74 BREWSTER’S WARBLER. 
That is, if an /’, pW pw (in plumage a chrysoptera) mate with the hybrid 
pW Pw (leucobronchialis) of the same generation, their 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, 
by this theory 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 required 
of a working hypothesis? 
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, 7. e. they were either lewcobronchiales 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 F, 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 I’, leucobronchialis, pW PW, mated with an impure chry- 
soptera, pW pw. 
1Jt 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. 
