The Leach Petrels 



upon our list. Mr. Anthony informs me that the birds have recently (1922) been found 

 breeding on certain islands off the western coast of Lower California. 



Authorities. — Miller (Oceanodroma leucorhoa kadingi), Condor, vol. xx., 1918, 

 p. 211 (specimen taken off coast of s. Calif.); Oberholser, Proc. U. S. Nat. Mus., vol. 54, 

 1917, p. 171 (syst.; meas. ; distr.; etc.) ; Grinnell, Condor, vol. xx., 1918, p. 46 (syst.); 

 Loomis, Proc. Calif. Acad. Sci., ser. 4, vol. ii., pt. 2, no. 12, 1918, p. 160, part (syst.). 



The problems arising from a consideration of the distribution and interrelation- 

 ship of the Petrels of the genus Oceanodroma, are without exception the most fascinat- 

 ing as well as the most elusive and difficult of any presented by our science. 

 In particular, the central question of the forces controlling the differentiation of species 

 reaches in this group a commanding climax. Within its narrow limits most subtle 

 differences, which theoretically must exist, nevertheless defy detection, while other 

 differences are fairly startling in their abruptness. Thus, the dominant type, 

 0. leucorhoa leucorhoa (from which our 0. leucorhoa beali claims only most dubious dis- 

 tinction), dominates the islands along both shores of both the great oceans of the 

 Northern Hemisphere. It exists in countless multitudes on a thousand islands stretch- 

 ing through at least twenty degrees of latitude and along ten thousand miles of coastline; 

 and yet the differences alleged between extremes from the Commander Islands, off 

 the coast of Asia, or the Flanneries, off the coast of England, or the Olympiades, off 

 the coast of Washington, are so slight that the memory can scarcely retain them. 

 And suddenly, on a small group of islands off the coast of California, the Farallons, 

 appears another species absolutely distinct. From the Farallons to San Benito, 

 through a range of only ten degrees, four such indubitable species appear (including 

 Halocyptena microsoma, whose generic difference is based on a slight character), 

 and three forms of one of them, viz., 0. I. socorroensis, 0. I. kaedingi, and 0. I. macro- 

 dactyla, besides 0. leucorhoa (beali), which breeds on the Farallons and migrates way 

 beyond San Benito. There you have it! one species continuous over a highly diversified 

 coastal range of ten thousand miles in two oceans, and (accepting San Miguel as a 

 breeding station of 0. homochroa) four species or six races in a fairly uniform range of 

 500 miles along the coast of the Californias. 



The situation is, indeed, so extraordinary that we find ourselves obliged to at- 

 tempt some sort of reconstruction of recent Oceanodromine history. The species 

 Oceanodroma probably had its distributional, or evolutional, center here on the islands 

 of the southern Californias. High differentiation within a limited but interrupted 

 (i. e., island) area is sufficient evidence of ancient occupation. One species, the ances- 

 tral 0. leucorhoa, evolving perhaps on Guadalupe, finding its ancient home overcrowded, 

 set about searching for unoccupied territory. Passing over the ancient Farallons, 

 already occupied by homochroa, it found such territory in the newer islets of the North 

 Pacific Coast, successively released from glacial conditions. Here it flourished like 

 the green bay tree, taking on gradually the slightly different characters of larger size, 

 paler coloration, and deeper furcation of the tail, — 0. leucorhoa beali. From this 

 rich field, members of this group, wafted by some accident of wind or weather, reached 

 the North Atlantic Ocean, where, finding similar unoccupied territory, they spread 

 rapidly without sensible change of form, — 0. leucorhoa leucorhoa. 



Of course this outline is purely hypothetical, but it is not by any means fanciful; 

 for every student of evolutionary changes is agreed that uniformity in a given species, 

 over an extensive, highly diversified, interrupted area, argues both recency of occu- 

 pation and strength of stock. Moreover, the now thoroughly recognized and regular 

 north and south migrations of beali-leucorhoa in both oceans, argues a southern and 

 relatively recent origin of the species. Beali-leucorhoa is the dominant type of the 



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