36 PACIFIC COAST AVIFAUNA No, 12 
the Tring Museum tells me that in addition to the above differences, monorhis 
averages larger than socorroensis, and that he considers the two species not 
quite the same. <As for the ight gray or plumbeous shade of the head of our 
bird, and presumably the other, this should not be taken as a specific character. 
In freshly taken breeding birds it is very noticeable, but after the skin is laid 
away in a cabinet for a year or two, the head becomes as dark as the rest of the 
body. 
As is well known, this species has two extreme color phases. In one the 
rump is of the same color as the back and underparts, while in the other, the 
rump is almost entirely white. One of the latter was described under the name 
Oceanodroma monorhis chapmani (Berlepsch, Auk, xxm, 1906, p. 185). Every 
degree of intergradation occurs between the two types. I have studied this out 
as thoroughly as possible and although I have been unable to reach any positive 
conclusion in regard to the significance of this variation, I shall give the facts 
as they appear to me in order that they may form the basis from which others 
may work in the future. 
I judge that when, in a given species, there are two distinct phases with 
every degree of intergradation between, this species is in process of evolution 
from one type to the other. Thus socorroensis was originally either a dark- 
rumped or a white-rumped race, and is now evolving to the other extreme. Birds 
collected in 1902 by F. S. Daggett show a ratio of white- to dark-rumped birds of 
1 to 9. In 1910 I found the proportion of those which I took on the Coronados 
was in the neighborhood of 1 to 5, while in 1913 the ratio was not far from 1 to 
214. However, A. van Rossem (13) and L. M. Huey found that on August 13, 
1914, the ratio had gone back to about 1 to 4. J. Grinnell and F. 8. Daggett (7) 
seem to think that these petrels are descended from a wide-spread, white-rumped 
ancestor, and of course this may be true; but from the averages taken daring the 
first part of three nesting seasons, it will be seen that the proportion of lght- 
rumped birds is quite rapidly increasing, and it seems reasonable to conclude that 
in a relatively short time, a dark-rumped specimen of socorroensis will be rare. 
Another possible, though improbable, explanation of the occurrence of the 
two types, is that two species, a white- and a dark-rumped one, have bred together 
and hybridized too recently for the characters of the resulting hybrid to have 
become thoroughly fixed. If this was the case, the birds of one of the extreme 
phases should closely approach some other species, which, as far as I am aware, 1t 
does not do. Also the two phases occur among the breeding birds of both the 
Coronados and San Benito islands, and it does not appear likely that the cross- 
ing of two species would occur upon the two islands simultaneously. 
In order to discover whether there is any size difference between the two 
phases, I measured a series of a hundred and forty-three birds collected by D. R. 
Dickey, A. van Rossem, F. S. Daggett and myself. I divided the birds into four 
eroups according to the amount of white exhibited, calling them dark, medium, 
light and white. The number of birds in each group was, respectively, 45, 46, 19 
and 24. Measurements of the extremes, dark and white, show that the former | 
is slightly greater in length (taken only of birds in the flesh) wing, tail, bill, tar- 
sus, middle toe and fork of tail. This held good also in comparative group meas- 
