^°iqo4^n Grinnell, Chestuut-backcd Chickadee. 375 



linear appearance as shown at this day. But when the pioneer 

 invaders at the south reached the vicinity of Point Arena, they 

 met with somewhat changed temperature and consequent floral 

 conditions, but not so abrupt as to constitute a permanent barrier. 

 Doubtless the progress of invasion was retarded until adaptive 

 modifications evolved, which correlatively allowed of further inva- 

 sion, until the abrupt limits of the Santa Cruz District were 

 reached. 



San Francisco Bay and the Golden Gate seem to now form a 

 pretty effectual barrier between neglectus on the north and barlowi 

 on the south. At least, among the large number of skins examined 

 by me with this point in view, I can find none from one side that 

 can be confidently determined as being identical with the race on 

 the other. Neither chickadee has been found east of the bay, nor 

 anywhere nearly so far from the coast belt, except for one record 

 of a specimen taken in the fall at Haywards. This has been 

 reexamined and proved to be barlowi, as was to be expected from 

 its contiguity. However, the Golden Gate is so narrow that an 

 occasional crossing may take place. This was more probable 

 formerly, when the redwood timber grew up to the Gate on both 

 sides. Heermann in 1853 recorded the species from "San Fran- 

 cisco." But now, I think, the bird is unknown for several miles 

 on either side of the Gate. Doubtless this barrier accounts in 

 part for the origin of the distinct form barlowi within so short a 

 distance. 



As to the distance to which a species may invade, we can surmise 

 that, topography permitting, theoretically there is no limit so long 

 as adaptive modifications continually take place. The geographic 

 variation in Melospiza may be called to attention as an extreme 

 illustration. But practically, in the case of Par us rufescens bar- 

 lowi, much further invasion is improbable, because in adjoining 

 areas are already firmly established members of the same family 

 (^Biwlophiis, Psaltriparus, Chafncea) thoroughly adapted to prevail- 

 ing food conditions. No one of these could probably be successfully 

 competed against by a foreigner. Every animal tends to increase 

 at a geometric ratio, and is checked only by limit of food supply. 

 It is only by adaptations to different sorts of food, or modes of 

 food getting, that more than one species can occupy the same 



