No. 597] PRIMARILY UK ADAPTIVE VARIAXTS 



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black-backs as well as a ' 'foreign intermediate" between 

 the two divisions of that group. 3 



Many of the smaller gulls have a well-defined dark 

 hood in the adult. These hooded species may be divided 

 into two apparently natural groups, the first in which the 

 hood is black, the second in which it is dark brown. In 

 the former group Bonaparte's gull (Larus Philadelphia) 

 has a great deal of white in the primaries, making a 

 lengthwise band in the wing, conspicuous in life. Bona- 

 parte's gull is found in North America. In the brown- 

 headed Larus ridibundus of Europe and Asia the white 

 in the wing makes a similar conspicuous mark, so that 

 no one seeing the two species in life could fail to note the 

 great resemblance. They show distinctly what is ordi- 

 narily termed parallelism. Such more or less distantly 

 related, geographically separated parallels, not environ- 

 mental parallels, are of not infrequent occurrence. For 

 convenience we will call them "outcrops." 



Probably the closest relative of Larus Philadelphia is 

 Larus saundersi from the inland waters of China and 

 Mongolia, visiting the coast in winter. This would be a 

 foreign as opposed to an adjacent representative. 



Using, for the sake of familiarity, some of the same 

 material we have already considered, if we contrast 

 white-winged Larus Philadelphia, which is common in the 

 vicinity of New York, with its closest relative common 

 here, the laughing gull Larus atricilla, we will find that 

 the two are as different as the limits of the natural black- 

 hooded group of which they are both members will allow. 

 The white primaries of Philadelphia are contrasted with 

 the unusually dark primaries of atricilla, the mantle of 

 the former is pale, that of the latter dark, and there is 

 considerable difference in size. Such contrasted co- 

 Avi^fino. nll^« fl rp nf frenuent occurrence, let us call them 



