NO. 56S] SHORTER ARTICLES AND DISCUSSIOX 249 



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, 

 at least as regards birds and mammals. It is even conceivable 

 that ; with sufficient refinement in methods, the inquirer might 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 of 

 the species; in other words to analyze the "environmental com- 

 plex" 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 con- 

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

 obstruction affects directly the individuals of a species en- 

 countering 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 in the case of purely aquatic mammals, and 

 bodies of water to purely terrestrial, especially xerophilous, 

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

 the degree of impassability. Oceans and continents are most 

 perfect, and affect a large proportion of the species. The com- 

 paratively narrow Colorado River is a barrier of the first rank, 

 but only to a certain few desert rodents. Mechanical barriers, 

 where they exist at all, are clearly recognizable. 



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

 mammals of a whole continent, that by far the greater number 

 of species are delimited in range without any reference to actual 

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

 short of coast lines. The barriers here concerned are intangible, 

 but nevertheless powerful. By their action the spread of species, 

 genera and families is held in check as surely as by any tangible 

 obstruction . 



By these invisible barriers the individual may not necessarily 

 be stopped at all, as with animals of free locomotion; but the 

 species is affected. For example, the mocking bird in its Cali- 

 fornian distribution is closely confined to those parts of the state 

 possessing certain definite climatic features; but vagrant indi- 

 viduals, especially in autumn, occur far beyond the limits of 

 these restrictive conditions. Carnivorous mammals are well 



