CH. III. VEGETATION OP THE OPPOSITE COASTS. 59 



of the Strait is too incomplete to speak confidently on this 

 point. On the other hand, however, we may with some 

 certainty assert that comparatively many well-marked 

 species found on the southern side of the Strait are limited 

 to the African shore, and have not been able to spread into 

 Europe. From the accessible materials we find at least 

 thirty-eight species belonging to this category, of which 

 the large majority are species spread over a wide area in 

 Northern Africa. 



In attempting to draw inferences from these facts, it 

 is necessary to bear in mind that the region where they 

 occur — the southern part of the Iberian peninsula, and the 

 opposite comer of Marocco — is remarkable for the variety 

 of its flora, and for the large number of distinct species, 

 each inhabiting a very restricted area. To those who 

 suppose that the presence of numerous plants in two 

 neighbouring districts, which are limited to one or the 

 other, but are not common to both, is to be regarded as 

 evidence for the existence of a physical barrier between 

 them, an objector might reply that we have no more 

 right to affirm that it is the prolonged existence of the 

 Strait between Europe and Africa that has prevented the 

 extension of so many species from one continent to the 

 other, than we have to maintain that two neighbouring 

 mountain groups, such as the Sierra Nevada of Granada 

 and the Serrania de Eonda, each possessing a number of 

 peculiar species, must have been formerly isolated by the 

 sea, as otherwise the species would have been intermixed. 



In answer to this objection, it may, with some plausi- 

 bility, be urged that a large majority of the species with 

 restricted areas are mountain plants ; that there is much 

 reason to believe that most of these peculiar species did 

 originate within insulated areas, at a time when these 

 were separated by the sea from neighbouring masses, where 

 the conditions of Ufe for each organism must have been 

 somewhat different ; and that in a few instances local 

 peculiarities of soil, either chemical or mechanical, may 



