EFFICIENCY OF KLAMATH GAP AS A BARRIER TO BOREAL 

 SPECIES COMPARED WITH THAT OF PITT RIVER AND FEATHER 

 RIVER GAPS COLLECTIVELY. 



Ill view of the iiiiriowiiess of Jvlaniatli Gap, a break of less tliaii 50 

 miles, separating the boreal fauna of Shasta from that of the Cascades, 

 compared with the breadth of the combined Pitt lliver and Feather 

 River gaps, about 100 miles, separating Shasta from the boreal fauna 

 of the Sierra Nevada northwest of Honey Lake, one might expect Shasta 

 to share more species with the Cascades than with the Sierra. The con- 

 trary is true. The Featiier River Gap, as elsewhere explained (p. 70), is 

 ineffective compared with the others; the branches of Feather River do 

 not cut comi^letely through the mountains, and the gap is merely a low 

 part of the range, with the Honey Lake ridge and small boreal-capped 

 peaks projecting here and there as stepping stones between the main 

 Sierra and Mount Lassen. Pitt River Gap is deeper, cutting completely 

 through the range between Lassen and Shasta, forming a boreal break 

 about 60 miles in width, and there is no apparent reason why it should 

 not be as effective a barrier as Klamath Gap, although from the stand- 

 point of zone distribution it does not cut so low and therefore has a 

 slightly cooler summer climate, in consequence of which it is less effect- 

 ive. But this difference is insufficient to explain the really great dis- 

 parity in potency of the two, for in checking the extension of boreal 

 species Klamath Gap has proved far more effective. 



Passing over the species common to Shasta and the Sierra-Cascade 

 system as a whole (see p. 73), only three of the ten distinctively Sierra 

 mammals which reach the northern end of the Sierra fail to reach 

 Shasta, and two if not all of these bridge the Feather River Gap and 

 reach Mount Lassen, which is separated from Shasta by only the Pitt 

 River Gap. On the other hand, not one of the ten distinctively Cascade 

 mammals which occur at the extreme south end of the Cascade Range 

 has been able to cross the narrow Klamath Gap to Shasta. 



If the number of distinctive mammals of the Sierra-Cascade system 

 be reduced by subtracting those which are represented in the two 

 ranges by closely related forms ' 8 distinctive specific types will remain 



'Close discrimination of species and. subspecies is necessary in laying off the 

 minor subdivisions of faunas; and it is interesting from the zoological standpoint 

 to know which and how many of tlie specific types common to a given area have 

 undergone enough change in ]iart8 of that area to warrant separate recognition by 

 name, but from the standpoint of the distribution of specific types such details are 



of little value. 



83 



