VOL. T-] Distribution of Land Birds. 227 



ed the California Transition Region, I will attempt later to justify 

 this alteration of Dr. Merriam's map In restricting the Sonoran and 

 Californian Areas, and introducing a transition region, but I might 

 here call attention to a passage in Heilprin's work on the Distribu- 

 tion of Animals. Speaking of the Sonoran Transition' Region, he 

 says: **This tract, which, as already stated, comprises the peninsula 

 of Lower California, the State of Sonora in Mexico, New Mexico, 

 Arizona, and parts, not yet absolutely defined, of Nevada, Califor- 

 nia, Texas and Florida, is as far as the Mammalia and birds are 

 concerned not very clearly differentiated. The intermingling of 

 northern and southern elements, with a decided preponderance in 

 favor of the former, is very great, and the peculiarities insignificant." 



Before defining more in detail these life areas it may be of inter- 

 est to note something of the causes which have produced the pres- 

 ent distribution of birds in California. The State is traversed by 

 two mountain chains Inclosing the immense interior valley district 

 watered by the Sacramento and San Joaquin Rivers and their trib- 

 utaries. North of latitude forty -one degrees the country is mostly 

 mountainous and coniferous trees form the characteristic vegetation. 

 Near the coast the redwood is the most abundant tree. It is most 

 continuous in this northern region, but extends southward, follow- 

 ing for the most part the western slope of the Coast Range as far as 

 latitude thirty -six degrees. South of latitude thirty -nine its pres- 

 tige begins to be challenged by the oak, and as it extends south- 

 ward its ranks become less in number and thinned, until it merges 

 entirely into the oak groves. In the Sierra Nevada Mountains the 

 coniferous forests, consisting of pine, fir, spruce, cedar and the Big 

 Trees, extend in an almost unbroken chain from the northern limit 

 of the State south nearly to latitude thirty -four degrees, where 

 the two mountain ranges unite. Even here the coniferous belt does 

 not end but extends southward in a more or less broken line, 

 along the higher ranges and peaks, well into Lower California. 



The country considered above is that belonging to the Boreal 

 Province, and it will now be of interest to compare with it the ter- 

 ritory included In the Sonoran Province. Excluding the transition 

 region, its most marked physical feature is the absence of mount- 

 ains. The great interior valley region is a vast plain occupying the 

 center of the State and inclosed on all sides by mountains. The 

 valley of the Sacramento River Is almost entirely an oak grove, but 



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