274 



CHAPTER III 



Since the eustatic and isostatic movements of the shore-line in Pleistocene 

 time apparently were insufficient to create a bridge connecting Greenland and 

 Iceland with Europe, as described above, it is tempting to think of a prolonged per- 

 sistence of the Tertiary basalt plateau into part of the Pleistocene. The absence of 

 undoubtedly endemic species, and even true subspecies, in the insect fauna of 

 Iceland and Greenland prevents the acceptance of survival in situ since Tertiary 

 (Pliocene) time. Considering this, the present author (Lindroth, 193 1, p. 550-557) 

 arrived at the conclusion that the proposed land-connection most probably should 

 be dated to the last (Riss-Wiirm) interglacial period. 



Facts contradicting this view have since appeared in the form of the discovery 

 of fossiliferous deposits in Central Sweden from the said period. The included 

 insects (Lindroth, 1948) give evidence of a climate at least as warm as the present 

 one during part of the Riss-Wiirm Interglacial in Scandinavia and this cannot be 

 considered in accordance with a situation implying that the Gulfstream was 

 prevented by a land-bridge from entering the Norwegian Sea^. 



It should also be considered in this connection that the fauna of Iceland is 

 originally boreal, not arctic, the few exceptions {Colymbetes dolabratus, &c.) being 

 easily dispersed animals independent of any land-connection. The invasion of this 

 boreal fauna therefore took place in the warm middle part of some interglacial 

 period. Such a violent oscillation, from one stage: "land-bridge, blockade of 

 Gulfstream",— to another: "land-bridge sunk, 200 meters at least, Gulfstream 

 free passage",— cannot be accepted to have taken place in a time so short as the 

 temperate phase of an interglacial period. 



It seems that a "removal" of the land-bridge one step further back in time, to 

 the last but one Interglacial (Mindel-Riss), is the most likely solution. From a 

 biological point of view, the difficulty in accepting this idea seems to me to lie 

 not so much in the lack of endemics in Iceland-Greenland, as in the necessity 

 of accepting survival in these islands through two subsequent glaciations, the 

 first of them being the severe Riss. 



Yet if the land-bridge existed in the Mindel-Riss Interglacial, the climatic in- 

 fluence of the dislocated Gulfstream, at least on Iceland, ought to have been 

 highly increased and these favourable conditions may have lasted during part of 

 the following Riss glaciation. 



There is one special reason to think that in Iceland, to a lesser degree also in 

 Greenland, local conditions may have been favourable enough, even during Riss, 

 to make a survival possible: the presence of volcanic hot springs. Tuxen (1944, p. 

 189-199) has discussed this question and rightly concludes that a field of hot 



^ Vide also the table of climate and vegetation during the last interglacial period in Den- 

 mark by Jessen & Milthers (1928, p. 336). 



