The facts known as to the distribution of the last-named plant, and the allied species, _Periploca græca_, suggest speculations as to their past history that deserve some passing notice. The genus _Periploca_, which takes its name from the twining stems of the species first known to botanists, has its centre in the sub-tropical zone of the Asiatic continent. The single mainly western species is _Periploca lævigata_. This appears to be common in the Canary Islands, and grows freely in the tract now visited by us to the north-west of Mogador. It has been found in abundance on some rocky islands near the coast ot Sicily; but, in spite of the silky hairs attached to the seeds, it has not spread itself to neighbouring islands, nor to the Sicilian coast. It has been detected in two or three places in the south-east of Spain, and here and there in rocky places on the skirts of the desert in the interior of Algeria and Tunis. Finally, it was long ago found by Labillardière in one place on the coast of Syria. All this points to the former wide diffusion of a plant which no longer finds favourable conditions of existence, unless, perhaps, in the Canary Islands. Its presence in the interior of North Africa may possibly date from the period when it grew near the coast of a great gulf opening to the Atlantic; but it is not easy to understand how it has held its ground in a climate so different from that of its natural home. This plant has inherited from a remote ancestor a habit which is now of no service to it. The young branches near the root twine round any adjacent support; but as they grow older they become stiff and straight, and the taller specimens derive no adventitious support from this source. The history of _Periploca græca_, the only species known to the older botanists, is somewhat different. It is rather common in Georgia, and in parts of Persia and Asia Minor. Less common in Greece, it becomes extremely rare to the west of that limit, being found only in Montenegro, at one place in Dalmatia, at another in South-eastern Italy near Otranto, and, finally, in the pine-woods on the Tuscan coast near Pisa. These facts indicate the former wider extension of the species towards the west, and its gradual retreat towards its primitive home in Asia. But we have more direct evidence to that effect. The prints of leaves unmistakably belonging to this species are not uncommon in the quaternary deposits of the valley of the Arno. It may probably have flourished in thickets on the Monte Pisano, and on the Monte Nero near Leghorn, when these were islands in a tertiary sea, and gradually descended towards the Mediterranean as the coast line was advanced by a change of level, and by the formation of the deltas of the Arno and the Serchio.