186 CALIFORNIA ACADEMY OF SCIENCES. 



its boundary is not sharply defined, while in the chuck- 

 will's-widow ( Antrostomus carolinensis) it is a very broad 

 band. In the California jay (Aphelocoma californica) it 

 is sharply defined but is composed of a series of fine 

 'white streaks or spots, and hence is not a continuous 

 line. It hardly seems within the bounds of reason to 

 suppose that all the species which exhibit this mark 

 are the descendants of a common ancestor which pos- 

 sessed it, and that it has persisted without significance 

 for so long a time, although this would not perhaps be 

 any less conceivable than the retention by man of the 

 pineal eye. On the other hand, it can hardly be held 

 that the superciliary stripe is a recognition mark, be- 

 cause it is very often inconspicuous in life, although 

 sometimes very noticeable, as in the varied thrush (Hesper- 

 ocickla ncevia), where it forms one of the very best char- 

 acters for recognizing the bird instantly, even though 

 at a distance. Among many of the warblers, vireos and 

 sparrows, where it is present in closely allied species, it 

 would not serve, however, as a help in detecting the 

 species. 



There is one thing which seems to be of some sig- 

 nificance that this marking so universally present in 

 the most diverse groups is a streak and not a bar. This 

 fact together with the circumstance that it is frequently 

 present in the female or young and not in the adult, 

 and that the reverse is never (?) the case, would indicate 

 that it is a marking of great antiquity, and has prob- 

 ably no very great utilitarian significance, except in 

 exceptional cases where natural selection has made use 

 of it for a recognition mark. 



In general the markings of the head are lateral rather 

 than trail verse. They may be reduced, for the most 

 part, to certain types or modifications and combinations 

 of these. Plate IV represents the various patterns of 



