108 University of California Publications in Zoology [Vol. 12 



riers. These barriers consist of any sort of conditions less favorable 

 to the existence of the species than those in the center of abundance. 



Theoretically, sooner or later and in all directions, every species 

 is absolutely stopped. But as a matter of undoubted fact most barriers 

 are continually shifting, and the adaptability of the animals them- 

 selves may be also undergoing continual modification ; so that perfect 

 adjustment is beyond the limits of possibility so long as topography 

 and climate keep changing. The ranges of species may thus be con- 

 stantly shifting. Descent-lines may move about repeatedly over the 

 same general region, like sparks in the soot on the back of a brick 

 fireplace. 



Yet, in all of our studies, of but a few years' duration, the time 

 element is reduced almost to a negligible quantity, and we may look 

 upon the areas occupied by each species as, for the time of our 

 observation, fixed. We are thus enabled to compare one with another, 

 and because of the large number of the species, we can infer a good 

 deal as to the nature of barriers in general as regards birds and mam- 

 mals. It is even conceivable that with sufficient refinement in methods 

 the inquirer may in time find himself able, from a comparative study 

 of the ranges of rodents, for example, to establish the identity of all 

 of the external factors which have to do with the persistence of each 

 species; in other words to analyze the "environmental complex" into 

 its uttermost elements as regards the existing species of rodents in 

 their recent development. 



The most obvious kind of barrier to distribution is that consisting 

 of any sort of physical, or mechanical, obstruction. Such obstruction 

 affects directly the individuals of a species encountering it, either by 

 stopping their advance, or by destroying outright such as attempt 

 to cross it. As barriers of this nature are to be cited land to purely 

 aquatic mammals, and bodies of water to purely terrestrial, especially 

 xerophilous, mammals. In each case width of the barrier has to do 

 with degree of impassability. Oceans and continents are most perfect 

 and affect a large proportion of the species. The comparatively nar- 

 row Colorado River is a barrier of the first rank, but only to a certain 

 few desert rodents, as pointed out in a preceding chapter. Mechanical 

 barriers, where they exist at all, are clearly recognizable. 



It is to be observed, however, upon considering the birds and mam- 

 mals of a whole continent, that by far the greater numbers of species 

 are delimited in range irrespective of any direct dependence upon 

 actual land and water boundaries; more explicitly, their ranges fall 



