RECENT EMIGRATION iSi 



south ! You may fill South Devonshire with reed-beds 

 and with spinneys, suitable in every way for the Reed 

 Warbler and the Nightingale, yet the birds will not 

 emigrate southwards to fill them ; you may afforest 

 Ireland as densely as you like, as indeed much of it has 

 been afforested, yet you cannot tempt a single Wood- 

 pecker or a single Tawn}' Owl or Nuthatch to break 

 the Law of its dispersal or the conditions of its exten- 

 sion, either by emigrating south or by crossing a wide 

 water area to take up its abode in your forests. I do 

 not deny that this afforestation offers many advantages 

 of which species have readily availed themselves and 

 increased their area, but only is such an advantage 

 seized upon when it is also in harmony with an unvary- 

 ing Law of dispersal. Depend upon it, it is the same 

 Law of dispersal that governs the emigrations of species 

 in our islands now, just as it governed the movements 

 of species in other areas, and as we know it controlled 

 their extension in past ages. Not a single instance of 

 a species increasing its range in our area in a southerly 

 direction, or from north to south, can be named among 

 the long list of examples given above. Of course we 

 except instances of dispersal which may have an appar- 

 ent southern tendency in such species that have been 

 introduced into certain areas by artificial means — the 

 Red-legged Partridge, the Capercaillie, and some others, 

 that have been brought to the British Islands (and 

 maintained there) by human agency. 



There is another very interesting fact which must be 

 noticed in connection with this extension of range of so 

 many species in the British Islands. In no less than 

 seventeen cases out of the twenty-eight instances of 



