74 BRE\YSTER'S \YARBLER. 



That is, if an F2 pW p\v (in plumage a chrysoptera) mate with the hybrid 

 pW Pw [leucobronchialis) of the same generation, then- issue would be, on the 

 average, chrysoptera, leucobronchialis, pinus, and lawrencei in the relative propor- 

 tions of 3: 3: 1: 1. The only other possible unions which could produce Law- 

 rence's Warblers, if the union of two Brewster's Warblers be debarred, are 

 indicated above by the dotted lines, — altogether five combinations out of 

 twenty-two possibilities. It is evident, therefore, that Lawrence's Warbler, 

 bj- this theorv of its origin, must ever be a very much rarer bird than Brewster's 

 Warbler, and that such is really the case is well established by the records. The 

 hybrid theory illumined by the Mendelian Law of Heredity accounts not only 

 for the existence of Brewster's and Lawrence's Warblers but also for the relative 

 abundance of these two extraordinary forms. What more should be recjuired 

 of a working hj'pothesis? 



I regret that the crucial test afforded by the mating of H. pinus with H. 

 chrysoptera was not presented for study in Lexington last summer. The chances 

 of meeting with a case of this sort here are very remote, H. pinus being so rare 

 as almost to be classed as accidental. In place of the union of the two hypotheti- 

 cal parent species I had in both cases to deal with the union of the hypothetical 

 hybrid, H. leucobronchialis, with one of the parent species, H. chrysoptera. The 

 results were not devoid of interest. As has been shown, the offspring were all 

 like one or the other of the parents, i. e. they were either leucobronchiales or 

 chrysopterae. There were none that showed characters intermediate between 

 the parents. In other words Mendel's Law of Dominance was operative.' 



By our theory the union of an Fi leucobronchialis with a pure chrysoptera 

 should produce a mixed brood of leucobronchiales and chrysopterae and this was 

 the composition of one of the two broods of mixed parentage whose history has 

 been detailed in the foregoing pages. The same result would ensue from other 

 combinations, e. g. an F2 leucobronchialis, pW PW, mated with an impure chry- 

 soptera, pW pw. 



' It is a curious fact that intermediates between leucobronchialis and chrysoptera are almost un- 

 known. Mr. Brewster (Bull. Nuttall Orn. Club, 1881, 6, p. 219. and Dr. Fisher, (Id., p. 245) have 

 recorded a specimen of leucobronchialis with black auriculars like chrysoptera; Mr. Ridgway in his 

 article in the Auk, 1885, 2, p. 363, seems inadvertently to have referred to this case as two. Dr. Town- 

 send (Auk, 1908, 25, p. 65-66) mentions a female leucobronchialis in Mr. Brewster's collection with faint 

 grayish cheek patches. On the other hand leucobronchialis, especially in Connecticut, grades into 

 pinus by a complete series of intermediates. Whether the Law of Dominance would cease to operate 

 as a result of long continued breeding-in of the hybrid with pinus I leave to the consideration of 

 those who are better versed in Mendelism than I am. I have little doubt, after surveying the whole 

 genus Helminthophila and taking into account the color of the juvenile plumage of all the species, that 

 the yellowish under parts are an ancestral feature. That an ancestral character should be suppressed 

 as a recessive at the first crossing is not remarkable. The same thing has been shown to happen in 

 crossing breeds of barn-yard fowl. 



