1913] Grinnell-Swarfh: Birds and Mammals of San Jacinto 385 



According to the hypothesis already expressed, the detached 

 tracts of the Boreal zone on the mountain masses of southern 

 California are inhabited by residual faunas representative of 

 that which was spread continuously along the divide in earlier 

 times. The facts show that fewer types have been preserved 

 in one place than another, and further that the following state- 

 ment of a possible law appears to be justified: The smaller the 

 disconnected area of a given zone (or distributional area of any 

 other rani,-), tin fewer tht la/us which an persistent therein. 



Although formulated upon the basis of study in a restricted 

 region we believe the above generalization will prove of wide 

 applicability. Reference to the fauna of such a continental 

 archipelago as the Santa Barbara group of islands, shows that, 

 among themselves and as compared with the mainland, in rich- 

 ness of fauna as to species possessed (but not necessarily as to 

 individuals), a very similar state of affairs exists. But this rela- 

 tionship must not of course-be expected everywhere absolutely 

 without exception, because of occasional extraordinary circum- 

 stances of overwhelming effect. 



If our generalization is found to bear the test universally, 

 then it is certainly fallacious to establish lines of dispersal and 

 centers of distribution upon such data as the relative numbers of 

 types found to occupy the regions studied, unless size of the 

 areas in question Ik taken into account. 



It is highly probable that the extinction of species (and thus, 

 of course, of larger groups) has been hastened by restriction of 

 habitat. Comparison of areas of similar climatic conditions, with 

 regard to size and number of endemic forms (such as has been 

 made in the present article), comes very near to proving this 

 point. 



The query arises as to the cause of more rapid disappearance 

 of types in such cases, even though they may hold, each, un- 

 challenged possession of a separate ecologic niche. This cannot 

 be answered from positive data. But it seems inevitable that 

 the smaller the area available to a species (whether a sea-girt 

 oceanic island or a patch of the Boreal zone completely sur- 

 rounded by Sonoran. as on the San Jacintos) the greater the 

 chance for complete extinction of the species from such causes 



