446 



NATURE 



\_Sept. 8, 1 88 1 



Islands considered with regard to geological changes. ' In this 

 paper the British flora is considered to consist of assemblages of 

 plants from five distinct sources, which, with the exception of 

 one, immigrated during periods when the British Isles were 

 united to the continent of Europe, and have remained more or 

 less localised in England, in Scotland, or in Ireland. Of these 

 he considered the Pyrenean assemblage, which is confined to the 

 west of Ireland, to be the oldest, and to have immigrated, after 

 the eocene period, along a chain of now submerged mountains, 

 that extended across the Atlantic from Spain to Ireland, and 

 indeed formed the eastern boundary of an imaginary continent of 

 miocene age, v\liich extended to the Azores Islands, and beyond 

 them. This, the "Atlantis" of speculative geologists, has long 

 since been abandoned. Tlie second assemblage is of plants 

 characteristic of the Sonth-West of France, which now pre\'ail 

 in Devon, Cornwall, and the Channel Islands ; their immigration 

 he assigns to a miocene date, probably corresponding to the red 

 crag. The third assemblage is of plants of the North-East of 

 France, which abound in the chalk districts of the South-Eastern 

 counties of England ; their immigration is referred to the era of 

 the mammaliferous crag. The fourth is of Alpine plants now 

 found on the mountains of Scotland, Wales, and England ; these 

 were introduced mainly by floating ice from Scandinavia during 

 the glacial period, when the greater part of the British Isles 

 were sub.nerged, its mountain tops forming part of a chain of 

 islands in the glacial sea that extended to the coast of Norway ; 

 this was during the newer pliocene period. Lastly, the Germanic 

 plants were introduced during the upheaval of the British Islands 

 from the glacial ocean, and as the temperature was gradually 

 increasing ; these are spread over the whole islands, though more 

 abundant on the Eastern side. At the commencement of thi-. 

 immigration England was supposed to be continuous with the 

 Germanic plains, from which it was subsequently severed by tlie 

 formation of the English Channel. Also, at the commencement 

 of this immigration, Ireland was assumed to be continuous an ith 

 England, to be early severed by the formation of the Irish Sea ; 

 which severance, by interrupting the migration of Germanic 

 types, accounts for the absence of so many British animals in the 

 sister island. 



I have thus briefly related Forbes' views, to show how pro- 

 foundly he was impressed with the belief that geographical and 

 cUmatal conditions were the all-powerful controllers of the 

 migrations of animals and plants. Forbes was the reformer of 

 the science of geographical distribution.- 



Before the publication of the d DCtrine of the origin of species 

 by variation and natural selection, all reasoning on their distribu- 

 tion w as in subordination to the idea that these were permanent 

 and special creations ; just as, before it was shown that species 

 were often older than the inlands and mountains they inhabited, 

 naturalists had to make their theories accord with the idea that all 

 migration tooli place under existing conditions of land and sea. 

 Hitherto the modes of dispersion of species, genera, and families 

 had been traced ; but the origin of representative species, genera, 

 and families remained an enigma 3 ; these could be explained 

 only by the supposition that the localities where they occurred 

 presented conditions so similar that they favoured the creation of 

 similar organisms, which failed to account for representation 

 occurring in the far more numerous cases where there is no dis- 

 coverable similarity of physical conditions, and of their not 

 occuiTing in places ^A-here the conditions are similar. Now under 

 the theory of modification of species after migration and isola- 

 tion, their representation in distant localities is only a question 



' British Associaiiox Reports, 1845, pt. ii. p. 67, and Amials and Maga- 

 zine of Natu7-al H istory , vol. xvi. p. 126. This tlie author followed by a 

 much fuller exposition of the subject, which appeared in the Memoirs of the 

 Geological Survey of the United Kingdom, vol. i. p. 336(1846), entitled " On 

 the Connection between the distribution of the existing flora and fauna of the 

 British Isles, and the geological changes which have affected their area, 

 especially during the epoch of the northern drift." After many years 

 interval I have re-read this Memoir with increased pleasure and profit- The 

 stores of exact information which he collected concerning the plants, the 

 animals, and the geology of Europe and North America, appear to me to be 

 ■ no less remarkable than the skill with which he correlated them and educed 

 from the whole so many very original and in great part incontrovertible 

 conclusions. 



- I cannot dismiss the subject of the geography of the British flora without 

 aa allusion to the labours of Hewett Cottrell Watson, who, after a life devoted 

 to the topography of British plants, was laid in the grave only a month ago. 

 Watson was the first botanist who measured the altitudinal range of each 

 species, and. by a rigidly statistical method, traced their distribution in every 

 county, and grouped them according to their continental affinities, as well as 

 by the physical c -nditions of their habitats. 



^ The representation of species Forbes alludes to as *' an accident . . . 

 which has hitherto not been accounted for" (Mem. Geol. Survey, ;Vol. i. 

 P 350. 



of time and changed physical conditions. In fact, as Darwin 

 well sums up, all ' the leading facts of distribution are clearly 

 explicable under this theory ; such as the multiplicatio.i of new 

 forms ; the importance of barriers in forming and separating 

 zoological and botanical provinces ; the concentration of related 

 species in the same area ; the linking together under different 

 latitudes of the inhabitants of the plains and mountains, of the 

 forests, marshes, and deserts, and the linking of these with the 

 extinct beings which formerly inhabited the same areas ; and the 

 fact of different forms of life occurring in areas having nearly 

 the same physical conditions. 



With the establishment of the doctrine of the orderly evolu- 

 tion of species under known laws, I close this list of those 

 recognised principles of the science of geographical distribution 

 which must guide all who enter upon its pursuit. As Humboldt 

 was its founder, and Forbes its reformer, so we must regard 

 Darwin as its latest and greatest lawgiver. With their example, 

 and their conclusions to guide, advance becomes possible when- 

 ever discoveiy opens new paths, or study and reflection retraverse 

 the old ones. 



And it was not long before palseontology brought to the sur- 

 face new data for the study of the present and past physical 

 geography of the globe. 



This was the discovery in Arctic latitudes of fossil plants whose 

 existing representatives are to be found only in warm temperate 

 ones. To Arctic travellers and voyagers this discovery is wholly 

 due. Of these I believe I am correct in saying that Sir John 

 Richardson was the earliest, for he, in the year 1848, when 

 descending the McKenzie River to the Polar Sea in search of 

 the Franklin Expedition, found in lat. 65° N. beds of coal, be- 

 sides shales full of leaves of forest-trees belonging to such genera 

 as the maple, poplar, taxodium, oak, &c. In the narrative of 

 his journey - Richardson mentions these fossils, and figures some 

 of them; and in a subsequent work' he speaks of them as 

 " leaves of deciduous trees belonging to genera which do not in 

 the present day come so far north on the American contiiient by 

 ten or twelve degrees of latitude." This discovery vN-as followed, 

 in 1853, by the still more remarkable one, by Capt. M'Clure and 

 Sir Alexander Armstrong (during another search for Sir John 

 Franklin), of pine cones and acorns imbedded in the soil of 

 Banksland, in lat. 75° N., at an elevation of 300 feet above 

 the sea level. And again in 1854 Dr. Lyall found extensive 

 accumulations of similar fossils near Discoe in Greenland (lat. 

 70° N.), during the return of Sir Edward Belcher's searching 

 expedition. Nor are these fossils confined to America : they 

 have been found in Spitzbergen, in Siberia, and in many other 

 localities within the Polar area as well as south of it, proving 

 that forests of deciduous trees, in all respects like those of the 

 existing forests of the warm temperate regions, approached to 

 M-ithin ten degrees of the Pole. The first of these collections 

 critically examined was Dr. Lyall's ; it was communicated to 

 Prof. Heer of Zurich, the highest authority on the flora of the 

 Tertiary period, and described by him,* as were also subse- 

 quently all the other collections brought from the Arctic 

 regions.^ 



The examination of these fossil leaves revealed the wonderful 

 fact that, not only did they belong to genera of trees common 

 to the forests of all the three northern continents, such as planes, 

 beeches, ashes, maples, &c., but that they also included what 

 are now extremely rare and even local genera, as sequoia, liquid- 

 amber, magnolia, tulip-trees, gingkos, &c., proving that the 

 forests were of a more mixed character than any now existing. 

 These results opened up a new channel for investigating the 

 problem of distribution, and the first naturalist to enter it as a 

 botanist. Dr. Asa Gray, who pursued it with brilliant restilts, 

 embodied in a series of memoirs on the vegetation of the United 

 States of America, and of which my notice must be most brief. 



When studying the collections of Japanese plants Ijrought by 

 the ofiicers of Wilkes' expedition. Dr. Gray found cumulative 

 evidence of the strong affinity betweea the flora of Eastern Asia 



' Of the many pre-Darwinian writers on distribution who advoc.ated the 

 Lamarckian doctrine of evolution,! am not aware of any who suggested that 

 it would explain the existence of representative species, or indeed any 

 other of the phenomena of distribution. Von Baer, however, in the «ry 

 year of the publication of the first edition of the " Origin of Species," ex- 

 pressed his conviction, chieflj; grounded on the laws of geographical distil- 

 bution, that forms now specifically distinct have descended from a single 

 parent form. See "Origin of Species," ed. 5, Historical Sketch, p. 23. . 



= " Boat Voyage through Rupert's Land and in the Arctic Sea, vol 1. 

 p. 1 86. 3 " Polar Regions," p. 28g. ,^ 



* " Ueber die von Dr. Lyall in Gronland entdeckten fossUen PBanzen 

 Zurich Vierteljahrschr. vol vii. p 176 (1862). 



5 " Flora fossilis Arctica." 



