448 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



ment of species in the Eocky Mountains has been much less than in 

 Eastern America, and that a very small area would be left bare of life 

 on the return to a normal temperature ; consequently, the previous occu- 

 pants of the higher mountains would again return to their former do- 

 main, increased by refugees from the circumpolar continent of temperate 

 climate, driven southward by the increasing cold. 



Such being the case, it ought to be possible, with well-prepared lists 

 of the insects of the Plains and mountain regions, by comparison with 

 lists of the local faunae of other zoological districts of the continent, to 

 ascertain, with reasonable probability, the invasions from different direc- 

 tions by which, in the first place, the newly emerged land was colonized ; 

 and, in the second place, the modifications, either in distribution or in 

 structure, which have subsequently occurred. 



I have on another occasion* expressed my belief that the study of the 

 distribution of existing insects could give much information concerning 

 formef topographical and geographical changes in the surface of the 

 earth. I then gave several examples to show how the distribution of 

 species peculiar in their habits and structure confirmed what was already 

 known by geological investigation of the gradual evolution of the mid- 

 dle part of the continent. I will now advance the additional thesis, 

 that we may obtain somewhat definite information of the sequence,, 

 extent, and effects of geological changes in the more recent periods by 

 a careful study of the insect fauna in its totality. 



While these pages were being prepared, I received from Mr. T. Ver- 

 non Wollastont a copy of his excellent volume on the small Coleopte- 

 rous fauna of Saint Helena. This fauna, containing but 203 species, is 

 remarkable for the large predominance of EhyncJiophora, of the families- 

 Cossonidce and Anthribidce. It has, however, been greatly contaminated 

 by the introduction, through commerce, cf foreign species to the number 

 of 74, or nearly three-eighths of the number now known to inhabit the 

 island. The introduction of these 74 exotic species, in addition to the 

 other changes produced by human agency, must have greatly modified 

 the preexisting fauna, by repressing some and extinguishing others of 

 the aboriginal species. 



In the case of a portion of a continental area, such as is under consid- 

 eration for my present purpose, thcxproblems are by no means so simple. 

 The human agency in the introduction of foreign species is slight. The 



* Trans. Am. Assoc. Adv. Science, 1875, Detroit, President's Address. 



t Since writing the above paragraph, I have been informed of the death of this most 

 estimable and laborious investigator. The last of his publications was the memoir on 

 the Coleoptera of Saint Helena, referred to in the text. The monographs of the Cole- 

 opterous faunse of the Atlantic Islands by Mr. Wollastou are among the most complete 

 and exhaustive contributions to faunal Entomology published. Their full importance 

 can only be appreciated when more thorough investigations of the Beetles of the Amer- 

 ican and African Atlantic slopes are made and careful comparisons instituted. It 

 will then be found that several genera of the Atlantides which do not occur on the 

 other continent are represented in the American faunee. 



