex- 

 tends 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 B. atricristatus 



Forehead varies from deep black to dull Forehead white to buffish white. 



black, suffused with rusty brown. 



Crown and crest grey, slightly darker Crown and crest black, abruptly con- 



than the black. trasting 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 costari- 

 censis 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 



11 Bull. Amer. Mus. Nat. Hist., XXIII, 1907, p. 467- 



