Journal of a tour in Marocco and the Great Atlas

by Joseph Dalton Hooker; John Ball

2. We may believe in the trans-oceanic migration of some African species to the nearer islands, along with the transport of some Canarian species (those enumerated in [p. 416], and others which may be hereafter found) to the neighbouring continent.

3. An ancient extension of the continent to the Purpurariæ, leaving the other islands separated by deep sea.

It is an objection to the latter hypothesis that a profoundly deep ocean bed lies between the lines of 100 fathom soundings that girdle the islands and the African coast respectively; and that while the 100 fathom line extends about thirty miles from the coast of the continent, it is never more than five miles, rarely more than one or two, from those of the islands.

In favour of the hypothesis of trans-oceanic transport it may be remarked that the distance between the African coast and Fuertaventura is not more than seventy miles, and that a moderate change of level of about 600 feet would reduce that distance by one-half, while it would but slightly affect the interval that separates the Purpurariæ from the other islands.

Among the possible causes leading to an interchange of species between the Purpurariæ and the African coast the agency of man must not be omitted. The fishermen of those islands were formerly in the habit of visiting some points on the opposite coast, although intercourse of this kind has almost ceased in recent times.

It must be observed that our knowledge of the vegetation of the Canary Islands is yet incomplete. Although several additions to the Flora have been published by C. Bolle and others, no supplement to Webb’s ‘Phytographia’ has been published. Several additional species exist in herbaria, besides those that may be hereafter found.

So little is known of the geology of Marocco, that there are no data for ascertaining whether during antecedent geological periods it contained a more tropical Flora than now; but evidence in support of such a hypothesis is forthcoming in Madeira, where fossiliferous beds which have been referred to ‘some part of the Pliocene period’[14] have been discovered, containing leaves referable in part to existing species of Madeiran plants, and in part to extinct ones of tropical aspect;[15] and it is well ascertained that during preceding geological periods Western Europe was clothed with a vegetation that suggests a very much warmer climate than now prevails, and of which vegetation the Laurus nobilis in the south-west of the continent has been supposed to be a surviving representative.

In Grand Canary, also, Upper Miocene beds exist, containing numerous species of fossil shells, of which one is an Oregon species, and another tropical African; and in more recent deposits of the same Archipelago many shells have been found which no longer inhabit the adjacent seas, including tropical West African, Mozambique, and Mediterranean species.

We can form no conception of means of transport from the American continent that would transfer the parent species of Bowlesia and of the Bystropogons from the Andes to the Atlantic islands; and we can but hazard the assumption that, at some very distant date, these genera existed in more eastern parts of America, from whence seeds were transported across the ocean. On the other hand, the transport of parent forms or existing species from the continents of Europe and Africa to the Atlantic islands may have been much facilitated by greater extensions of land in bygone ages. Madeira, the Canaries, and the Cape de Verde Islands, are all supposed to stand on a submarine platform which skirts the coasts of Western Europe and North-Western Africa, and whose submerged margin immediately to the westward of the position of the islands descends rapidly to a profound depth. The westward margin of this platform was possibly the coast-line in Miocene times. An elevation of its surface of a few hundred feet would approximate the islands to the mainland very materially, and greatly facilitate transport. That they were, however, ever united to the continent is opposed to the views of most competent geologists. Lyell, speaking of this, says: ‘The general abruptness of the cliffs of all the Atlantic islands, coupled with the rapid deepening of the sea outside the 100 fathom line, are characters which favour the opinion that each island was formed separately by igneous eruptions, and in a sea of great depth.’ Moreover, the Azores, whose botany in so many respects resembles that of the other Atlantic islands, as distinguished from that of the continent, are enormously more distant from the mainland; and these islands stand on a platform of their own, separated from the continental one by an ocean of profound depth; so that any theory of transport which applies to the Canarian and Madeiran Archipelagos, should apply also to the Azorean.

It remains a point of some nicety to decide whether the Macaronesian islands should be regarded as a Botanical province apart from the Mediterranean, or a sub-division of the latter. The assemblage of American and Oriental genera which their Flora contains, together with the arboreous representatives of tropical Laurineæ, all so entirely foreign to the European Flora, would give it a title to be called a Botanical province; and to this as a further title is the prevalence of a considerable proportion of North European plants, in the Northern Archipelago especially. On the other hand, fully two-thirds of the species are typical of the Mediterranean Flora, and by far the majority of the remainder are derivative species of the same origin; so that, on the whole, I am disposed to regard it as a very distinct sub-division of the Mediterranean province, which owes its peculiarities partly to the conservation of types once common to West Europe and North Africa, but which have been eliminated in those regions, and partly to the effect of isolation and climate on the progeny of species still existing in those regions.