By SIR WILLIAM THISELTON-DYER, K.C.M.G., C.I.E. Sc.D., F.R.S.
The publication of "The Origin of Species" placed the study of Botanical Geography on an entirely new basis. It is only necessary to study the monumental "Geographie Botanique raisonnee" of Alphonse De Candolle, published four years earlier (1855), to realise how profound and far-reaching was the change. After a masterly and exhaustive discussion of all available data De Candolle in his final conclusions could only arrive at a deadlock. It is sufficient to quote a few sentences:--"L'opinion de Lamarck est aujourd'hui abandonee par tous les naturalistes qui ont etudie sagement les modifications possibles des etres organises...
"Et si l'on s'ecarte des exagerations de Lamarck, si l'on suppose un premier type de chaque genre, de chaque famille tout au moins, on se trouve encore a l'egard de l'origine de ces types en presence de la grande question de la creation.
"Le seul parti a prendre est donc d'envisager les etres organises comme existant depuis certaines epoques, avec leurs qualites particulieres."(Vol. II. page 1107.)
Reviewing the position fourteen years afterwards, Bentham remarked:--"These views, generally received by the great majority of naturalists at the time De Candolle wrote, and still maintained by a few, must, if adhered to, check all further enquiry into any connection of facts with causes," and he added, "there is little doubt but that if De Candolle were to revise his work, he would follow the example of so many other eminent naturalists, and...insist that the present geographical distribution of plants was in most instances a derivative one, altered from a very different former distribution." ("Pres. Addr." (1869) "Proc. Linn. Soc." 1868-69, page lxviii.)Writing to Asa Gray in 1856, Darwin gave a brief preliminary account of his ideas as to the origin of species, and said that geographical distribution must be one of the tests of their validity. ("Life and Letters", II. page 78.) What is of supreme interest is that it was also their starting-point.
He tells us:--"When I visited, during the voyage of H.M.S. "Beagle", the Galapagos Archipelago,...I fancied myself brought near to the very act of creation. I often asked myself how these many peculiar animals and plants had been produced: the simplest answer seemed to be that the inhabitants of the several islands had descended from each other, undergoing modification in the course of their descent." ("The Variation of Animals and Plants" (2nd edition), 1890, I. pages 9, 10.) We need not be surprised then, that in writing in 1845 to Sir Joseph Hooker, he speaks of "that grand subject, that almost keystone of the laws of creation, Geographical Distribution." ("Life and Letters", I. page 336.)Yet De Candolle was, as Bentham saw, unconsciously feeling his way, like Lyell, towards evolution, without being able to grasp it. They both strove to explain phenomena by means of agencies which they saw actually at work.
If De Candolle gave up the ultimate problem as insoluble:--"La creation ou premiere formation des etres organises echappe, par sa nature et par son anciennete, a nos moyens d'observation" (Loc. cit. page 1106.), he steadily endeavoured to minimise its scope. At least half of his great work is devoted to the researches by which he extricated himself from a belief in species having had a multiple origin, the view which had been held by successive naturalists from Gmelin to Agassiz. To account for the obvious fact that species constantly occupy dissevered areas, De Candolle made a minute study of their means of transport. This was found to dispose of the vast majority of cases, and the remainder he accounted for by geographical change. (Loc. cit. page 1116.)But Darwin strenuously objected to invoking geographical change as a solution of every difficulty. He had apparently long satisfied himself as to the "permanence of continents and great oceans." Dana, he tells us "was, I believe, the first man who maintained" this ("Life and Letters", III. page 247. Dana says:--"The continents and oceans had their general outline or form defined in earliest time," "Manual of Geology", revised edition. Philadelphia, 1869, page 732. I have no access to an earlier edition.), but he had himself probably arrived at it independently. Modern physical research tends to confirm it. The earth's centre of gravity, as pointed out by Pratt from the existence of the Pacific Ocean, does not coincide with its centre of figure, and it has been conjectured that the Pacific Ocean dates its origin from the separation of the moon from the earth.