THE CHIEF COLEOPTEROUS FA.UNJK. 



33 



depends on other causes than uniformity of condition ©f life 

 within its limits. I cannot doubt that if there had been an 

 isolated communication between the Indo-African districts and 

 the North Pole, we should there have had a fauna related to 

 and developed out of that fauna, and wholly distinct from the 

 other faunas of the arctic regions. It is continuity of soil or 

 freedom of intercommunication which has produced the present 

 uniformity of fauna in the arctic regions ; but where minor inter- 

 ruptions exist, or old, barriers or conditions equivalent to a bar- 

 rier formerly existed, there are also subdivisions in the character 

 of the fauna, and in the position of these minor divisions we see 

 the operation of these laws and are able to trace the existence 

 and former position of the barriers. Thus we find two minor 

 subfaunas in Arctic America, an eastern and a western one. Two 

 causes may have produced these. One of these may have been 

 the sea which, it can scarcely be doubted, formerly existed be- 

 tween the G-ulf of Mexico and the Polar Sea, in the line of the 

 Missouri and Mackenzie rivers ; another may have been that the 

 ground now occupied by one of these subfaunas was under water 

 at a later period than the other, so that it was peopled at a dif- 

 ferent date from it. Probably both contributed to produce the 

 present arrangement of the subfaunas to the east and west of the 

 Mackenzie Eiver. That there was a barrier there, and that 

 that side was still supplied with the same general type (though 

 with minor deviations), is to be explained by their having re- 

 ceived their species from the same general stock, but coming to 

 it from different directions, the one from the east, the other 

 from the west. That the minor differences to which I allude 

 are, in the case of North America, to be referred to this cause, 

 and not to mere gradual increase of variation arising from in- 

 crease of distance, seems to be a legitimate inference from the 

 fact that while the whole of the north of North America, without 

 exception, belongs to the Europeo-Asiatic type, there are a 

 number of European genera which occur in North-east America, 

 and not in North-west, and a few which occur in North-west, 

 and not in North-east America. 



In the Appendix I have given a list of genera of Coleoptera 

 which inhabit both sides of the Pacific, and do not occur in the 

 Atlantic States of the American Continent, and also of a list of 

 some species of other genera, similarly distributed. These are 

 almost literally taken from my friend Dr. Leconte's Reports in 



LINN. PEOC. — ZOOLOGY, VOL. XI. 3 



