164 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



already on p. 1 14. And in the final analysis it must be remembered 

 that a single disseminule in many millennia may be sufficient to 

 ' plant ' a species. 



But whereas the idea of certain plants persisting in tiny areas for 

 hundreds of thousands of years, unchanged and unaffected by the 

 coming and going of ice-ages and hordes of other migrants, is scarcely 

 in accordance with our saner biological reflection, some persistence 

 on ice-free tracts, for example through the last glacial maximum, 

 may well have taken place. And restocking may indeed have been 

 helped by plants surviving on temporary sheltered ' islands ' sur- 

 rounded by ice, or on ice-free coastal strips — thence to recolonize 

 surrounding areas on release from the bondage of glaciation. On 

 the other hand, the plants surviving on mountain nunataks would 

 be mainly arctic or high-alpine ones accustomed to rigorous con- 

 ditions and unlikely to recolonize lastingly the surrounding plains 

 on recession of the ice and marked amelioration of conditions. Thus, 

 as was pointed out by Professor G. E. Du Rietz [in Hit.), ' Scan- 

 dinavian botanists have believed more in glacial survival in coastal 

 areas than on nunataks ', and such an ' hypothesis of glacial survival ' 

 would seem more reasonable ; nor are the criticisms of the nunatak 

 hypothesis which apply to North America necessarily valid in 

 northern Europe where conditions tend to be different. Many 

 persisting plants, however, appear to become depleted in biotypes 

 and, owing to a consequent narrowness of ecological amplitude, 

 rather passive ; some become reactivated by cross-breeding fol- 

 lowing release from glaciation, aggressively recolonizing surrounding 

 areas from their vegetated ' islands '. Thus, although the nunatak 

 hypothesis in its strictest form seems unsound and indeed unneces- 

 sary, some residue of it may well be valid and still found useful, 

 in spite of the fact that the recolonization of deglaciated areas 

 appears in general to have taken place by immigration of plants 

 that tided over the inimical period in distant (usually southern) 

 areas. Thence the plants subsequently migrated, for the most part 

 by gradual stages. 



Persistence through the Pleistocene, or at least through its latest 

 phases, may also help to explain the existence of ' arctic ' plants on 

 mountains far to the south, although in some cases migrating birds 

 are probably responsible. In other instances, the similarity to arctic 

 conditions evidently enabled properly acclimatized plants to retreat 

 to the mountain tops, even as they were able to persist near the 

 margin of the ice and follow it north. For in both these situations 



