12 



NATURE 



\JMay I, 1S79 



at work are upon stratigraphical evidence certainly of the 

 ages to which they are ascribed. 



We have a limited Thanet sand flora ; a considerable 

 insight into the Woohvich and Reading Beds flora, 

 obtained from Dulwich, Reading, Newhaven ; an Old- 

 haven flora from Bromley; an extensive London clay flora 

 from Sheppey ; a Lower Bagshot flora from Alum Bay, 

 Studland, and Corfe; a Middle Bagshot flora from Bourne- 

 mouth and Bovey Tracey ; upper eocene floras from 

 Hordwell, Gurnet Bay, &c. All these will be embraced 

 in the monograph now in course of publication by 

 Ettingshausen and myself 



The nearly unbroken sequence seen in the eocene floras 

 extends into the miocene. There is no great break in 

 passing from one to the other when we compare them 

 over many latitudes, and but little change, beyond that 

 brought about by altered temperature or migration. If 

 tertiary floras of difl^erent ages are met with in one area, 

 ^eat changes on the contrary are seen, and these are 

 mainly due to progressive changes in climate. From 

 middle eocene to miocene the heat imperceptibly dimin- 

 ished. Very gradually the tropical members of the flora 

 disappeared ; that is to say, they migrated, for most of their 

 types, I think, actually survive at the present day, many 

 but very slightly ahered. Then the sub-tropical members 

 decreased, and the temperate forms, never quite absent 

 even in the middle eocenes, preponderated. As decreasing 

 temperature drove the tropical forms south, the more 

 northern must have pressed closely upon them. The 

 northern eocene, or the temperate floras of that period, 

 must have pushed, from their home in the far north, more 

 and more south as climates chilled, and at last in the 

 miocene time, occupied our latitudes. The relative pre- 

 ponderance of these elements, I bcliere, will assist in 

 determining the age of tertiary deposits in Europe, more 

 than any minute comparisons of species. Thus it is 

 useless to seek in the Arctic regions for eocene floras, 

 as we know them in our latitudes, for during the tertiary 

 period, the climatic conditions of the earth did not permit 

 their growth there. Arctic fossil floras of temperate and 

 therefore to Heer miocene aspect, are in all probability of 

 eocene age, and what has been recognised in them as a 

 newer or miocene facies, is due to their having been first 

 studied in Europe, in latitudes which only became fitted 

 for them in miocene times. 



When stratigraphical eviderce is silent or inconclusive, 

 this unexpected persistence and migration of plant-types 

 or species throughout the tertiaries, should be remem- 

 bered, and the degrees of latitude in which they are found 

 should be well considered before conclusions are pub- 

 lishea respecting their age. 



I need not here point out how completely this theory 

 accords with that of the dispersion or migration of species 

 from a northerly centre, so ably treated of by Asa Gray, 

 Dawson, Dyer, Saporta, Hooker, and in fact by all who 

 have pondered upon the subject, excepting Heer, for I hope 

 to write a few words upon this at a future time. Before 

 quitting it, for the present, Heer may as well learn that I 

 am not alone in my opinions, for Prof. J. W. Dawson, of 

 Montreal, considers with me that the reference of the 

 beds in Greenland to miocene is not warranted by com- 

 parison with the tertiary plants of America. 



" Immediately above these upper cretaceous beds we 

 have the great lignite tertiary of the west — the Laramie 

 group of recent American reports— abounding in fos- 

 sil plants, at one time regarded as miocene, but now 

 known to be lower eocene, though extending upward 

 toward the miocene age. These beds, with their charac- 

 teristic plants, have been traced into the British territory 

 north of the 49th parallel, and it has been shown that 

 their fossils are identical with those of the McKenzie River 

 Valley, described by Heer as miocene, and probably also 

 with those of Alaska, referred to the same age. Now 

 this tnfly eocene flora of the temperate and northern 



parts of America has so many species in common with 

 that called miocene in Greenland, that its identity can 

 scarcely be doubted. These facts have led to scepticism 

 as to the miocene age of the upper plant-bearing beds of 

 Greenland, and more especially Mr. J. Starkie Gardner 

 has ably argued, from comparison with the eocene flora 

 of England and other considerations that they are really 

 of that earlier date." 1 



Private correspondence has already informed me that 

 others now share in these views. 



Not content with withering my theories as to the eocene 

 age of part of his miocene Arctic floras, Heer tilts against 

 my explanation of the former higher temperatures which 

 are known to have prevailed in our own and more 

 northern latitudes. My explanation is, however, justi- 

 fied by our experience of what we conceive to be natu- 

 ral laws, and does not contradict that experience, and. 

 Heer has no theory to set up in its place. 



The differences in the temperatures of the seas wash-^ 

 ing Arctic lands in the same latitudes are seen to alte 

 the isothermal temperatures of their coasts to the extenfi 

 of 27° ; that is to say, the coasts which are refriger-l 

 ated by the descending ice-Iadcn currents are 27° colde 

 than the shores of the North Cape, which are washed by 

 an ascending current. With this fact and its causes.^ 

 palpably before us, we are justified in inferring that ifi 

 the cold currents were shut off from these coasts, their ' 

 temperature would rise by some 27". The cold cur- 

 rents were shut off in the eocene time, for plants and J 

 animals passed freely between Europe and America, and ' 

 therefore the temperature of the northern eocene lands 

 may haye been from this cause some 27° higher. Buq 

 the Arctic eocene floras only required about 20° highe 

 temperature, and the cause invoked is therefore more 

 than sufficient. 



Heer agrees with me that the higher temperature a| 

 the North Cape is due to warmer sea, and that continents 

 extending far south also have their influence. He objectsj 

 that Spitzbcrgen, being within the influence of the GulT 

 Stream, has a temperature of only 7° above the mean ol 

 its latitude. But then Spitzbergen is not shut in by thfl 

 Gulf Stream, but only washed along one shore by it, and 

 that after its current had been enfeebled and refrigerated] 

 to the last degree by the icy water it has to press through. 

 Yet slight as the cause then is, it raises the isotherm o{j 

 Spitzbergen 7°. He again objects that the closing 

 these outlets would stop the flow of the Gulf Strear 

 This, however, would not be the case completely, 

 long as any difference existed between the temperature td 

 the north, and that under the tiopics, a circulation would 

 continue and would only cease when the whole .•\tlantic 

 north of the Equator, had reached a uniform heat. Nc 

 streams only, but the whole Atlantic from the Equate 

 northwards, would be enormously wanned, and evefl 

 parts of continents most remote from seas, would fed 

 the influence. 



This theory if true, Heer says, is at all events not 

 original. In that case, so much the more likely to be true^ 

 but it is original to me.^ It is true that very many theorie 

 have been put forward to account for former temperatures 

 and some of these have been based upon altered distribM 

 tions of land and sea. But while some required changij 

 in the level of the sea, and others involved entirely nove 

 continental areas, none have been supported by any kind 

 of proof, either that the supposed changes had actualW 

 taken place, or were even competent to account for thjj 

 former temperatures. The theory 1 have ventured to pu 

 forward is only absurd in its simplicity. The Atlantic 

 may be likened to a great bath heated by the sun, fron 

 which we may shut off the cold taps either partially 

 entirely, from one or from both ends, thereby producing" 



* "The Genesis and Migrations of Plants," by J. W. Dawson. The 

 Princeton Review, 1879, p. 282. 

 2 Natukk, v. I. xix. p. 123. 



