Introduction 



A Scandinavian biologist travelling in Eastern Canada is at once struck by the 

 close affinity of the nature of this country with that of his native land. Not only are 

 the soft, levelled profiles of the landscape similar, as are the heavily ground rocks, 

 the gentle hills of moraine, the countless lakes and ponds, the wide areas of bogs and 

 fens— all vestiges of the simultaneous periods of glaciation— but the main stock 

 of native animals and plants seems identical too. 



This far-reaching coincidence is indisputable— and yet, largely fictitious. The 

 specialist in zoological or botanical taxonomy, after a closer comparison, in most 

 cases will find that the European and the North American representatives of the 

 "species" after all are not quite identical. Whether he is inclined to regard them 

 as different subspecies or prefers a specific separation, is often a matter of judgment 

 only. 



An excellent illustration of this is given by arboreous plants. There is no dif- 

 ficulty in using a common trivial name for almost any kind of tree occurring in 

 northern regions of the two continents: in both of them grow alder, ash, aspen, 

 birch, elm, hazel, maple, oak, pine, rowan, spruce, and several others; but no 

 indigenous kind of tree is identical on both sides of the Atlantic— provided 

 the common juniper {Juniperus communis) is not counted as a tree. With the 

 possible exception of one of the birches {Betula tortuosa) and perhaps one alder 

 {Alnus incana), botanists even regard all of them as different species. 



The number of "Eur-American" species (the word used here regardless of a 

 possible occurrence in Asia) may be greater within any of the two following groups 

 of organisms: 



(a) arctic or subarctic animals and plants which, on the whole, often tend to 

 a circumpolar distribution; 



(b) animals and plants at a lower evolutionary stage, e.g. many insects, arachnids, 

 tardigrades, "worms", rotatorians, etc.; mosses, lichens, fungi, etc. These may 

 in part be "older" as species and thus have had more time at their disposal for 

 distribution. Due to their generally reduced size, often combined with the power 



