BOTANY OF THE OUTER HEBRIDES. 571 



to this Lat. of 59 degrees or as we descend from Alpine 

 heights to sea level. Within these limits there are very 

 many plants found in the Outer Hebrides common to the 

 Inner Hebrides and mainland. These must have spread 

 to the Outer Hebrides before separation from the Inner 

 Hebrides and mainland took place in post-glacial times. 

 Seeds may be carried by birds, by winds and other agencies 

 across stretches of water such as the Minch, but the uni- 

 formity of the distribution of these plants over the whole 

 chain of the Outer Hebrides could only have taken place 

 when there was continuity of land surface over the whole 

 archipelago of the Western Isles and the mainland of 

 Scotland. More than one period of post-glacial depres- 

 sion is believed by geologists to have occurred which 

 lowered the levels of the British Isles to at least 150 feet 

 thus destroying or altering the flora of the shore lines. 

 This period was followed by a period of upheaval which 

 was sufficiently great to form a pathway for a new migra- 

 tion of species from the mainland. Hence the flora of 

 the Hebrides is not of the type which is found peculiar to 

 Oceanic islands. 



LIST OF HEBRIDEAN PLANTS FOUND UP TO N. LAT. 59. 



Galium cruciatum Potamogeton pusillus 



Epilobium parviflorum perfoliat. 



Ulmus montana Primula veris 



Cardamine sylvatica Epilobium tetragonum 



Callitriche peduncul. Potamogeton filiformis 



Myosotis repens Erythrea littoralis 



Thlaspi arvense luncus compressus 



Stachys ambigua Sagina maritima 



Hypericum elodes Ruppia 



Pinguicula lusitanica Polygonum Raii 



Lastraea Fcenisecti Beta Maritim. 



Asplenium Maritim. Eryngium Maritim. 



Pyrethrum Cerastium tetrandum 



Carex extensa Jasione montana 



distans Osmunda regal is 



panculata Scolopendrum vulgare 



Scirpus fluitans Lamium incisum 



Zaunichellia palustris Sonchus Asper 



