INTRODUCTION 



ASKELL Love 



Institut Botanique de I'Universite de Montreal, Montreal, Canada 



The dispersal of plants and animals is one of the great problems in the field 

 of evolution. It is also one of the most fascinating questions of the biological 

 sciences, since it is concerned not only with understanding a distant past but 

 also with knowledge of present conditions. In addition, it is not a problem to 

 be solved by the biologist alone; his conclusions must be confirmed by aid of 

 palynology telling us about biological and climatical changes in the recent 

 past, and by aid of historical geology regarding the more distant past. The 

 history and evolution of all living beings is closely related to their distribution, 

 which in turn is intimately associated with the geological history of continents 

 and oceans. 



It has long been known that a considerable number of species of plants and 

 animals belonging to seemingly identical species inhabit both sides of the 

 Atlantic Ocean. Already Humboldt has raised the interesting question 

 whether any of these species are originally common to both continents or 

 whether those species, externally so similar as to be known by the same 

 name, are in fact identical to each other. 



The analogy of the animal kingdom seems to favor the negative of this 

 question, since no quadruped or terrestrial bird, and even no reptile and not 

 even an insect is said to be naturally common to the equinoctial regions of 

 the Old and New Worlds. The same may be true also for higher plants in 

 these regions. But as we go farther north and approach higher latitudes, the 

 probabihty of finding animals and plants of identical species on both sides of 

 the Ocean becomes increasingly greater. In northern Europe there are rare 

 plants, not related to any others on that continent, but of identically the same 

 species as are widespread in North America; similarly, there are plants in 

 eastern North America, whose closest or even identical relatives all occur in 

 Europe. The same is also the case for some lower animals which are unable to 

 fly or swim across the Ocean and are as confined to a terrestrial habitat as 

 ever any plants. 



The observation that identical animals and plants occur on both sides of the 

 North Atlantic led, late in the last century, to the launching of a theory of 

 so-called Pleistocene survival. However, this theory did not solve any ques- 

 tions, it only moved the problem farther back in time. The outstanding 

 unsolved problem in historical biogeography in the North Atlantic still is, 



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