Sept. 8, i8Si] 



NATURE 



447 



aad Eastern North America, to the exclusion of the western 

 half of that continent ; and also that Europe and Western Asia 

 did not share in this affinity. But what especially attracted his 

 attention was, that this affinity did not depend only on a few 

 identical or representative genera, but upon many endemic 

 genera of exceptional clmracter, and often consisting of only 

 two almost identical species. This led to a rigorous comparison 

 of th :ise plants with the fossils from the Arctic regions whose 

 affinities had been determined by Heer, and with others which 

 had been meanwhile accumulating in the United States, and had 

 been described by Lesquereux ; and the result was what I may 

 call an abridged outline history of the flora of North America 

 in its relations to the physical geography of that country, from 

 the Cretaceous to the present time. 



The latest researches which have materially advanced our 

 kn 3wledge of the laws of distribution are those of Prof. Blytt 

 of Christiania. His essay on "The Immigration of the Nor- 

 wegian Flora during alternately Rainy and Dry Periods " has 

 for its object to define and localise the various assembliges of 

 plants of which that fljra is composed, and to ascertain their 

 mother-country and the sequence of their introduction. The 

 problem is that of Prof. Forbes, which I have already describid 

 to you, only substituting Norway for the British Isles. Both 

 these authors invoke the glacial period to account for the dis- 

 persion of Arctic plants, both deal with a rising land, both 

 assume that immigration took place over land ; but Prof. Blytt 

 finds another and mo t powerful controlling agent, in alternating 

 period ; of greater moisture and comparative drouj;ht, of which 

 the Norwegian peat bogs afford ample proof. These bogs were 

 formed during the rise of the land, as the cold of the glacial 

 period declined. They are found at various heights above the 

 sea in Norway ; the most elevated of them are of course the 

 oldest, and contain remains of the earliest immigrants. The 

 lowest are the newest, and contain remains of the latest intro- 

 duced plants only. The proofs of the alternating wet and dry 

 seasons rest on the fact that the different Layers of peat in each 

 bog present widely different characters, contain the remains of 

 diBerent assemblages of plants, and these chai-acters recur in the 

 same order in all the bogs. First there is a layer of wet spongy 

 peat, with the remains of bog-mosses and aquatic plants ; this 

 gradually passes upwards into a layer of dry soil containing the 

 remains of many land plants, and prostrate tninks of trees, 

 showing that the countiy was forested. To this succeeds wet 

 spongy peat as before, to be again covered with dry peaty soil 

 and tree trunks, &c. , and so on. From an examination of the 

 plant remains in these formations Prof. Blytt draws the following 

 conclusions : — 



The Norwegian flora began with an immigration of Arctic 

 plants during a dry period, evidence of which he finds in the 

 presence of the remains of these beneath the lowest layer of peat. 

 As the climate became warmer and the land rose, a rainy period 

 set in, accompanied by an immigration of sub-Arctic plants 

 (juniper, mountain ash, aconites, &c. ), which to a great extent 

 replaced the Arctic flora, which is impatient of great wet. This 

 was the period of the first peat -bog formation. It was followed 

 by a dry period, during which the bogs gradually dried up ; 

 while with the increasing warmth, deciduous trees and their 

 accompanying herbaceous vegetation were introduced. The 

 succeeding rainy season produced a second peat-formation, 

 killing and burying the decidujui trees, the increasing warmth 

 at the same time bringing in the Atlantic flora, characterised by 

 the holly, foxglive, and other plants now confined in Norway 

 to the rainy Atlantic coast. To this succeeded a third period of 

 drought, when the bogs dried up, and pine-forests with then- 

 accompanying plants immigrate I into Norway, to be in like 

 manner destroyed and buried by bog earth during the next fol- 

 lowing rainy period ; and it was during these last alternations 

 that the subboreal plants now affecting the lowest south-eastern 

 districts of Norway were introduced ; and the sub-Atlantic 

 plants, the most southern of all the types which are confined to 

 the extreme south of the country. 



It would be premature to regard all Prof. Blytt's recurrent 

 period . as irrefragably established, or his correlations of these 

 with the several floras as fully proved ; but there is no doubt, I 

 think, tliat he has brought forward a vera causa to account for 

 the alternation of dry country witli wet country plants in Norway, 

 and one that must ha\'e both actively promoted the first introduc- 

 tion of these into that country, asalo influenced then subsequent 

 localisation. It would strengthen Prof. Blytt's conclu ions very 

 much if his alternating periods of rain a id drought should be 



found to harjionisc with Mr. CroU's recmrent astronomical 

 periods, and with Mr. Geikie's fluctuations of temperature diu-ing 

 the decline of the Glacial epoch: so would also the finding in 

 the bogs of Scotland a repetition of the conditions which obtain 

 in those of Norway ; and there are so very maoy points of 

 resemblance in the physical geography and vegetation of these 

 two countries that I do not doubt a comparison of their peat 

 formations would yield most instructive results. 



Thus far all the knowledge we have obtained of the agenti 

 controlling geographical distribution have been derived from 

 observations and researches on northern animals and plants, 

 recent aad tertiary. Turning naw to the southern hemisphere, 

 tlie phenomena of distribution are much more difficult of 

 explanation. Geographically speaking, there is no Antarctic 

 flora except a few lichens and sea-weeds. The plants called 

 Antarctic,' from their analogy with the Arctic, are very few in 

 number, and nowhere cro.s the 62" of south latitude. They are, 

 in so far as they are endemic, confined to the southern islands of 

 the great southern ocean, and the mountains of South Chili, 

 Au tralia, Tasmania, and New Zealand ; wltilst the few non- 

 endemic are species of the nearest continents, or are identical 

 with temperate northern or with sub-Arctic or even Arctic 

 species. Like the Arctic flora, the Antarctic Ls a very imiform 

 one round the globe, the same species, in many cases, especially 

 the non-endemic, occurring on everj' island, though there are 

 sometimes thousands of miles of ocean between the nearest of 

 these. And, as many of the island plants reappear on the 

 mountains above mentioned, far to the north of their island 

 homes, it is inferred on these groimds, as well as on astron .)mical 

 and geological, that there was a glacial period in the southern 

 temperate 2one as well as in the northern. 



The south temperate flora is a fourfold one. South America, 

 South Africa, Australia, and New Zealand contain each an 

 assemblage of plants differing more by far amongst themselves 

 than do the floras of Europe, North Asia, and North America ; 

 they contain, in fact, few species in common, except the Ant- 

 arctic ones that inhabit their mountains. These south te.nperate 

 ])lants have their representative species and genera on the moun- 

 tains of the tropics, each in their own meridian only, and there 

 they meet immigrants from all latitudes of the northern hemi- 

 sphere. Thu. the plants of Fuegia extend northward along the 

 Andes, ascending as they advance. Australian genera rcaiipear 

 on the lofty mountain of Kini-balu in Borneo ; New Zealand 

 ones on the mountains of New Caled niia ; and the most inter- 

 esting herbarium ever brought from Central Africa, that of Mr. 

 Joseph Thomson, from the highlands of the lake districts, con- 

 tains many of the endemic genera, and even species of the Cape 

 of Good Hope. Nor does the northern representation of the 

 south temperate flora cease within the tropics ; it extends to the 

 middle north temperate zone ; Chilian genera reappearing in 

 Mexico and California ; South African in North Afi-ica, in the 

 Canary Island-, and even in Asia Minor ' ; and Australian in 

 the Khasia Mountains of East Bengal, m East China and 

 Japan. 



So too there is a representation of genera in the southern tem- 

 perate continents, feeble numerically compared to what the 

 north presents, but strong in other respects. This is shown by 

 the families of Proteacere, Cycadea:, and RestiaceK, abounding 

 ia South Africa and AusU-aUa alone, though not a single species 

 or even genu; of these famiUes is coimnon to the two countries ; 

 by New Zealand, with a fl ara differing in almost every element 

 from the Chilian, yet having a few species of both calceolaria 

 and fuchsia, genera otherwi e purely American; whilst as 

 regards Au-tralia and New Zealand, it is difficult to say which 

 are the most puzzling, the contrasts or the similarities which their 

 animal and vegetable productions present. 



These features of the vegetation of the south temperate and 

 Antarctic regions, though they simulate those of the north 

 temperate and Arctic, may not originate from precisely similar 

 causes. In the absence of such evidence as the fossil animals 

 and plants of the north affords,^ there is no proof that the Ant- 



' For ace lunts of the Antarctic flora see the "Botany" of the Antarctic 

 Expedition of Sir James Ross, where the relations of the fl jras of the southern 

 hemisphere wah the Antarctic are discussed in introductory chapters. 



= Pelar.^onium Endliclt4rinnum in the Taurus is a remarkable instance. 



» The only fossil leaves hitherto found in higher southern latitudes are 

 those of beeches, closely allied to existing southern specie!, brought by 

 Darwin from Fuegia. In one locality alone beyond the forest region of tlie 

 south have fossil plants been found ; there were silicified trunks of trees in 

 lava beds of Kerguelen's Island (discovered by myself forty years ago). It 

 is deeply to be regretted that searches for shales containing fossils wefe not 



