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CHAPTER III 



several students, whereas others supposed that human transport has been respons- 

 ible for their occurrence in North America. 



This applies to two ants, Tetramorium caespitum L. and its parasite, Anergates 

 atratulus Schenck. Wheeler, and others, assumed that the Tetramorium, and con- 

 sequently also its parasite, had been introduced from Europe with the early colo- 

 nists and both species are still regarded as adventive by Smith (195 1, p. 823). 

 This opinion was strongly opposed by Creighton (1950, p. 289): —"The discovery 

 of Anergates in America rules out the possibility that caespitum might have been 

 imported to this country"; and (I.e., p. 244):— "—I cannot agree that the first 

 advent of caespitum on this continent is a result of importation. To do so implies 

 that Anergates has also been imported. I believe that it can be demonstrated that 

 the probability for this having occurred is too remote to be credible." 



It seems to me that Creighton has not sufficiently considered the nature of 

 ballast transport. If ballast was taken in great quantities from heaps of rubbish 

 in a port {vide pp. 165, 200) it is not at all unlikely that a whole Tetramorium nest, 

 with inquilines and parasites, might have been brought on board. Though scat- 

 tered and disturbed during the process, it might very well have reorganized itself 

 during the long journey (p. 162) and become unloaded with the ballast on some 

 North American shore after arrival. It should also be remembered that the myr- 

 mecophilous Isopod (Woodlouse) Platyarthrus hoffmanseggi Brandt apparently 

 reached North America in the same way. Yet, Anergates atratulus is so rare in 

 Britain that it was probably carried across from the European mainland. 



A much more intricate case is that of the snail Cepaea {Helix) hortensis O. F. Miill. 

 (fig. 29). This was likewise long regarded as a European introduction into North 

 America {vide Johnson, 1906, p. 79-80) though, after it had been found in "pre- 

 Columbian kitchen-midding deposits", Pilsbry (1894, p. 321) suggested that the 

 importation might have taken place with the Vikings of the eleventh century. 

 Later, subfossils of proposed greater age were recorded:— "in the glacial Pleisto- 

 cene of Maine" (Dall, 1910, p. 20); "in a shell heap— associated with bones of the 

 large extinct mink— Mustela macrodon Prentiss.", on an island in Penobscot 

 Bay, Maine (Johnson, 1915, p. 131); "in a prehistoric shell-heap on Mahone Bay, 

 about 75 miles west of Halifax, N. S." (Wurtemberg, 1919, p. 71). Since this in- 

 formation was published, conchologists and zoogeographers have almost unani- 

 mously accepted Cepaea hortensis as indigenous in North America (Scharff, 1909, 

 p. 18-20; 191 1, p. 39; Taylor, 191 1, p. 361-362; 0kland, 1927, p. 342 a.f.). Pilsbry 

 (1939, p. 8) also changed his view but left open the question of whether the present 

 separated occurrences of the species should be regarded as remnants of a wide 

 Holarctic area, or if they should be explained as being the result of migration 

 along a former transatlantic land-bridge, as advocated by Scharff. Wegener (1929, 



