76 BREWSTER'S WARBLER 



itely ascertained facts. The interbreeding of leucobronchialis with 

 pinus, and with chrysoptera, of pinus with chrysoptera, and of lawrencei 

 with />mw.r is recorded on unquestionable evidence. Here alone, there- 

 fore, we have indisputable knowledge of sets of relations which in 

 their subsequent stages are bound to produce the most varied results, 

 accounting for every phase of plumage of the lawrencei type of which 

 we have any knowledge. 



It is difficult, however, to understand how the interbreeding of 

 a yellow-breasted (pinus) with a black-breasted bird (chryso- 

 ptera) can, even ultimately, produce a white-breasted bird (leucobron- 

 chialis). Though I at one time believed this to be probable, fresh 

 evidence has led me to accept the theory of dichromatism as satisfac- 

 torily accounting for those birds which are pinus minus yellow. Such 

 specimens have the wing-bar virtually white, and, as above stated, 

 differ from typical pinus only in the absence of yellow. In the collec- 

 tion of Dr. L. B. Bishop, there is a series of these birds in which this 

 leucochroic phase of pinus is connected with the typical yellow bird 

 by a perfectly graduated set of intergrades. (See Bishop's 11 admir- 

 able presentation of the case.) 



H. chrysoptera, as we have seen, breeds with pinus in either its 

 yellow (true pinus) or white (leucobronchialis) phase, in the latter 

 case, doubtless producing the results which have made it difficult to 

 accept the theory of dichromatism ; as, for example, when a specimen 

 is a leucochroic pinus in all parts except the wing-bars, which, 

 reversing the change from yellow and olive-green to white and gray 

 throughout the rest of the plumage, turn from white to yellow. 



The great variability of this character, however, may safely be 

 attributed to the effects of hybridization. Thus we have otherwise 

 typical specimens of pinus with the broad yellow wing-band of chrys- 

 optera, while chrysoptera may show the two white bars of pinus. In 

 H. lawrencei, similar conditions may be found and, as has just been 

 said, they also exist in the bird known as leucobronchialis. 



A strong argument for the theory of hybridity among these 

 Warblers, is that they have been found breeding only where their 

 ranges overlap. This same fact, it is true, may be presented as evi- 

 dence against the theory of dichromatism. Dichromatism, however, 

 does not necessarily occur throughout the range of a species, nor 

 does the absence of leucochroic breeding specimens of pinus from a 

 part of its range where chrysoptera does not occur, prove that they 

 do not exist. 



