° 1914 ] SwARTH, California Forms of Psaltriparus. 515 



contrasted faunas were found encroaching to a marked extent into 

 their neighbor's territory, Colorado Desert forms into the San 

 Diegan district, and vice versa. In the particular case in point 

 we can well conceive of the coast bush-tits throughout this pecu- 

 liarly attenuated strip of appropriate territory being dominated by 

 the sheer numbers of continually dispersing individuals of cali- 

 fornicus, and the distinguishing subspecific characters of the birds 

 suffering in consequence. 



South of San Francisco Bay the humid coast strip (Santa Cruz 

 district) though narrow, is sharply cut off from the violently con- 

 trasted San Joaquin Valley, which is besides almost destitute of 

 possibly competing bush-tits, hence there is every reason for the 

 coast birds preserving their distinctive characters. To the north- 

 ward, beyond Marin County, the coast region is also definitely sepa- 

 rated from the interior valleys; but immediately north of San 

 Francisco Bay, in southern Marin and Sonoma counties, the moun- 

 tains, which here closely approach the coast, become so low and 

 broken as to lose much of their effectiveness as barriers, and, as sug- 

 gested above, there is little to hinder the influx of birds from the 

 Sacramento Valley, densely populated with bush-tits of the sub- 

 species califoniicus. It may well be that not only the peculiar 

 observed conditions among the bush-tits, but also similar instances 

 encountered among certain other birds of the same region, are to be 

 explained by the peculiar combination of circumstances at this point. 

 These conditions briefly summarized, are: Marked restriction of 

 territory appropriate to the humid coast races, ineffective barriers 

 interposed against complementary forms of much greater numbers 

 occupying adjacent territory, and continual encroachment of 

 indi\'iduals (the radiating overflow) of the latter subspecies. 



Northcentral California. Small series of skins at hand from the 

 Siskiyou Mountains, in extreme northern Siskiyou County, and 

 from the Trinity Mountain region in southeastern Siskiyou County 

 and northwestern Shasta County, prove rather difficult to allocate. 

 It seems apparent, however, that these birds must be considered 

 as intergrades between minimus and calif ornicus, though obviously 

 nearer the former. In fact certain of the specimens are not to be 

 distinguished from typical examples of minimus in comparable 

 plumage, though others from the same localities e\adently tend 



