176 MICHIGAN SURVEY, 1908. 



"The Europeo-Asiatlc Beetle-faima^ does not stop even at Japan; it passes over 

 into North America by Behring's Straits, or rather, I should say, it is found in 

 North America on the other side of Behring's Straits. In Russian America we 

 have a fresh crop of Europeo- Asiatic form, genera and species; and here another 

 noteworthy circumstance presents itself. It is generally taken for granted that 

 there is a uniform homogeneous arctic- fauna which extends all around the arctic 

 circle. It is so, and it is not so. It is so on the large scale, but not so on the 

 small. The arctic fauna is subject to the laws of spreading by continuity and 

 stoppage by barriers just the same as any other fauna. I have elsewhere endeav- 

 ored to show that the mammalian fauna of Greenland is Europeo-arctic as dis- 

 tingushed from Americano-arctic. I maintain that the homogeneity of a fauna 

 depends on other causes than uniformity of condition of 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 intercommuni- 

 cation which has produced the present uniformity of fauna in the arctic regions; 

 but were minor interruptions exist, or old barriers or conditions equivalent to 

 a barrier formerly existed, there are also subdivisions in the character of the fauna, 

 and in the position of these minor divisions we see the oi>eration of these laws 

 and are able to trace the existence and former position of the barriers. Thus 

 we iind 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 between the 

 Gulf of Mexico and the Polar Sea, in the line of the Missouri and Mackenzie 

 rivers; another may have been that tbe 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 different date from it. Probably both contributed to produce the 

 present arrangement of the subfaunas to the east and west of the Mackenzie River. 

 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 

 received their species from the same general stock, but coming to it from dif- 

 ferent 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, 

 this cause, and not to mere gradual increase of variation arising from increase 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-Asiatlc 

 type, there are a number of European genera which occur in North-east America, 

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

 North-ea^t America, pp. 32-33. 



•'Returning to the Asiatic terminus of the microtypal stirps, let us now en- 

 deavor to trace its further course. The genus Blaps, which is a characteristic 

 feature in the Coleopterous fauna of Central Asia, will furnish us with the means.' 

 It may be taken as a representative case applicable to other species also, although 

 it is the most striking instance which occurs to me. Upwards of 100 different 

 species of Blaps, out of a total of about 150, have been described as inhabiting 

 ithe country between Southern Russia, Mongolia, and Mantchourla. Now if we 

 cross to California in continuation of the same line we have not Blaps, but we 

 have Blaps's brother and he has been a twin. We have Eleodes, its perfect 

 -counterpart and representative; and it is to be observed that while the facies 

 of the species actuEilly inhabiting California is entirely that of Blaps, a number 

 •of species which are found in Kansas and on the eastern flanks of the Rocky 

 Mountains have a somewhat different facies; and I should add that the supposi- 

 Ttion that these are stragglers from the Californian shores is strengthened by the 

 iact that the genus does not occur to the east of the Missouri; other Heteromerous 

 forms, reminding us of Mediterranean and Asiatic species, occur in California, 

 and the whole of the north-west of America has a greater preponderance of the 

 microtypal stirps than perhaps occurs east of the Rocky Mountains, pp. 36-37. 



"Next step to the south of California comes Mexico. It also is largely supplied 

 with Eleodes; and although some of the showiest and finest non-microtypal Col- 



" ' I was unable in my 'Geogi-aphiea! Distribution of Mammals' to adopt Dr. Sclater's terminology 

 of Palaearctic, Neoarctic, &c., because we did not agree in the extent and limits of our regions; and 

 now, of course, in this paper I can still less do so, 'as a principal effect of my hypothesis, it it be 

 sound, must be to still further break down their limits and destroy their solidity." 



