BARRIERS TO MIGRATION 1043 



related species are normally found in adjoining districts, and not 

 generally in the same or in widely separated districts. It is uni- 

 versally true that in such cases the species of a genetic series 

 within the same cJiroiiofaiina, or fauna of the same geological time 

 division, differ more widely the further they are removed in space, 

 and are closely related in immediately adjoining loco faunas. This 

 is beautifully illustrated by the mutations of the Hawaiian Acha- 

 tinellidas, a group of brilliantly colored tree snails, different species 

 of which inhabit the different rugged valleys incised in the margins 

 of the extinct volcanic craters of these islands. The most nearly 

 related mutations of these snails live- in adjoining valleys, where, 

 nevertheless, they are almost wholly isolated from the inhabitants 

 of the neighboring valleys. Here the amount of divergence in 

 characters between the occupants of two valleys can, as Gulick 

 says, be roughly estimated by measuring the number of miles be- 

 tween the valleys. Recently Gulick has formulated this in the fol- 

 lowing law : "Forms that are most nearly related, and are, there- 

 fore, the least subject to sexual and impregnational isolation, are 

 distributed in such a manner that their divergence is directly pro- 

 portional to their distance from each other, which is also the mea- 

 sure of the time and degree of their geographical isolation ; while 

 those most manifestly held apart by sexual instincts and impregna- 

 tional incompatibilities do not follow this law." ( Gulick-i 9 :.?.?/.) 

 The trout of the modern (Holocenic) chronofauna furnishes a 

 good illustration of this law. Thus Salmo clarki, the cut-throat 

 trout of the Columbia and Alissouri rivers — which interlace at their 

 headwaters — has its nearest relatives in the basin of Utah (S. vir- 

 ginalis) and in the Platte ( S. stoinias). "Next to the latter is 

 Salmo spilurus of the Rio Grande, and then Salmo plenriiicus 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 is onward in space across the barrier." (Jordan-25: 

 547.) The distribution of the fossil eurypterids of the American 

 Siluric in such a manner as to parallel the distribution of the trout 

 above mentioned has been cited as a proof of the river habitat of 

 these organisms. (Grabau-i8.) It should be emphasized, how- 

 ever, that isolation cannot always be determined to be a factor in 

 the production of species, and, indeed, if variation is orthogenetic, 

 the development of new mutations in a progressive series can very 

 well go on within one area. Thus many of the pronounced muta- 

 tions of Planorbis in the Steinheim strata occurred within the same 

 Tertiary locofauna. In the Eocenic locofauna of the Paris basin 

 a large number of successive ruutations in each of a number of 



