EFFICIENCY OF KLAMATH GAP AS A BARRIER TO BOREAL 
SPECIES COMPARED WITH THAT OF PITT RIVER AND FEATHER 
RIVER GAPS COLLECTIVELY. 
In view of the narrowness of Klamath Gap, a break of less than 50 
miles, separating the boreal fauna of Shasta from that of the Cascades, 
compared with the breadth of the combined Pitt River and Feather 
River gaps, about 100 niles, 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 istrue. The Feather River Gap, as elsewhere explained (p.70), is 
ineffective compared with the others; the branches of Feather River do 
not cut completely 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 tbe 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 Lasser, 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 
JClose 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 the specific types common to a given area have 
undergone enough change in parts 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 
