THE UAXARIAX FLORA. 419 



Fuertaventura is not more than seventy miles, and that a 

 moderate change of level of about 600 feet would reduce that 

 distance by one-half, while it would but slightly affect the 

 interval that separates the Purpurarise from the other islands. 



Among the possible causes leading to an interchange of 

 species between the Purpurarise and the African coast the 

 agency of man must not be omitted. The fishermen of those 

 islands were formerly in the habit of visiting some points on 

 the opposite coast, although iutercourse of this kind has almost 

 ceased in recent times. 



It must be observed that our knowledge of the vegetation 

 of the Canary Islands is yet incomplete. Although several 

 additions to the Flora have been published by 0. Bolle and 

 others, no supplement to Webb's ' Phytographia ' has been 

 published. Several additional species exist in herbaria, besides 

 those that may be hereafter found. 



So little is known of the geology of Marocco, that there are 

 no data for ascertaining whether during antecedent geological 

 periods it contained a more tropical Flora than now; but 

 evidence in support of such a hypothesis is forthcoming in 

 Madeira, where fossiliferous beds which have been referred to 

 'some part of the Pliocene period' ' have been discovered, con- 

 taining leaves, referable in part to existing species of Madeiran 

 plants, and in part to extinct ones of tropical aspect ; ^ and it 

 is well ascertained that during preceding geological periods 

 Western Europe was clothed with a vegetation that suggests a 

 very much warmer climate than now prevails, and of which 

 vegetation the Laurus nobilis in the south-west of the continent 

 has been supposed to be a surviving representative. 



In Grand Canaxy, also. Upper Miocene beds exist, con- 

 taining numerous species of fossil shells, of which one is an 

 Oregon species, and another tropical African ; and in more re- 

 cent deposits of the same Archipelago many shells have been 

 found which no longer inhabit the adjacent seas, including 

 tropical West African, Mozambique, and Mediterranean species. 



We can form no conception of means of transport from the 

 American continent that would transfer the parent species of 

 Bowlesia and of the Bystropoc/ons from the Andes to the 



' Lyell's Principles of Geology, ed. 11, vol. ii. p. 410. 



' Lyell's Students Elements of Geology, ed. 2, pp. 538, 539. 



E B 2 



