had been set down for reading at the Linnean Society on the same day as Darwin's； but had to give way。 In this he advocated the fixity of species。 He withdrew it after hearing Darwin's。 We can hardly realise now the momentous effect on the scientific thought of the day of the announcement of the new theory。 Years afterwards （1882） Bentham； notwithstanding his habitual restraint； could not write of it without emotion。 〃I was forced； however reluctantly； to give up my long…cherished convictions； the results of much labour and study。〃 The revelation came without preparation。 Darwin； he wrote； 〃never made any communications to me in relation to his views and labours。〃 But； he adds； I。。。fully adopted his theories and conclusions； notwithstanding the severe pain and disappointment they at first occasioned me。〃 （〃Life and Letters〃； II。 page 294。） Scientific history can have few incidents more worthy。 I do not know what is most striking in the story； the pathos or the moral dignity of Bentham's attitude。
Darwin necessarily restricted himself in the 〃Origin〃 to establishing the general principles which would account for the facts of distribution； as a part of his larger argument； without attempting to illustrate them in particular cases。 This he appears to have contemplated doing in a separate work。 But writing to Hooker in 1868 he said：〃I shall to the day of my death keep up my full interest in Geographical Distribution； but I doubt whether I shall ever have strength to come in any fuller detail than in the 〃Origin〃 to this grand subject。〃 （〃More Letters〃； II。 page 7。） This must be always a matter for regret。 But we may gather some indication of his later speculations from the letters； the careful publication of which by F。 Darwin has rendered a service to science； the value of which it is difficult to exaggerate。 They admit us to the workshop； where we see a great theory； as it were； in the making。 The later ideas that they contain were not it is true public property at the time。 But they were communicated to the leading biologists of the day and indirectly have had a large influence。
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。 Bentham has stated this in a striking way： 〃Fixed and immovable as is the individual plant； there is no class in which the race is endowed with greater facilities for the widest dispersion。。。Plants cast away their offspring in a dormant state； ready to be carried to any distance by those external agencies which we may deem fortuitous； but without which many a race might perish from the exhaustion of the limited spot of soil in which it is rooted。〃 （Pres。 Addr。（1869）； 〃Proc。 Linn。 Soc。〃 1868…69； pages lxvi； lxvii。）
I have quoted this passage from Bentham because it emphasises a point which Darwin for his purpose did not find it necessary to dwell upon； though he no doubt assumed it。 Dispersal to a distance is； so to speak； an accidental incident in the life of a species。 Lepidium Draba； a native of South…eastern Europe； owes its prevalence in the Isle of Thanet to the disastrous Walcheren expedition； the straw…stuffing of the mattresses of the fever…stricken soldiers who were landed there was used by a farmer for manure。 Sir Joseph Hooker （〃Royal Institution Lecture〃； April 12； 1878。） tells us that landing on Lord Auckland's Island； which was uninhabited； 〃the first evidence I met with of its having been previously visited by man was the English chickweed； and this I traced to a mound that marked the grave of a British sailor； and that was covered with the plant； doubtless the offspring of seed that had adhered to the spade or mattock with which the grave had been dug。〃
Some migration from the spot where the individuals of a species have germinated is an essential provision against extinction。 Their descendants otherwise would be liable to suppression by more vigorous competitors。 But they would eventually be extinguished inevitably； as pointed out by Bentham； by the exhaustion of at any rate some one necessary constituent of the soil。 Gilbert showed by actual analysis that the production of a 〃fairy ring〃 is simply due to the using up by the fungi of the available nitrogen in the enclosed area which continually enlarges as they seek a fresh supply on the outside margin。 Anyone who cultivates a garden can easily verify the fact that every plant has some adaptation for varying degrees of seed…dispersal。 It cannot be doubted that slow but persistent terrestrial migration has played an enormous part in bringing about existing plant…distribution； or that climatic changes would intensify the effect because they would force the abandonment of a former area and the occupation of a new one。 We are compelled to admit that as an incident of the Glacial period a whole flora may have moved down and up a mountain side； while only some of its constituent species would be able to take advantage of means of long…distance transport。
I have dwelt on the impor