OTHER THEORIES OF SPECIES-FORMING. 259 



are birds-of-the-year, which thus, obeying the 'mad impulse,' are 

 led away from their birthplace into new country, where they may 

 take up their permanent abode, and be less likely to compete with 

 their parents or others of their kind. Then, too, cross-breeding 

 of distinctly related individuals is more likely. The records of 

 the Santa Cruz chickadee outside of its regular breeding range, are 

 all of August to October dates (Hay wards, Gilroy, San Jose, etc.). 



"Thus, as above indicated, by the occupancy of new territory 

 the number of individuals which can be supported will corre- 

 spondingly grow. Hence a vigorous colony will spread out along 

 lines of least resistance, being hindered by slight faunal changes, 

 but completely checked only by topographic or abrupt climatic 

 barriers. Parus hudsonicus and its near relative Parus rufescens 

 are boreal species, the former inhabiting the Hudsonian Zone and 

 the latter a certain portion of the Canadian. It seems reasonable 

 to suppose that rufescens differentiated in the northern part of the 

 humid coast belt, which has been called the Sitkan District. This 

 is a faunal subdivision of the Canadian Zone, and its northern 

 part approximates more closely Hudsonian conditions than south- 

 erly. Granting that the early centre of differentiation and distri- 

 bution of Parus pre-hudsonicus rufescens was in the northern part 

 of the Sitkan District, then the route of emigration would be con- 

 fined to the narrow southward extension of that faunal area. The 

 habitat of Parus rufescens thus gradually acquired the long north 

 and south 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 invasion, until the abrupt limits of the Santa Cruz Dis- 

 trict 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 reex- 

 amined and proved to be barlowi, as was to be expected from its 

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

 sional crossing may take place: This was more probable formerly, 

 when the redwood timber grew up to the Gate on both sides. 



