In answer to this objection, it may, with some plausibility, 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 life for each organism must have been somewhat different; and that in a few instances local peculiarities of soil, either chemical or mechanical, may explain the fact that a particular species is limited to a very small district. These considerations do not, however, fully explain the known facts regarding some regions of the earth possessing an exceptional number of peculiar species confined to small areas, the most remarkable of which are Asia Minor, South Africa, South-western Australia, and that which we are now discussing; and in weighing the evidence afforded by the floras of the opposite coasts as bearing on the probable duration of such a barrier as the Strait of Gibraltar, it is best to leave out of account all species that are not known to be widely distributed. Here our very limited knowledge of the flora of North Marocco opposes a considerable difficulty. Subject to such light as future observation will throw upon the subject, it may be said that, so far as mere botanical evidence goes, we should infer that the barrier was not present at the time when the great majority of the existing plants spread into this region; but that it has been established long enough to oppose a limit to the further diffusion of many species that otherwise would, in all probability, be found on both sides of the Strait, thus indicating a period geologically recent, but very ancient as compared with the historic record. On the following morning we crossed the bay to Gibraltar, and, still finding no means of conveyance to Tangier, endeavoured to console ourselves by botanising on the ‘Rock.’ Later in the day the impatience natural to the British traveller induced us to open negotiations for the hire of one of the numerous tug steamers that make handsome profits by helping becalmed ships through the Strait. The first demand of one hundred dollars helped to moderate our ardour; and, though the more reasonable sum of forty-five dollars was afterwards named by another merchant, we finally decided to remain a second night in Europe, and await the ordinary steamer on the following day.