1916] Taylor: Beavers of Western North America 485 



of which resembled the parent stock and others of which exempli- 

 fied potentially new species. For example, when Leptinotarsa tln<>it- 

 lineata was so treated there was a break-up in the next generation 

 into Leptinotarsa immaculothorax, L. pallida, and the unmodified 

 L. decemlineata. The evidence from the zoogeography of the 

 higher vertebrates clearly indicates, as has been repeatedly stated 

 and implied, that isolation of one portion of a parent stock and 

 subjection of it to the conditions of one new environment, results in 

 the differentiation of but one new form, and that until further 

 migration has effected the further isolation of the stock and its 

 subjection to some other environment there is no further polytypic 

 evolution. 



A further implication contained in the work of Tower should be 

 mentioned here. The modifications were definitely brought about in 

 his experimental work through the action of the unaccustomed 

 environmental stimulus during the period of growth and matura- 

 tion of the germ-cells, and at no other time. This invites one to 

 the suggestion that the environment may do its work by bringing 

 pressure to bear directly or indirectly on the germ-cells, and that 

 environmental-somatic impacts may be of no moment in speciation 

 if they are not conveyed to the germ-cells. The failure of environ- 

 mental-somatic impacts to be translated into environmental-germinal 

 impacts may explain the permanence of subspecies amid conditions 

 very different from those to which they are accustomed. Ammosper- 

 mophilus leucurus leucurus, for example, having its center of dis- 

 tribution on the Colorado Desert, ranges notably over into the 

 damper San Diegan faunal area, maintaining its desert character- 

 istics even under the moister conditions (see Grinnell and Swarth, 

 1913, pp. 391-394). Evidently, however, such a translation of 

 environmental-somatic impacts into environmental-germinal impacts 

 has taken place in most cases in the past, and may perhaps be 

 anticipated to take place, where it has not yet done so, at some time 

 in the future. 



As water, constantly dropping, wears away the stone, so 

 the environment, through constant and continuous re-impression, 

 seems to find at last some avenue to the germ-cell, until, as believed 

 by most students of zoogeography, it becomes true that differentia- 

 tion has come to be a part of the connotation of geographical 

 isolation. 



That the differentiation of which geographic isolation is a condi- 



