The general aspect of the floras is nearly identical, but there is enough of difference to show that for a long period a barrier has existed sufficient to limit the diffusion of many characteristic species. Of these we found three on the hill near Algeciras—_Rhododendron ponticum_, _Sibthorpia europæa_, and _Helianthemum lasianthum_, a fine species with large yellow flowers, approaching a Cistus in stature and habit. A much longer list of European plants that have not passed into Africa might be made if all the known species found between Gibraltar and Trafalgar were taken into account; but it might with some reason be objected, that our knowledge of the African side 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 corner 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 Ronda, each possessing a number of peculiar species, must have been formerly isolated by the sea, as otherwise the species would have been intermixed.