If Darwin laid the foundation, the present fabric of Botanical Geography must be credited to Hooker. It was a happy partnership. The far-seeing, generalising power of the one was supplied with data and checked in conclusions by the vast detailed knowledge of the other. It may be permitted to quote Darwin's generous acknowledgment when writing the "Origin":--"I never did pick any one's pocket, but whilst writing my present chapter I keep on feeling (even when differing most from you) just as if I were stealing from you, so much do I owe to your writings and conversation, so much more than mere acknowledgements show." ("Life and Letters", II. page 148 (footnote).) Fourteen years before he had written to Hooker: "I know I shall live to see you the first authority in Europe on...Geographical Distribution." (Ibid. I. page 336.) We owe it to Hooker that no one now undertakes the flora of a country without indicating the range of the species it contains. Bentham tells us: "After De Candolle, independently of the great works of Darwin...the first important addition to the science of geographical botany was that made by Hooker in his "Introductory Essay to the Flora of Tasmania", which, though contemporaneous only with the "Origin of Species", was drawn up with a general knowledge of his friend's observations and views." (Pres. Addr.
(1869), "Proc. Linn. Soc." 1868-69, page lxxiv.) It cannot be doubted that this and the great memoir on the "Distribution of Arctic Plants" were only less epoch-making than the "Origin" itself, and must have supplied a powerful support to the general theory of organic evolution.
Darwin always asserted his "entire ignorance of Botany." ("More Letters", I. page 400.) But this was only part of his constant half-humorous self-depreciation. He had been a pupil of Henslow, and it is evident that he had a good working knowledge of systematic botany. He could find his way about in the literature and always cites the names of plants with scrupulous accuracy. It was because he felt the want of such a work for his own researches that he urged the preparation of the "Index Kewensis", and undertook to defray the expense. It has been thought singular that he should have been elected a "correspondant" of the Academie des Sciences in the section of Botany, but it is not surprising that his work in Geographical Botany made the botanists anxious to claim him. His heart went with them. "It has always pleased me," he tells us, "to exalt plants in the scale of organised beings." ("Life and Letters", I. page 98.) And he declares that he finds "any proposition more easily tested in botanical works (Ibid. II. page 99.) than in zoological."In the "Introductory Essay" Hooker dwelt on the "continuous current of vegetation from Scandinavia to Tasmania" ("Introductory Essay to the Flora of Tasmania", London, 1859. Reprinted from the "Botany of the Antarctic Expedition", Part III., "Flora of Tasmania", Vol I. page ciii.), but finds little evidence of one in the reverse direction. "In the New World, Arctic, Scandinavian, and North American genera and species are continuously extended from the north to the south temperate and even Antarctic zones; but scarcely one Antarctic species, or even genus advances north beyond the Gulf of Mexico" (page civ.). Hooker considered that this negatived "the idea that the Southern and Northern Floras have had common origin within comparatively modern geological epochs." (Loc. cit.) This is no doubt a correct conclusion. But it is difficult to explain on Darwin's view alone, of alternating cold in the two hemispheres, the preponderant migration from the north to the south. He suggests, therefore, that it "is due to the greater extent of land in the north and to the northern forms...having...been advanced through natural selection and competition to a higher stage of perfection or dominating power."("Origin of Species" (6th edition), page 340; cf. also "Life and Letters", II. page 142.) The present state of the Flora of New Zealand affords a striking illustration of the correctness of this view. It is poor in species, numbering only some 1400, of which three-fourths are endemic.
They seem however quite unable to resist the invasion of new comers and already 600 species of foreign origin have succeeded in establishing themselves.
If we accept the general configuration of the earth's surface as permanent a continuous and progressive dispersal of species from the centre to the circumference, i.e. southwards, seems inevitable. If an observer were placed above a point in St George's Channel from which one half of the globe was visible he would see the greatest possible quantity of land spread out in a sort of stellate figure. The maritime supremacy of the English race has perhaps flowed from the central position of its home.
That such a disposition would facilitate a centrifugal migration of land organisms is at any rate obvious, and fluctuating conditions of climate operating from the pole would supply an effective means of propulsion. As these became more rigorous animals at any rate would move southwards to escape them. It would be equally the case with plants if no insuperable obstacle interposed. This implies a mobility in plants, notwithstanding what we know of means of transport which is at first sight paradoxical.