294 THE MIGRATION OF BRITISH BIRDS 



XIII. The development of "Arctic " species. 



XI\'. Migration a result of Normal Increase, and not 

 initiated by winter conditions, or retreat from adverse 

 conditions. 



The object of the present work has been to a great 

 extent to demonstrate a hitherto undiscovered Law 

 governing the dispersal of species. That Law forbids 

 the southern emigration of Arctic and North Temperate 

 forms to Antarctic and South Temperate latitudes. So 

 far as I can ascertain, every biologist of note insists 

 upon this Southern Emigration. The " arctic element 

 in South Temperate floras," to quote Dr. Wallace, has 

 been repeatedly brought forward in support of this view ; 

 and so generally has this interpretation of the facts 

 been accepted by naturalists, that to question its truth 

 seems little less than a rank biological heresy. If these 

 facts have been correctly interpreted, then our Law of 

 Dispersal cannot apply to floras ; animals may bow 

 submissive to its edicts, but plants may set it at defiance 

 and demonstrate its impotency. I hope, however, pre- 

 sently to show that not only does the phenomenon of 

 "Arctic" types in the Southern Hemisphere conform to 

 this Law of Dispersal, but that it actually illustrates it in 

 no uncertain way. 



Sir Joseph Hooker graphically describes a " continuous 

 current of vegetation" extending from Scandinavia to 

 Tasmania ; a second very similar current occurs along 

 the mountain systems of North and South America ; 

 whilst a third stretches across the highlands of the 

 African continent. This flora, which has been described 

 as the " Scandinavian," is universally admitted by 

 botanists to possess astonishing colonizing power, due 



