species and (b) the migration of floras. This he did independently, by his own self-thought,' as Darwin termed it. His views are apparent in his earlier publications but are most fully set forth in his 'Introductory Essay to the Flora Tasmaniæ,' dealing with the Antarctic flora as a whole.

His study of Darwin's plants from the Galapagos Islands and their relation to those of other tropical islands and of the South-American continent brought him into close relation with Darwin, whom he visited in 1847. This was the beginning of their memorable intimacy and continuous exchange of letters (contained in these volumes and the similar Life and Letters' of Darwin). These letters were really conversations as to endless botanical details-enquiries made and answered, criticisms and arguments submitted by one to the other. They form a record of surpassing interest to all future generations of biologists. Hooker's stores of knowledge of fact in every department of botanical science were of essential service to Darwin, while Darwin's marvellous fecundity in original suggestions as to the explanation and the significance of facts and his remorseless criticism of those suggestions by appeal to other facts and to experiment, were a perennial stimulus to Hooker, who was himself a theorist, a generaliser-what is sometimes called a philosopher'-of large outlook. Lyell wrote in 1859 to Hooker of the Introductory Essay to the Flora Tasmania': 'I have just finished the reading of your splendid Essay on the Origin of Species, as illustrated by your wide botanical experience, and think it goes far to raise the variety-making hypothesis to the rank of a theory, as accounting for the manner in which new species enter the world.' And Darwin wrote, I have finished your Essay. To my judgment it is by far the grandest and most interesting essay on subjects of the nature discussed I have ever read.'

Hooker was the earliest prominent naturalist to declare his adhesion to the theory of the Origin of Species by Natural Selection set forth by Darwin in his historic volume of 1859, but his complete adhesion to it was only arrived at by long and minute discussion with Darwin of his data, his arguments and inferences, extending over some years both before and after 1859,

in which the two naturalists were in constant communication. It must be borne in mind that Darwin's theory of the survival of favoured varieties by natural selection was something additional to the hypothesis of the derivative origin of species which Hooker had supported. Darwin's theory gave an explanation of that derivation, and showed it to be the necessary result of existing natural causes.

Hooker continued during the next twenty-two years to take a leading part in the development of an understanding of the geographical distribution of organisms on the earth's surface in the light of Darwin's great doctrine of Natural Selection. He was at times much perplexed by the attempt to demarcate natural phytogeographical provinces and sub-provinces, as distinct from merely topographical areas; and finally he seems to have come to the same conclusion as that which he reached in the classification of the vegetable kingdom adopted by him in the monumental work which he produced in collaboration with Bentham, the Genera Plantarum' (3 vols, octavo, 1862-83). This conclusion was that, while we are still seeking a closer knowledge of the phyletic connexions of the floras and faunas of the world, it is, in view of practical purposes (that is to say, for facilitating the accumulation and orderly arrangement of our knowledge), better to adopt a frankly arbitrary series of groups and provinces agreed upon and accepted because they are traditional and serviceable for purposes of reference, than to assume prematurely that we are in a position to define the limits and connexions of all natural phyto-geographical provinces and of all phyletic groups. To do this we have not yet (he thought) sufficient knowledge, though we already see clearly much of the outlines and the needful lines of enquiry.

The means and the causes of the migration of plants were matters of extreme importance in the great problem of distribution and the closely connected problem of the changes of land and water on the earth's surface. These were the subject of speculation and enquiry by both Darwin and Hooker. Hooker had at first put forward the hypothesis of a lost circumpolar continent in order to account for the facts of plant distribution in the southern hemisphere. But Darwin favoured the view

of the persistence even from Silurian times of the great continental masses at present existing, and the radiation from the northern temperate and sub-arctic region of successive floras by spreading along the cold mountain chains which extend through the tongue-like southward projections of continental land-to-day traceable as South America, Africa, and Indo-Malaya. Transport of seeds, etc., by ocean currents, by wind and by birds and other such agencies was shown experimentally by Darwin to be possible in many cases, but the emergence and submergence of large tracts of land as bridges or connexions across the deep ocean-beds were rejected by him. Hooker writes to Darwin in 1881

'Were you not the first to insist on this [the permanence since the Silurian period of the present continents and oceans], or at least to point this out? Do you not think that Wallace's summing-up of the proof of it is good? I know I once disputed the doctrine or rather could not take it in; but let that pass' (vol. ii, p. 224).

He goes on to say, in reference to the address which he was preparing for the British Association meeting at York, in which after many years' labour he expressed his final conclusions on Geographical Distribution,

'I must wind up with the doctrine of general distribution being primarily from north to south with no similar general flow from south to north-thus supporting the doctrine which has its last expression in Dyer's essay read before the Geograph. Society and referred to in my last R.S. address (1879).'

The conclusions at present held on this great subject, which so long occupied Hooker's attention as well as that of his friends Darwin and Wallace, are fully and admirably stated by Hooker's son-in-law and successor at Kew in his article on the Distribution of Plants in the last edition of the Encyclopedia Britannica'-an essay which permanently associates the name of Sir William Thiselton Dyer with those of Hooker and Darwin as a great master in this many-sided field of scientific speculation.

While Hooker never ceased to carry on by his own individual work and that of his staff the preparation Vol. 230.-No. 457.

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and publication of systematic Floras' of all parts of the British Empire, with a view to a full understanding of the origin of species and their geographical distribution (perhaps we should reverse the order of those terms), his botanical work was by no means limited to this. The 'Life' gives a full picture of his activities, which we may briefly summarise by mentioning some of his publications, while his letters, there reproduced, to his father, to Lyell, Darwin, Harvey (of Dublin), Bentham, Bryan Hodgson, Asa Gray, Huxley, Paget, and a host of other friends and fellow-workers, reveal the methods of his scientific work as well as his aims and struggles, the steps of his official aud public career and his family life. From them too we can gather his views not only on scientific problems but on art, literature, politics, education and religion.

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From the long list of his works (other than those already cited) we select first that on 'The Rhododendrons of Sikkim-Himalaya' (1849–51), edited by his father from material sent home by him while he was away collecting, drawing and mapping in the Himalayas. It is a sample of the beauty of form and colour which entrances the true naturalist however austere may be his devotion (as was Hooker's) to pure science. He writes, 'It is a far grander and better book than even I expected. . . . All the Indian world is in love with my Rhododendron book.' Then we have his Himalayan Journals; ' notes of a naturalist in Bengal, the Sikkim and Nepal Himalayas, the Khasia Mountains, etc.' (1854, re-issued 1905), a book like Darwin's 'Voyage of the Beagle' and Wallace's 'Malay Archipelago' for all to read and enjoy ; his Students' Flora of the British Islands' (1870), which has run through three editions; and his 'Primer of Botany' (1876), which has been reprinted twenty times in three editions-'the rashest and most profitable of all my undertakings,' as he called it in a letter to Asa Gray. His paper 'On the Diatomaceous Vegetation of the Antarctic Ocean' (Brit. Assoc. Reports, 1847) was the forerunner of that study of oceanic deposits which many years later became (especially in connexion with the voyage of the Challenger') a great and important branch of research. Similarly his papers on Stigmaria and Lepidostrobi in the memoirs of the Geological Survey,

1848, were the starting-point of the study of the tissues of ancient fossil plants by means of the microscope. He was the first to have sections of fossils cut sufficiently transparent for that purpose, a method which in the hands of a later generation has yielded very important results.

In the domain of Physiology, besides some other contributions, there stands out his remarkable work on the attraction, capture and digestion of insects by the Pitcher Plants (Brit. Assoc. Reports, Belfast, 1874, and 'Nature,' 1870). The work was suggested by Darwin when investigating the carnivorous habits of the Sundew (Drosera). Experiments as to the digestive ferment and microscopical investigation of the glands, etc., were made by Hooker, aided by Dyer, at Kew. In the special study and exploration of remarkable morphological characters, Hooker's investigation of the root parasites known as Balanophorea-curiously simple in structure, without leaves or petals-formerly thought to be allied to the Fungi but shown by Hooker to be degenerate mistletoes, is a sample of his morphological work (On the structure and affinities of the Balanophoreæ, Linnæan Society Transactions, 1856). He made acquaintance with these strange plants both in New Zealand and in the Himalayas.

But the most striking thing which he did in this way was his description of the morphology, development and histology and the determination of the affinities of a weird-looking South-African plant discovered by Dr Welwitsch in dry country inland from Walfisch Bay, and sent by him to Kew. Hooker named it after its discoverer; and specimens of it (since received through other travellers) have been kept in cultivation ever since in one of the hot-houses at Kew ('On Welwitschia, a new genus of Gnetaceæ,' Trans. Linn. Soc., 1863). Hooker's triumph in this investigation was that of showing, by microscopic examination of the tissues and of the reproductive structures and their development, that this strange-looking plant is one of the Gnetaceæ, a family including the little European Ephedra and grouped with the Cycads, the Gingko trees and the Conifers in the great assemblage called Gymnosperms. In the Life and Letters' we have a delightful picture (which will stir the sympathy of every morphologist) of his excitement,