The species common to Macaronesia and Marocco exclusively, are in so far as is at present known:—
| Helianthemum canariense, Jacq. | Sonchus acidus, Schousb. (In Lancerotte, only a single plant, possibly introduced) |
| Polycarpia nivea, Ait. (also occurs in C. de Verde) | Lithospermum microspermum, Boiss. |
| Zygophyllum Fontanesii, Webb | Linaria sagittata, Poir. |
| ?Cytisus albidus, DC. | Chenolea canariensis, Moq. |
| Ononis angustissima, Lam. (?A form of 0. Natrix) | Salix canariensis, Chr. Sm. (rather uncertain) |
| Astragalus Solandri, Lowe (Madeira only) | Romulea grandiscapa, Webb. (Perhaps only a var., but Baker keeps it) |
| Astydamia canariensis, DC. | Asparagus scoparius, Lowe. (Not quite certain) |
| Bowlesia oppositifolia | |
| Odontospermum odorum, Schousb. |
Although it would be out of place here to discuss all the questions raised by this slight sketch of the peculiarities of the Canarian Flora, there are some of them which so intimately bear upon the Maroccan as to awaken attention.
The wonderful development in the Canaries of endemic species belonging for the most part to Mediterranean types, points to the very early introduction of the parent forms of these, and the long isolation both of the Archipelago and its separate islets. It is in accordance with generally accepted views, to assume that the endemic species of each genus have been derived from parent forms originally introduced into one or more of the islets; and that as the descendants of these species spread over the Archipelago they were exposed to different conditions in each islet, resulting in their varying, and in the segregation and conservation of different local varieties each in its own insular birth-place; a supposition which is in accordance with the fact that those endemic species are really very local, many being confined to a single islet. In Marocco the parent forms of its Flora would be exposed to no such diverse conditions, and the areas in which varieties occurred, not being isolated, would be exposed both to invasion on all sides by other plants, and to destruction by agencies that affected the whole surrounding country, as drought, floods, insects, and birds.
The tropical types in the Canaries, with the exception of the Egypto-Arabian and the trees mentioned under V. c., are chiefly weeds of wide distribution, which have not reached Marocco, because of its want of ports and its limited commerce.
Finally the Dracæna, together with the tropical trees of Myrsineæ, Sapotaceæ (in Madeira), and Laurineæ, and the Egypto-Arabian types, suggest the hypothesis that at a very remote period these and many other plants of warmer and damper regions flourished in the area included in North-West Africa and its adjacent islands, and that they have been expelled from the continent by altered conditions of climate, but have been preserved in the more equable climate and more protected area of the Atlantic Islands.
Ball, who has given me valuable aid on many points discussed in this article, directs my attention to the important differences that exist between the vegetation of the eastern group of the Canary Islands—Fuertaventura, Lanzarote, and the adjacent islets—the ‘Purpurariæ’ of authors, and the western group, including Teneriffe, Grand Canary, &c.
In the first place, nearly all the characteristic Canarian types are absent in the eastern group. Out of fifty-four genera above enumerated as present in the Canaries but wanting in Marocco, two are in the Canaries confined to the eastern islands: one of these, Traganum, is an African desert type, probably to be found in South Marocco; the other, Melianthus, a Southern African plant, and scarcely indigenous. Of the remainder Plocama alone is certainly present, and three other generic types probably exist in that group; while forty-eight genera, including eight out of nine peculiar to the Canaries, are apparently absent. In the next place several characteristic desert plants, such as Oligomeris subulata, Ononis vaginalis, Convolvulus Hystrix, and Traganum nudatum, are present in the ‘Purpurariæ,’ but absent from the western islands.
Although the Flora of the Purpurariæ is incompletely known, and our acquaintance with that of the neighbouring African coast between the rivers Sous and Draha is extremely imperfect, these facts tend to prove that there is a closer botanical relationship between the eastern islands and the adjoining continent than there is between them and the western portion of the Canarian Archipelago. Such relationship might be brought about in three different ways.
1. The greater dryness and heat of the eastern islands may have favoured the immigration of African forms, and at the same time led to the destruction, or weeding out, of the characteristic Canarian types. In this case the cause would be of a purely local and climatic character.