ORIGIN OF THE BRITISH FLORA 211 



Discontinuous Floras. The present distribution of 

 alpine plants is an illustration of what is known as a 

 discontinuous flora. Such a flora is limited to certain 

 widely separated spots, between which no plants of the 

 same kind are found at all. Most plants have a wide 

 range, being found, more or less frequently, in many 

 adjacent countries ; but each plant has its limits of dis- 

 tribution. These limits are determined by the barriers 

 which it cannot pass e.g., mountain chains, masses of 

 water, unfavourable climate or soil. But although the 

 presence of these barriers may explain the distribution 

 of most plants, it does not explain the existence of plants 

 in discontinuous areas. To understand this, we need to 

 find out something of the past history of the land, its 

 ohanges of contour, and alterations of level, as well as 

 the secular changes of climate to which it has been sub- 

 jected. All these things have produced in the past an 

 ebb and flow of the vegetation, and with each change, 

 physical or climatic, stragglers were left behind, which 

 have given rise to the discontinuous floras of to-day. 



The Lusitanian Flora. More striking still than alpines 

 are certain plants found in the south-west of England 

 and in the west of Ireland, which do not occur elsewhere 

 nearer than Spain and Portugal. For this reason the 

 term " Lusitanian " has been applied to these plants, 

 from Lusitania, the old Latin name for Portugal. How 

 are we to account for their presence in the British Isles ? 

 It is not likely that they crossed the intervening sea, 

 although it is possible that some may have been intro- 

 duced by birds e.g., Arbutus Unedo by early invaders, 

 or in merchandise by boats from Mediterranean ports. 

 It is more probable, however, that most of these species 

 arrived naturally by land during the time when the west 

 of England was much nearer the coast of Europe than it 

 is to-day. We know, as a fact, that Ireland was once 

 not only joined to England and England to the Con- 

 tinent, but that to the south of our islands dry land 

 extended at that time to the westward far beyond the 

 present limits of France. Over this land, now sub- 

 merged, low-lying, and enjoying a climate remarkably 

 mild and moist, these plants may have crept up north- 

 wards from Portugal to the south-western parts of 

 Britain, and, conditions having remained suitable } they 



