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 light 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 know^n, 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, xxin, 1906, p. 185). Every 

 degree of intergradation occurs between the two types. I have studied this out 

 MS thorouglily 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 wh'en, 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 ¥. 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 

 21/2. 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. S. 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 light- 

 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, it 

 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 

 groups 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- 



