November 3, 1905.] 



SCIENCE. 



547 



which obtains with the species at large, and 

 under these conditions new forms are de- 

 veloped. This takes place rapidly when 

 the conditions of life are greatly changed 

 so that a new set of demands are made on 

 the species, and those not meeting them are 

 at once destroyed. The process is a slow 

 one, for the most part, when the barrier in 

 question interrupts the flow of life without 

 materially changing its conditions. But 

 this is practically a universal rule : A bar- 

 rier which prevents the intermingling of 

 members of a species will with time alter 

 the relative characters of the groups of 

 individuals thus separated. These groups 

 of individuals are incipient species and 

 each may become in time an entirely dis- 

 tinct species if the barrier is really insur- 

 mountable. 



In regions broken by few barriers, mi- 

 gration and interbreeding being allowed, 

 we find widely distributed species, homo- 

 geneous in their character, the members 

 showing individual fluctuation and climatic 

 etlPects, but remaining uniform in most 

 regards, all representatives slowly changing 

 together in the process of adaptation by 

 natural selection. In regions broken by 

 barriers which isolate groups of individuals 

 we find a great number of related species, 

 though in most cases the same region con- 

 tains a smaller number of genera or fam- 

 ilies. In other words, new species will be 

 formed conditioned on isolation, though 

 these same barriers may shut out altogether 

 forms of life which would invade the open 

 district. 



Thus throughout the eastern United 

 States, imbroken by important barriers, 

 there is but one true species of chipmunk, 

 Tamias striatus, and one species of shore- 

 lark (Eronopliila alpestris). In Cali- 

 fornia, broken by many barriers of various 

 sorts, there are a dozen or more different 

 kinds of chipmunks, species and subspecies. 

 But in the eastern states the fauna at large 



is much greater, because many types of 

 birds and other animals have found en- 

 trance there, forms which are excluded 

 from California by the barriers which sur- 

 round that region. 



In the great water basin of the Missis- 

 sippi many families of fishes occur and 

 very many species are diffused throughout 

 almost the whole area, occurring in all 

 suitable waters. Once admitted to the 

 water basin, each one ranges widely and 

 each tributary brook has many species. In 

 the streams of California, small and iso- 

 lated, the number of genera or families is 

 much smaller. Each species, unless run- 

 ning to the sea, has a narrow range, and 

 closely related species are not found in the 

 same river. 



The fact last mentioned has a very broad 

 application and may be raised to the dig- 

 nity of a general law of distribution. 



Given any species in any region, the 

 nearest related species is not likely to be 

 found in the same region nor in a remote 

 region, but in a neighboring district sepa- 

 rated from the first by a barrier of some 

 sort. 



The nearest relative of the Tahoe trout 

 (Salmo henshawi) is the cut-throat trout 

 (Salmo clarki), its parent form, which is 

 found in the Columbia and Missouri, the 

 two rivers interlacing at their fountain 

 heads. The nearest relatives of the cut- 

 throat are two forms apparently descended 

 from it, Salmo virginalis, of the basin of 

 Utah, and Salmo stomias, of the Platte. 

 Xext to the latter is Salmo spilurus, of Rio 

 Grande, and then Salmo pleuriticus, of the 

 Colorado. The latter in turn may be the 

 parent of the Twin Lakes trout, Salmo 

 macdonaldi. Always the form next away 

 from the parent stock is onward in space 

 across the barrier. 



The nearest relative to almost any 

 Hawaiian species of fish is found in Poly- 

 nesia; that to any Polynesian species is 



