ECOLOGY OF NORTHERN MICHIGAN. 53 



Royale are forms of western and northwestern distribution. There are 

 Conspicuous examples of this: 



1. The snail listed as Limnaea suniassi Bd., according to Mr. Bryant 

 Walker, although probably entitled to rank as a distinct variety or species, 

 is very closely related to L. summassi Bd. a peculiarly western form originally 

 described from British Columbia. 



2. The range of the ant, Camponotus herculeanus L. var. Whymperi 

 Forel, according to Dr. Wheeler, is not well known, but it has been previously 

 found in the mountains of Colorado and Britsh Columbia. 



3. The specimens of the Garter Snake, Thamnophis sirtalis parietalis, 

 taken on the island strikingly resemble in coloration western forms from 

 Washington and Colorado, and not those of southern Michigan, that are 

 sometimes referred to this variety. 



4. Although not found on the island, the Michigan Grayling, Thymallus 

 ontariensis, may also be mentioned in this connection. According to Jordan 

 and Evermann ('96, p. 518), this variety "represents a detached colony 

 left from the post glacial extension of the range of T. signifer, of which it 

 was a variety." The range of T. signifer is given as "Mackenzie River 

 to Alaska and the Artie Ocean." 



5. Another instance of the same nature was the finding of the Devils Club, 

 Echinopanax horridus Decsene, by Wheeler ('01, p. 620) on the north end 

 of the island in 1900. The range of this plant, as given by Macoun ('83, 

 p. 189), is the north coast of America and in Alaska; being abundant 

 west, but extremely rare east of the Rocky mountains. 



The occurrence of these western and northwestern elements in the biota 

 of Isle Royale is emphasized because, as was said before, the majority 

 of the species are those of the northeastern North American type. What- 

 ever may be the explanation of the occurrence of these western and north- 

 western forms this far to the east, an important factor is, no doubt, the 

 peneplain nature of the country, which probably formed an extensive 

 highway for boreal forms along the ice margin during the retreat of the 

 last ice sheet. 



PART III. REFERENCES. 



Adams, Chas. C. 



1902. Postglacial Origin and Migration of the Life of Northeastern 

 North America. Jour. Geog., I, No. 8 and 9. 



1905. The Postglacial Dispersal of the North American Biota. Biol. 

 Bull., IX, No. 1, pp. 53-71. 

 Bailey, Vernon. 



1896. Tamarack Swamps as Boreal Islands. Science, N. S., Ill, No. 59, 

 pp. 250-251. 

 Cowles, H. C. 



1899. The Ecological Relations of the Vegetation on the Sand Dunes 

 of Lake Michigan. Bot. Gaz., XXVII, Nos. 2, 3, 4 and 5. 



1901. The Influence of Underlying Rocks on the Character of the Vege- 

 tation. Bull. Am. Bureau Geog., II, June and December. 



1901. The Plant Societies of Chicago and Vicinity. Bull. Geog. Soc. 

 of Chicago, No. 2. 



