734 EEPORT — 1881. 



of 300 feet above the sea-level. And again in 1854, Dr. Lyall found exten- 

 sive accumulations of similar fossils near Discoe in Greenland (lat. 70° N.), during 

 the return of Sir Edv^ard 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 within ten degrees of the Pole. The first of these 

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

 Heer of Zurich, the highest authority on the flora of the tertiary period, and described 

 by him,' as were also subsequently 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, liqiii- 

 damber, 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 was a botanist. Dr. Asa Gray, who pursued it with brilliant results, em- 

 bodied in a series of memoirs on the vegetation of the United States of America, 

 of which my notice must be most brief. 



When studying the collections of Japanese plants brought by the officers of 

 Wilkes' expedition, Dr. Gray found cumulative evidence of the strong affinity 

 between the flora of Eastern Asia and 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 character, and often consisting of only 

 two almost identical species. This led to a rigorous comparison of those 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 

 State's, and had been described by Lesquereux ; and the result was what I may call 

 an abr'ido-ed 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 knowledge of the 

 laws of distribution are those of Prof. Blytt, of Christiania. His essay on ' the 

 immigration of the Norwegian flora during alternately rainy and dry periods, has 

 for its' object to define and localise the various assemblages of plants of which that 

 flora is composed, and to ascertain their mother-country and the sequence of their in- 

 troduction. The problem is that of Prof. Forbes, which I have already described 

 to vou only substituting Norway for the British Isles. Both these authors invoke 

 the" o-lacial period to account for the dispersion of Arctic plants, both deal with a 

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

 flnds another and most powerful controlling agent, in alternating periods of greater 

 moisture and comparative drought, of which the Norwegian peat-bogs afford ample 

 proof. These boo-s 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 Nor- 

 way • 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 introduced plants only. The proofs of the alternating wet and dry seasons 

 rest on the fact, that the diiferent layers of peat in each bog present widely different 

 characters, contain the remains of different assemblages of plants, and these 

 characters' 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, 



• ' Ueber die von Dr. Lyall in Gronland entdeckten fossilen Pflanzen,' Zurich 

 ViertcljalirsscUr. vol. vii. p. 176 (1862). 

 2 Florii fossilis Arctica. 



